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[gawk-diffs] [SCM] gawk branch, master, updated. e6b05afd9971b457c0b4690


From: Arnold Robbins
Subject: [gawk-diffs] [SCM] gawk branch, master, updated. e6b05afd9971b457c0b46907a91185b66be8ff4e
Date: Fri, 17 Aug 2012 09:38:44 +0000

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- Log -----------------------------------------------------------------
http://git.sv.gnu.org/cgit/gawk.git/commit/?id=e6b05afd9971b457c0b46907a91185b66be8ff4e

commit e6b05afd9971b457c0b46907a91185b66be8ff4e
Merge: 3b23e17 76cc4d2
Author: Arnold D. Robbins <address@hidden>
Date:   Fri Aug 17 12:38:04 2012 +0300

    Merge branch 'extgawk'

diff --cc ChangeLog
index d4dc5e4,dd0dad2..1cc23e7
--- a/ChangeLog
+++ b/ChangeLog
@@@ -1,7 -1,597 +1,601 @@@
+ 2012-08-12         Arnold D. Robbins     <address@hidden>
+ 
+       * gawkapi.h: Make the versions enum constants instead of defines.
+ 
+ 2012-08-11         Andrew J. Schorr     <address@hidden>
+ 
+       * awkgram.y (add_srcfile): It is now a fatal error to load the
+       same file with -f and -i (or @include).
+       * TODO.xgawk: Update to reflect this change.
+ 
+ 2012-08-10         Arnold D. Robbins     <address@hidden>
+ 
+       * FUTURES, TODO.xgawk: Updates.
+ 
+ 2012-08-08         Arnold D. Robbins     <address@hidden>
+ 
+       * configure.ac: Add -DNDEBUG to remove asserts if not developing.
+ 
+       * gawkapi.h: Document how to build up arrays.
+       * gawkapi.c (api_sym_update): For an array, pass the new cookie
+       back out to the extension.
+ 
+       * awk.h (IOBUF): Move struct stat into IOBUF_PUBLIC.
+       (os_isreadable): Change to take an IOBUF_PUBLIC.
+       * gawkapi.h (IOBUF_PUBLIC): Received struct stat.
+       (INVALID_HANDLE): Moves to here.
+       * io.c (iop_alloc): Stat the fd and fill in stat buf.
+       (iop_finish): Use passed in stat info.
+ 
 +2012-08-05         Arnold D. Robbins     <address@hidden>
 +
 +      * README.git: More stuff added.
 +
+ 2012-08-01         Arnold D. Robbins     <address@hidden>
+ 
+       * io.c (iop_finish): New function.
+       (iop_alloc): Add errno_val parameter. Move code into iop_finish.
+       Add large explanatory leading comment.
+       (after_beginfile): Rework logic. Check for input parser first, then
+       check for invalid iop.
+       (nextfile): Organize code better. Call iop_alloc then iop_finish.
+       (redirect): Call iop_alloc, find_input_parser, iop_finish.
+       (two_way_open): Call iop_alloc, find_input_parser, iop_finish.
+       (gawk_popen): Call iop_alloc, find_input_parser, iop_finish.
+       (find_input_parser): Set iop->valid if input parser takes control.
+       (get_a_record): Rework setting RT to use macros.
+ 
+ 2012-07-29         Andrew J. Schorr     <address@hidden>
+ 
+       * awk.h (set_RT_to_null, set_RT): Removed.
+       * gawkapi.h (api_set_RT): Removed.
+       (get_record): Signature changed in input parser struct.
+       * gawkapi.c (api_set_RT): Removed.
+       * io.c (set_RT_to_null, set_RT): Removed.
+       (get_a_record): Adjustments for new API for input parser.
+ 
+ 2012-07-29         Arnold D. Robbins     <address@hidden>
+ 
+       * awk.h (os_isreadable): Adjust declaration.
+       (struct iobuf): Add new member `valid'.
+       * io.c (iop_alloc): Remove do_input_parsers parameter, it's
+       always true. Adjust logic to set things to invalid if could not
+       find an input parser.
+       (after_beginfile): Use valid member to check if iobuf is valid.
+       Don't clear iop->errcode.
+       (nextfile): Adjust logic to clear errcode if valid is true and
+       also to update ERRNO.
+       (redirect): Check iop->valid and cleanup as necessary, including
+       setting ERRNO.
+       (two_way_open): Ditto.
+       (gawk_popen): Ditto.
+       (devopen): Remove check for directory.
+ 
+ 2012-07-27         Andrew J. Schorr     <address@hidden>
+ 
+       * io.c (find_input_parser): Issue a warning if take_control_of fails.
+ 
+ 2012-07-27         Arnold D. Robbins     <address@hidden>
+ 
+       * awk.h (set_RT): Change to take a NODE * parameter.
+       * io.c (set_RT): Change to take a NODE * parameter.
+       * gawkapi.h: Change open hook to input parser in comment.
+       * gawkapi.c (api_set_RT): Adjust call to set_RT.
+ 
+ 2012-07-26         Arnold D. Robbins     <address@hidden>
+ 
+       * awk.h (set_RT_to_null, set_RT): Declare functions.
+       (os_isreadable): Declare function.
+       * io.c (set_RT_to_null, set_RT): New functions.
+       (iop_close): Init ret to zero.
+       * gawkapi.c (api_register_input_parser): Check for null pointer.
+       (api_set_RT): New function.
+       * gawkapi.h (api_set_RT): New function.
+ 
+ 2012-07-26         Andrew J. Schorr     <address@hidden>
+ 
+       * gawkapi.h (IOBUF_PUBLIC): Document the get_record and close_func
+       API.
+       (awk_input_parser_t) Change can_take_file argument to const, and
+       document the API.
+       * io.c (get_a_record): Document that the caller initializes *errcode
+       to 0, and remote the test for non-NULL errcode.
+ 
+ 2012-07-26         Andrew J. Schorr     <address@hidden>
+ 
+       * gawkapi.c (api_sym_update_scalar): Fix some minor bugs.  Was
+       not updating AWK_NUMBER when valref != 1.  And strings were not
+       freeing MPFR values.
+ 
+ 2012-07-25         Arnold D. Robbins     <address@hidden>
+ 
+       Start refactoring of IOBUF handling and turn "open hooks"
+       into "input parsers".
+ 
+       * awk.h (IOP_NOFREE_OBJ): Flag removed.
+       (register_input_parser): Renamed from register_open_hook.
+       * ext.c (load_ext): Make sure lib_name is not NULL.
+       * gawk_api.c (api_register_input_parser): Renamed from
+       api_register_open_hook.
+       * gawk_api.h (api_register_input_parser): Renamed from
+       api_register_open_hook.  Rework structure to have "do you want it"
+       and "take control of it" functions.
+       * io.c (iop_alloc): Remove third argument which is IOBUF pointer.
+       Always malloc it. Remove use of IOP_NOFREE_OBJ everywhere.
+       (find_input_parser): Renamed from find_open_hook.
+       (nextfile): Don't use static IOBUF.
+       (iop_close): Call close_func first. Then close fd or remap it
+       if it's still not INVALID_HANDLE.
+       (register_input_parser): Renamed from register_open_hook.
+       Use a FIFO list and check if more than one parser will accept the
+       file. If so, fatal error.
+ 
+ 2012-07-25         Andrew J. Schorr     <address@hidden>
+ 
+       * configure.ac: Instead of using acl_shlibext for the shared library
+       extension, define our own variable GAWKLIBEXT with a hack to work
+       correctly on Mac OS X.
+       * Makefile.am (SHLIBEXT): Use the value of GAWKLIBEXT instead of
+       acl_shlibext.
+ 
+ 2012-07-24         Arnold D. Robbins     <address@hidden>
+ 
+       * configure.ac: Add crude but small hack to make plug-ins work
+       on Mac OS X.
+ 
+ 2012-07-20         Arnold D. Robbins     <address@hidden>
+ 
+       * gawkapi.h: Rework table to not take up so much space.
+       * gawkapi.c (api_sym_update_scalar): Rework optimization code
+       to clean up the function.
+ 
+ 2012-07-17         Andrew J. Schorr     <address@hidden>
+ 
+       * gawkapi.h: Add comments explaining new api_create_value and
+       api_release_value functions.
+       * gawkapi.c (sym_update_real): Allow updates with AWK_SCALAR and
+       AWK_VALUE_COOKIE types.  After creating a regular variable,
+       remove the call to unref(node->var_value), since this is not
+       done elsewhere in the code (see, for example, main.c:init_vars).
+       If the update is for an existing variable, allow any val_type
+       except AWK_ARRAY (was previously disallowing AWK_SCALAR and
+       AWK_VALUE_COOKIE for no apparent reason).
+       (api_sym_update_scalar): The switch should return false for an
+       invalid val_type value, so change the AWK_ARRAY case to default.
+       (valid_subscript_type): Any scalar value is good, so accept any valid
+       type except AWK_ARRAY.
+       (api_create_value): Accept only AWK_NUMBER and AWK_STRING values.
+       Anything else should fail.
+ 
+ 2012-07-17         Arnold D. Robbins     <address@hidden>
+ 
+       Speedup:
+ 
+       * awk.h (r_free_wstr): Renamed from free_wstr.
+       (free_wstr): Macro to test the WSTRCUR flag first.
+       * node.c (r_free_wstr): Renamed from free_wstr.
+ 
+       Support value cookies:
+ 
+       * gawkapi.h (awk_val_type_t): Add AWK_VALUE_COOKIE.
+       (awk_value_cookie_t): New type.
+       (awk_value_t): Support AWK_VALUE_COOKIE.
+       (api_create_value, api_release_value): New function pointers.
+       * gawkapi.c (awk_value_to_node, api_sym_update_scalar,
+       valid_subscript_type): Handle AWK_VALUE_COOKIE.
+       (api_create_value, api_release_value): New functions.
+ 
+ 2012-07-16         Arnold D. Robbins     <address@hidden>
+ 
+       * gawkapi.c (awk_value_to_node): Support AWK_SCALAR.
+       (api_sym_update_scalar): Performance improvements.
+ 
+ 2012-07-12         Arnold D. Robbins     <address@hidden>
+ 
+       Allow creation of constants. Thanks to John Haque for the
+       implementation concept.
+ 
+       * gawk_api.h (api_sym_constant): Create a constant.
+       * gawk_api.h (api_sym_update_real): Renamed from api_sym_update.
+       Add is_const paramater and do the right thing if true.
+       (api_sym_update, api_sym_constant): Call api_sym_update_real
+       in the correct way.
+       (set_constant): New function.
+ 
+ 2012-07-11         Andrew J. Schorr     <address@hidden>
+ 
+       * gawkapi.h: Fix typo in comment.
+       (awk_value_t): Type for scalar_cookie should be awk_scalar_t,
+       not awk_array_t.
+       (gawk_api): Add new api_sym_lookup_scalar function.
+       (sym_lookup_scalar): New wrapper macro for api_sym_lookup_scalar hook.
+       * gawkapi.c (api_sym_lookup_scalar): New function for faster scalar
+       lookup.
+       (api_impl): Add entry for api_sym_lookup_scalar.
+ 
+ 2012-07-11         Andrew J. Schorr     <address@hidden>
+ 
+       * gawkapi.c (awk_value_to_node): Change to a switch statement
+       so AWK_SCALAR or other invalid type is handled properly.
+       (valid_subscript_type): Test whether a value type is acceptable
+       for use as an array subscript (any scalar value will do).
+       (api_get_array_element, api_set_array_element, api_del_array_element):
+       Use new valid_subscript_type instead of restricting to string values.
+ 
+ 2012-07-11         Arnold D. Robbins     <address@hidden>
+ 
+       Lots of API work.
+ 
+       * gawkapi.h: Function pointer members renamed api_XXXX and
+       macros adjusted. More documentation.
+       (awk_valtype_t): New AWK_SCALAR enum for scalar cookies.
+       (awk_scalar_t): New type.
+       (awk_value_t): New member scalar_cookie.
+       (api_sym_update_scalar): New API function.
+       (erealloc): New macro.
+       (make_const_string): New macro, renamed from dup_string.
+       (make_malloced_string): New macro, renamed from make_string.
+       (make_null_string): New inline function.
+       (dl_load_func): Add call to init routine through pointer if
+       not NULL.
+ 
+       * gawkapi.c (awk_value_to_node): Assume that string values came
+       from malloc.
+       (node_to_awk_value): Handle AWK_SCALAR.
+       (api_sym_update): Ditto.
+       (api_sym_update_scalar): New routine.
+       (api_get_array_element): Return false if the element doesn't exist.
+       Always unref the subscript.
+       (remove_element): New helper routine.
+       (api_del_array_element): Use it.
+       (api_release_flattened_array): Ditto.
+       (api_impl): Add the new routine.
+ 
+ 2012-07-11         Andrew J. Schorr     <address@hidden>
+ 
+       * gawkapi.c (api_sym_update): Allow val_type to be AWK_UNDEFINED
+       for setting a variable to "", i.e. dupnode(Nnull_string).
+ 
+ 2012-07-10         Andrew J. Schorr     <address@hidden>
+ 
+       * awkgram.y (add_srcfile): Lint warning message for a previously loaded
+       shared library should say "already loaded shared library" instead
+       of "already included source file".
+ 
+ 2012-07-08         Arnold D. Robbins     <address@hidden>
+ 
+       * gawkapi.h (set_array_element): Use index + value instead
+       of element structure. Matches get_array_element.
+       (set_array_element_by_elem): New macro to use an element.
+       * gawkapi.c (api_set_array_element): Make the necessary adjustments.
+ 
+ 2012-07-04         Arnold D. Robbins     <address@hidden>
+ 
+       * awkgram.y (tokentab): Remove limit on number of arguments
+       for "and", "or", and "xor".
+       * builtin.c (do_and, do_or, do_xor): Modify code to perform the
+       respective operation on any number of arguments. There must be
+       at least two.
+ 
+ 2012-06-29         Arnold D. Robbins     <address@hidden>
+ 
+       * gawkapi.h: Improve the documentation of the return values
+       per Andrew Schorr.
+ 
+ 2012-06-25         Arnold D. Robbins     <address@hidden>
+ 
+       * TODO.xgawk: Updated.
+       * awk.h (track_ext_func): Declared.
+       * awkgram.y (enum defref): Add option for extension function.
+       (struct fdesc): Add member for extension function.
+       (func_use): Handle extension function, mark as extension and defined.
+       (track_ext_func): New function.
+       (check_funcs): Update logic for extension functions.
+       * ext.c (make_builtin): Call track_ext_func.
+ 
+ 2012-06-24         Andrew J. Schorr     <address@hidden>
+ 
+       * TODO.xgawk: Most of IOBUF has been hidden.
+       * gawkapi.h (IOBUF): Remove declaration (now back in awk.h).
+       (IOBUF_PUBLIC): Declare new structure defining subset of IOBUF fields
+       that should be exposed to extensions.
+       (gawk_api): Update register_open_hook argument from IOBUF to
+       IOBUF_PUBLIC.
+       * awk.h (IOBUF): Restore declaration with 5 fields moved to new
+       IOBUF_PUBLIC structure.
+       (register_open_hook): Update open_func argument from IOBUF to
+       IOBUF_PUBLIC.
+       * gawkapi.c (api_register_open_hook): Ditto.
+       * io.c (after_beginfile, nextfile, iop_close, gawk_pclose): Some fields
+       such as fd and name are now inside the IOBUF public structure.
+       (struct open_hook): Update open_func argument from IOBUF to
+       (register_open_hook): Ditto.
+       (find_open_hook): opaque now inside IOBUF_PUBLIC.
+       (iop_alloc): fd and name now in IOBUF_PUBLIC.
+       (get_a_record): If the get_record hook returns EOF, set the IOP_AT_EOF
+       flag.  Access fd inside IOBUF_PUBLIC.
+       (get_read_timeout): File name now inside IOBUF_PUBLIC.
+       * interpret.h (r_interpret): File name now inside IOBUF_PUBLIC.
+       * ext.c (load_ext): No need to call return at the end of a void
+       function.
+ 
+ 2012-06-24         Arnold D. Robbins     <address@hidden>
+ 
+       * ext.c (load_ext): Don't retun a value from a void function.
+       * gawkapi.c (api_set_array_element): Set up vname and parent_array.
+ 
+ 2012-06-21         Arnold D. Robbins     <address@hidden>
+ 
+       More API and cleanup:
+ 
+       * awk.h (stopme): Make signature match other built-ins.
+       * awkgram.y (stopme): Make signature match other built-ins.
+       (regexp): Minor edit.
+       * gawkapi.c (api_set_argument): Remove unused variable.
+       Set parent_array field of array value.
+       * TODO.xgawk: Update some.
+ 
+       Remove extension() builtin.
+ 
+       * awk.h (do_ext): Removed.
+       (load_ext): Signature changed.
+       * awkgram.y (tokentab): Remove do_ext.
+       Change calls to do_ext.
+       * ext.c (load_ext): Make init function a constant.
+       * main.c (main): Change calls to do_ext.
+ 
+ 2012-06-20         Arnold D. Robbins     <address@hidden>
+ 
+       Restore lost debugging function:
+ 
+       * awkgram.y (stopme): Restore long lost debugging function.
+       * awk.h (stopme): Add declaration.
+ 
+       API work:
+ 
+       * ext.c (get_argument): Make extern.
+       * awk.h (get_argument): Declare it.
+       * gawkapi.c (api_set_argument): Call it. Finish off the logic.
+       (api_get_argument): Refine logic to use get_argument.
+       * gawkapi.h (set_argument): New API.
+ 
+ 2012-06-19         Arnold D. Robbins     <address@hidden>
+ 
+       Remove code duplication in gawkapi.c from msg.c:
+ 
+       * awk.h (err): Add `isfatal' first parameter.
+       * awkgram.y (err): Adjust all calls.
+       * msg.c (err): Adjust all calls. Move fatal code to here ...
+       (r_fatal): From here.
+       * gawkapi.c: Remove code duplication and adjust calls to `err'.
+ 
+       Handle deleting elements of flattened array:
+ 
+       * awk.h (get_argument): Remove declaration.
+       * ext.c (get_argument): Make static.
+       * gawkapi.h (awk_flat_array_t): Make opaque fields const. Add
+       more descriptive comments.
+       * gawkapi.c (release_flattened_array): Delete elements flagged
+       for deletion. Free the flattened array also.
+ 
+       Add additional debugging when developing:
+ 
+       * configure.ac: Add additional debugging flags.
+       * configure: Regenerated.
+ 
+ 2012-06-18         Arnold D. Robbins     <address@hidden>
+ 
+       * gawkapi.h (get_array_element): Restore `wanted' paramater.
+       (awk_element_t): Use awk_value_t for index. Add awk_flat_array_t.
+       (flatten_array): Change signature to use awk_flat_array_t;
+       (release_flattened_array): Change signature to use awk_flat_array_t;
+       * gawkapi.c (api_sym_update): Handle case where variable exists already.
+       (api_get_array_element): Restore `wanted' paramater and pass it
+       on to node_to_awk_value.
+       (api_set_array_element): Revisse to match changed element type.
+       (api_flatten_array): Revise signature, implement.
+       (api_release_flattened_array): Revise signature, implement.
+ 
+ 2012-06-17         Arnold D. Robbins     <address@hidden>
+ 
+       API Work:
+ 
+       * gawkapi.h (get_array_element): Remove `wanted' parameter.
+       (r_make_string): Comment the need for `api' and `ext_id' parameters.
+       * gawkapi.c (api_sym_update): Move checks to front.
+       Initial code for handling arrays. Still needs work.
+       (api_get_array_element): Implemented.
+       (api_set_array_element): Additional checking code.
+       (api_del_array_element): Implemented.
+       (api_create_array): Implemented.
+       (init_ext_api): Force do_xxx values to be 1 or 0.
+       (update_ext_api): Ditto.
+ 
+ 2012-06-12         Arnold D. Robbins     <address@hidden>
+ 
+       API Work:
+ 
+       * gawkapi.h (awk_value_t): Restore union.
+       (get_curfunc_param): Renamed to get_argument. Return type changed
+       to awk_bool_t. Semantics better thought out and documented.
+       (awk_atexit, get_array_element): Return type now void.
+       (sym_lookup): Return type now void. Argument order rationalized.
+       * gawkapi.c (node_to_awk_value): Return type is now awk_bool_t.
+       Semantics now match table in gawkawpi.h.
+       (api_awk_atexit): Return type now void.
+       (api_sym_lookup): Return type is now awk_bool_t. Change parameter
+       order.
+       (api_get_array_element): Return type is now awk_bool_t.
+ 
+       Further API implementations and fixes for extension/testext.c:
+ 
+       * awk.h (final_exit): Add declaration.
+       * ext.c (load_ext): Change `func' to install_func.
+       * gawkapi.c: Add casts to void for id param in all functions.
+       (api_sym_update): Finish implementation.
+       (api_get_array_element): Start implementation.
+       (api_set_array_element): Add error checking.
+       (api_get_element_count): Add error checking, return the right value.
+       * main.c (main): Call final_exit instead of exit.
+       (arg_assign): Ditto.
+       * msg.c (final_exit): New routine to run the exit handlers and exit.
+       (gawk_exit): Call it.
+       * profile.c (dump_and_exit): Ditto.
+ 
+ 2012-06-10         Andrew J. Schorr     <address@hidden>
+ 
+       * TODO.xgawk: Addition of time extension moved to "done" section.
+ 
+ 2012-06-10         Andrew J. Schorr     <address@hidden>
+ 
+       * gawkapi.c (api_update_ERRNO_string): Treat boolean true as a request
+       for TRANSLATE, and false as DONT_TRANSLATE.
+ 
+ 2012-06-06         Arnold D. Robbins     <address@hidden>
+ 
+       * cint_array.c (tree_print, leaf_print): Add additional casts
+       for printf warnings.
+ 
+       * awk.h (update_ext_api): Add declaration.
+       * gawkapi.c (update_ext_api): New function.
+       * eval.c (set_LINT): Call update_ext_api() at the end.
+       * gawkapi.h: Document that do_XXX could change on the fly.
+ 
+       * awk.h (run_ext_exit_handlers): Add declaration.
+       * msg.c (gawk_exit): Call it.
+ 
+ 2012-06-05         Arnold D. Robbins     <address@hidden>
+ 
+       * ext.c (load_ext): Remove use of RTLD_GLOBAL. Not needed in new
+       scheme. Clean up error messages.
+ 
+ 2012-06-04         Arnold D. Robbins     <address@hidden>
+ 
+       * configure.ac: Remove use of -export-dynamic for GCC.
+       * configure: Regenerated.
+ 
+ 2012-05-30         Arnold D. Robbins     <address@hidden>
+ 
+       * main.c (is_off_limits_var): Minor coding style edit.
+       * gawkapi.c (awk_value_to_node): More cleanup.
+       (node_to_awk_value): Use `wanted' for decision making.
+       (api_sym_update): Start implementation. Needs more work.
+       General: More cleanup, comments.
+       * gawkapi.h (api_sym_update): Add additional comments.
+ 
+ 2012-05-29         Arnold D. Robbins     <address@hidden>
+ 
+       * gawkapi.c (node_to_awk_value): Add third parameter indicating type
+       of value desired. Based on that, do force_string or force_number
+       to get the "other" type.
+       (awk_value_to_node): Clean up the code a bit.
+       (get_curfunc_param): Move forcing of values into node_to_awk_value.
+       (api_sym_lookup): Add third parameter indicating type of value wanted.
+       (api_get_array_element): Ditto.
+       * gawk_api.h: Additional comments and clarifications. Revise APIs
+       to take third 'wanted' argument as above.
+       (awk_value_t): No longer a union so that both values may be accessed.
+       All macros: Parenthesized the bodies.
+       * bootstrap.sh: Rationalize a bit.
+ 
+ 2012-05-26         Andrew J. Schorr     <address@hidden>
+ 
+       * Makefile.am (include_HEADERS): Add so gawkapi.h will be installed.
+       (base_sources): Add gawkapi.h so that it is in dist tarball.
+       * TODO.xgawk: Update.
+       * main.c (is_off_limits_var): Stop returning true for everything
+       except PROCINFO.
+ 
+ 2012-05-25         Arnold D. Robbins     <address@hidden>
+ 
+       * main.c (is_off_limits_var): New function to check if a variable
+       is one that an extension function may not change.
+       * awk.h (is_off_limits_var): Declare it.
+       * gawkapi.c (api_sym_lookup): Use it.
+ 
+       * bootstrap.sh: Touch various files in the extension directory also.
+ 
+ 2012-05-24         Andrew J. Schorr     <address@hidden>
+ 
+       * gawkapi.h (awk_param_type_t): Remove (use awk_valtype_t instead).
+       (awk_ext_func_t): Pass a result argument, and return an awk_value_t *.
+       (gawk_api.get_curfunc_param): Add a result argument.
+       (gawk_api.set_return_value): Remove obsolete function.
+       (gawk_api.sym_lookup, gawk_api.get_array_element): Add a result
+       argument.
+       (gawk_api.api_make_string, gawk_api.api_make_number): Remove hooks,
+       since access to gawk internal state is not required to do this.
+       (set_return_value): Remove obsolete macro.
+       (get_curfunc_param, sym_lookup, get_array_element): Add result argument.
+       (r_make_string, make_number): New static inline functions.
+       (make_string, dup_string): Revise macro definitions.
+       (dl_load_func): Remove global_api_p and global_ext_id args,
+       and fix SEGV by setting api prior to checking its version members.
+       (GAWK): Expand ifdef to include more stuff.
+       * gawkapi.c (node_to_awk_value): Add result argument.
+       (api_get_curfunc_param): Add result argument, and use awk_valtype_t.
+       (api_set_return_value): Remove obsolete function.
+       (awk_value_to_node): New global function to convert back into internal
+       format.
+       (api_add_ext_func): Simply call make_builtin.
+       (node_to_awk_value): Add result argument, and handle Node_val case.
+       (api_sym_lookup, api_get_array_element): Add result argument.
+       (api_set_array_element): Implement.
+       (api_make_string, api_make_number): Remove functions that belong on
+       client side.
+       (api_impl): Remove 3 obsolete entries.
+       * TODO.xgawk: Update to reflect progress.
+       * Makefile.am (base_sources): Add gawkapi.c.
+       * awk.h: Include gawkapi.h earlier.
+       (api_impl, init_ext_api, awk_value_to_node): Add declarations
+       so we can hook in new API.
+       (INSTRUCTION): Add new union type efptr for external functions.
+       (extfunc): New define for d.efptr.
+       (load_ext): Remove 3rd obj argument that was never used for anything.
+       (make_builtin): Change signature for new API.
+       * awkgram.y (load_library): Change 2nd argument to load_ext
+       from dlload to dl_load, and remove pointless 3rd argument.
+       * main.c (main): Call init_ext_api() before loading shared libraries.
+       Change 2nd argument to load_ext from dlload to dl_load, and remove
+       pointless 3rd argument.
+       * ext.c (do_ext): Remove pointless 3rd argument to load_ext.
+       (load_ext): Remove 3rd argument.  Port to new API (change initialization
+       function signature).  If initialization function fails, issue a warning
+       and return -1, else return 0.
+       (make_builtin): Port to new API.
+       * interpret.h (r_interpret): For Op_ext_builtin, call external functions
+       with an awk_value_t result buffer, and convert the returned value
+       to a NODE *.  For Node_ext_func, code now in extfunc instead of builtin.
+ 
+ 2012-05-21         Andrew J. Schorr     <address@hidden>
+ 
+       * configure.ac: Remove libtool, and call configure in the
+       extension subdirectory.  Change pkgextensiondir to remove the
+       version number, since the new API has builtin version checks.
+       * TODO.xgawk: Update.
+       * ltmain.sh: Removed, since libtool no longer used here.
+ 
+ 2012-05-19         Andrew J. Schorr     <address@hidden>
+ 
+       * TODO.xgawk: Update to reflect progress and new issues.
+       * main.c (main): Add -i (--include) option.
+       (usage): Ditto.
+       * awkgram.y (add_srcfile): Eliminate duplicates only for SRC_INC
+       and SRC_EXTLIB sources (i.e. -f duplicates should not be removed).
+       * io.c (find_source): Set DEFAULT_FILETYPE to ".awk" if not defined
+       elsewhere.
+ 
+ 2012-05-15         Arnold D. Robbins     <address@hidden>
+ 
+       * awk.h: Include "gawkapi.h" to get IOBUF.
+       * gawkapi.h: Considerable updates.
+       * gawkapi.c: New file. Start at implementing the APIs.
+ 
+ 2012-05-13         Andrew J. Schorr     <address@hidden>
+ 
+       * TODO.xgawk: Update to reflect recent discussions and deletion of
+       extension/xreadlink.[ch].
+ 
  2012-05-11         Arnold D. Robbins     <address@hidden>
  
        Sweeping change: Use `bool', `true', and `false' everywhere.
diff --cc doc/ChangeLog
index b786eca,869ead2..38b8ad9
--- a/doc/ChangeLog
+++ b/doc/ChangeLog
@@@ -1,14 -1,38 +1,49 @@@
 +2012-08-14         Arnold D. Robbins     <address@hidden>
 +
 +      * gawk.texi: Fixed a math bug in the chapter on multiple
 +      precision floating point. Thanks to John Haque.
 +
 +2012-08-12         Arnold D. Robbins     <address@hidden>
 +
 +      * gawk.texi: Merged discussion of numbers from Appendix C into
 +      the chapter on arbitrary precision arithmetic. Did some surgery
 +      on that chapter to organize it a little better.
 +
+ 2012-08-10         Arnold D. Robbins     <address@hidden>
+ 
+       * awkcard.in, gawk.1, gawk.texi: Updated. Mostly for new API stuff
+       but also some other things.
+       * gawk.texi (Derived Files): New node.
+ 
+ 2012-08-01         Arnold D. Robbins     <address@hidden>
+ 
+       * Makefile.am (install-data-hook): Install a dgawk.1 link to the
+       man page also. Remove it on uninstall.
+ 
+ 2012-07-29         Andrew J. Schorr     <address@hidden>
+ 
+       * gawk.texi: Document that RT is set by getline.
+ 
+ 2012-07-04         Arnold D. Robbins     <address@hidden>
+ 
+       * gawk.texi, gawk.1, awkcard.in: Document that and(), or(), and
+       xor() can all take any number of arguments, with a minimum of two.
+ 
+ 2012-06-10         Andrew J. Schorr     <address@hidden>
+ 
+       * gawk.texi: Rename gettimeofday function to getlocaltime, since
+       the new time extension will provide gettimeofday.
+ 
+ 2012-05-24         Andrew J. Schorr     <address@hidden>
+ 
+       * gawk.texi, gawk.1: Replace references to dlload with dl_load.
+       But much more work needs to be done on the docs.
+       
+ 2012-05-19         Andrew J. Schorr     <address@hidden>
+ 
+       * gawk.texi, gawk.1: Document new -i option, and describe new default
+       .awk suffix behavior.
+ 
  2012-04-01         Andrew J. Schorr     <address@hidden>
  
        * gawk.texi: Replace documentation of removed functions update_ERRNO and
diff --cc doc/gawk.info
index 498c766,bcbdb04..54a8ccf
--- a/doc/gawk.info
+++ b/doc/gawk.info
@@@ -1,4 -1,4 +1,4 @@@
--This is gawk.info, produced by makeinfo version 4.13 from gawk.texi.
++This is gawk.info, produced by makeinfo version 4.13 from foo.texi.
  
  INFO-DIR-SECTION Text creation and manipulation
  START-INFO-DIR-ENTRY
@@@ -33,28992 -33,28702 +33,456 @@@ texts being (a) (see below), and with t
       software freedom."
  
  
--File: gawk.info,  Node: Top,  Next: Foreword,  Up: (dir)
--
--General Introduction
--********************
--
--This file documents `awk', a program that you can use to select
--particular records in a file and perform operations upon them.
--
--   Copyright (C) 1989, 1991, 1992, 1993, 1996, 1997, 1998, 1999, 2000,
--2001, 2002, 2003, 2004, 2005, 2007, 2009, 2010, 2011, 2012 Free
--Software Foundation, Inc.
--
--
--   This is Edition 4 of `GAWK: Effective AWK Programming: A User's
--Guide for GNU Awk', for the 4.0.1 (or later) version of the GNU
--implementation of AWK.
--
--   Permission is granted to copy, distribute and/or modify this document
--under the terms of the GNU Free Documentation License, Version 1.3 or
--any later version published by the Free Software Foundation; with the
--Invariant Sections being "GNU General Public License", the Front-Cover
--texts being (a) (see below), and with the Back-Cover Texts being (b)
--(see below).  A copy of the license is included in the section entitled
--"GNU Free Documentation License".
--
--  a. "A GNU Manual"
--
--  b. "You have the freedom to copy and modify this GNU manual.  Buying
--     copies from the FSF supports it in developing GNU and promoting
--     software freedom."
--
--* Menu:
--
--* Foreword::                       Some nice words about this
--                                   Info file.
--* Preface::                        What this Info file is about; brief
--                                   history and acknowledgments.
--* Getting Started::                A basic introduction to using
--                                   `awk'. How to run an `awk'
--                                   program. Command-line syntax.
--* Invoking Gawk::                  How to run `gawk'.
--* Regexp::                         All about matching things using regular
--                                   expressions.
--* Reading Files::                  How to read files and manipulate fields.
--* Printing::                       How to print using `awk'. Describes
--                                   the `print' and `printf'
--                                   statements. Also describes redirection of
--                                   output.
--* Expressions::                    Expressions are the basic building blocks
--                                   of statements.
--* Patterns and Actions::           Overviews of patterns and actions.
--* Arrays::                         The description and use of arrays. Also
--                                   includes array-oriented control statements.
--* Functions::                      Built-in and user-defined functions.
--* Internationalization::           Getting `gawk' to speak your
--                                   language.
--* Arbitrary Precision Arithmetic:: Arbitrary precision arithmetic with
--                                   `gawk'.
--* Advanced Features::              Stuff for advanced users, specific to
--                                   `gawk'.
--* Library Functions::              A Library of `awk' Functions.
--* Sample Programs::                Many `awk' programs with complete
--                                   explanations.
--* Debugger::                       The `gawk' debugger.
- * Language History::               The evolution of the `awk'
-                                    language.
- * Installation::                   Installing `gawk' under various
-                                    operating systems.
- * Notes::                          Notes about `gawk' extensions and
-                                    possible future work.
- * Basic Concepts::                 A very quick introduction to programming
-                                    concepts.
- * Glossary::                       An explanation of some unfamiliar terms.
- * Copying::                        Your right to copy and distribute
-                                    `gawk'.
- * GNU Free Documentation License:: The license for this Info file.
- * Index::                          Concept and Variable Index.
- 
- * History::                        The history of `gawk' and
-                                    `awk'.
- * Names::                          What name to use to find `awk'.
- * This Manual::                    Using this Info file. Includes
-                                    sample input files that you can use.
- * Conventions::                    Typographical Conventions.
- * Manual History::                 Brief history of the GNU project and this
-                                    Info file.
- * How To Contribute::              Helping to save the world.
- * Acknowledgments::                Acknowledgments.
- * Running gawk::                   How to run `gawk' programs;
-                                    includes command-line syntax.
- * One-shot::                       Running a short throwaway `awk'
-                                    program.
- * Read Terminal::                  Using no input files (input from terminal
-                                    instead).
- * Long::                           Putting permanent `awk' programs in
-                                    files.
- * Executable Scripts::             Making self-contained `awk'
-                                    programs.
- * Comments::                       Adding documentation to `gawk'
-                                    programs.
- * Quoting::                        More discussion of shell quoting issues.
- * DOS Quoting::                    Quoting in Windows Batch Files.
- * Sample Data Files::              Sample data files for use in the
-                                    `awk' programs illustrated in this
-                                    Info file.
- * Very Simple::                    A very simple example.
- * Two Rules::                      A less simple one-line example using two
-                                    rules.
- * More Complex::                   A more complex example.
- * Statements/Lines::               Subdividing or combining statements into
-                                    lines.
- * Other Features::                 Other Features of `awk'.
- * When::                           When to use `gawk' and when to use
-                                    other things.
- * Command Line::                   How to run `awk'.
- * Options::                        Command-line options and their meanings.
- * Other Arguments::                Input file names and variable assignments.
- * Naming Standard Input::          How to specify standard input with other
-                                    files.
- * Environment Variables::          The environment variables `gawk'
-                                    uses.
- * AWKPATH Variable::               Searching directories for `awk'
-                                    programs.
- * AWKLIBPATH Variable::            Searching directories for `awk'
-                                    shared libraries.
- * Other Environment Variables::    The environment variables.
- * Exit Status::                    `gawk''s exit status.
- * Include Files::                  Including other files into your program.
- * Loading Shared Libraries::       Loading shared libraries into your program.
- * Obsolete::                       Obsolete Options and/or features.
- * Undocumented::                   Undocumented Options and Features.
- * Regexp Usage::                   How to Use Regular Expressions.
- * Escape Sequences::               How to write nonprinting characters.
- * Regexp Operators::               Regular Expression Operators.
- * Bracket Expressions::            What can go between `[...]'.
- * GNU Regexp Operators::           Operators specific to GNU software.
- * Case-sensitivity::               How to do case-insensitive matching.
- * Leftmost Longest::               How much text matches.
- * Computed Regexps::               Using Dynamic Regexps.
- * Records::                        Controlling how data is split into records.
- * Fields::                         An introduction to fields.
- * Nonconstant Fields::             Nonconstant Field Numbers.
- * Changing Fields::                Changing the Contents of a Field.
- * Field Separators::               The field separator and how to change it.
- * Default Field Splitting::        How fields are normally separated.
- * Regexp Field Splitting::         Using regexps as the field separator.
- * Single Character Fields::        Making each character a separate field.
- * Command Line Field Separator::   Setting `FS' from the command-line.
- * Field Splitting Summary::        Some final points and a summary table.
- * Constant Size::                  Reading constant width data.
- * Splitting By Content::           Defining Fields By Content
- * Multiple Line::                  Reading multi-line records.
- * Getline::                        Reading files under explicit program
-                                    control using the `getline' function.
- * Plain Getline::                  Using `getline' with no arguments.
- * Getline/Variable::               Using `getline' into a variable.
- * Getline/File::                   Using `getline' from a file.
- * Getline/Variable/File::          Using `getline' into a variable from a
-                                    file.
- * Getline/Pipe::                   Using `getline' from a pipe.
- * Getline/Variable/Pipe::          Using `getline' into a variable from a
-                                    pipe.
- * Getline/Coprocess::              Using `getline' from a coprocess.
- * Getline/Variable/Coprocess::     Using `getline' into a variable from a
-                                    coprocess.
- * Getline Notes::                  Important things to know about
-                                    `getline'.
- * Getline Summary::                Summary of `getline' Variants.
- * Read Timeout::                   Reading input with a timeout.
- * Command line directories::       What happens if you put a directory on the
-                                    command line.
- * Print::                          The `print' statement.
- * Print Examples::                 Simple examples of `print' statements.
- * Output Separators::              The output separators and how to change
-                                    them.
- * OFMT::                           Controlling Numeric Output With
-                                    `print'.
- * Printf::                         The `printf' statement.
- * Basic Printf::                   Syntax of the `printf' statement.
- * Control Letters::                Format-control letters.
- * Format Modifiers::               Format-specification modifiers.
- * Printf Examples::                Several examples.
- * Redirection::                    How to redirect output to multiple files
-                                    and pipes.
- * Special Files::                  File name interpretation in `gawk'.
-                                    `gawk' allows access to inherited
-                                    file descriptors.
- * Special FD::                     Special files for I/O.
- * Special Network::                Special files for network communications.
- * Special Caveats::                Things to watch out for.
- * Close Files And Pipes::          Closing Input and Output Files and Pipes.
- * Values::                         Constants, Variables, and Regular
-                                    Expressions.
- * Constants::                      String, numeric and regexp constants.
- * Scalar Constants::               Numeric and string constants.
- * Nondecimal-numbers::             What are octal and hex numbers.
- * Regexp Constants::               Regular Expression constants.
- * Using Constant Regexps::         When and how to use a regexp constant.
- * Variables::                      Variables give names to values for later
-                                    use.
- * Using Variables::                Using variables in your programs.
- * Assignment Options::             Setting variables on the command-line and a
-                                    summary of command-line syntax. This is an
-                                    advanced method of input.
- * Conversion::                     The conversion of strings to numbers and
-                                    vice versa.
- * All Operators::                  `gawk''s operators.
- * Arithmetic Ops::                 Arithmetic operations (`+', `-',
-                                    etc.)
- * Concatenation::                  Concatenating strings.
- * Assignment Ops::                 Changing the value of a variable or a
-                                    field.
- * Increment Ops::                  Incrementing the numeric value of a
-                                    variable.
- * Truth Values and Conditions::    Testing for true and false.
- * Truth Values::                   What is ``true'' and what is ``false''.
- * Typing and Comparison::          How variables acquire types and how this
-                                    affects comparison of numbers and strings
-                                    with `<', etc.
- * Variable Typing::                String type versus numeric type.
- * Comparison Operators::           The comparison operators.
- * POSIX String Comparison::        String comparison with POSIX rules.
- * Boolean Ops::                    Combining comparison expressions using
-                                    boolean operators `||' (``or''),
-                                    `&&' (``and'') and `!' (``not'').
- * Conditional Exp::                Conditional expressions select between two
-                                    subexpressions under control of a third
-                                    subexpression.
- * Function Calls::                 A function call is an expression.
- * Precedence::                     How various operators nest.
- * Locales::                        How the locale affects things.
- * Pattern Overview::               What goes into a pattern.
- * Regexp Patterns::                Using regexps as patterns.
- * Expression Patterns::            Any expression can be used as a pattern.
- * Ranges::                         Pairs of patterns specify record ranges.
- * BEGIN/END::                      Specifying initialization and cleanup
-                                    rules.
- * Using BEGIN/END::                How and why to use BEGIN/END rules.
- * I/O And BEGIN/END::              I/O issues in BEGIN/END rules.
- * BEGINFILE/ENDFILE::              Two special patterns for advanced control.
- * Empty::                          The empty pattern, which matches every
-                                    record.
- * Using Shell Variables::          How to use shell variables with
-                                    `awk'.
- * Action Overview::                What goes into an action.
- * Statements::                     Describes the various control statements in
-                                    detail.
- * If Statement::                   Conditionally execute some `awk'
-                                    statements.
- * While Statement::                Loop until some condition is satisfied.
- * Do Statement::                   Do specified action while looping until
-                                    some condition is satisfied.
- * For Statement::                  Another looping statement, that provides
-                                    initialization and increment clauses.
- * Switch Statement::               Switch/case evaluation for conditional
-                                    execution of statements based on a value.
- * Break Statement::                Immediately exit the innermost enclosing
-                                    loop.
- * Continue Statement::             Skip to the end of the innermost enclosing
-                                    loop.
- * Next Statement::                 Stop processing the current input record.
- * Nextfile Statement::             Stop processing the current file.
- * Exit Statement::                 Stop execution of `awk'.
- * Built-in Variables::             Summarizes the built-in variables.
- * User-modified::                  Built-in variables that you change to
-                                    control `awk'.
- * Auto-set::                       Built-in variables where `awk'
-                                    gives you information.
- * ARGC and ARGV::                  Ways to use `ARGC' and `ARGV'.
- * Array Basics::                   The basics of arrays.
- * Array Intro::                    Introduction to Arrays
- * Reference to Elements::          How to examine one element of an array.
- * Assigning Elements::             How to change an element of an array.
- * Array Example::                  Basic Example of an Array
- * Scanning an Array::              A variation of the `for' statement. It
-                                    loops through the indices of an array's
-                                    existing elements.
- * Controlling Scanning::           Controlling the order in which arrays are
-                                    scanned.
- * Delete::                         The `delete' statement removes an
-                                    element from an array.
- * Numeric Array Subscripts::       How to use numbers as subscripts in
-                                    `awk'.
- * Uninitialized Subscripts::       Using Uninitialized variables as
-                                    subscripts.
- * Multi-dimensional::              Emulating multidimensional arrays in
-                                    `awk'.
- * Multi-scanning::                 Scanning multidimensional arrays.
- * Arrays of Arrays::               True multidimensional arrays.
- * Built-in::                       Summarizes the built-in functions.
- * Calling Built-in::               How to call built-in functions.
- * Numeric Functions::              Functions that work with numbers, including
-                                    `int()', `sin()' and
-                                    `rand()'.
- * String Functions::               Functions for string manipulation, such as
-                                    `split()', `match()' and
-                                    `sprintf()'.
- * Gory Details::                   More than you want to know about `\'
-                                    and `&' with `sub()',
-                                    `gsub()', and `gensub()'.
- * I/O Functions::                  Functions for files and shell commands.
- * Time Functions::                 Functions for dealing with timestamps.
- * Bitwise Functions::              Functions for bitwise operations.
- * Type Functions::                 Functions for type information.
- * I18N Functions::                 Functions for string translation.
- * User-defined::                   Describes User-defined functions in detail.
- * Definition Syntax::              How to write definitions and what they
-                                    mean.
- * Function Example::               An example function definition and what it
-                                    does.
- * Function Caveats::               Things to watch out for.
- * Calling A Function::             Don't use spaces.
- * Variable Scope::                 Controlling variable scope.
- * Pass By Value/Reference::        Passing parameters.
- * Return Statement::               Specifying the value a function returns.
- * Dynamic Typing::                 How variable types can change at runtime.
- * Indirect Calls::                 Choosing the function to call at runtime.
- * I18N and L10N::                  Internationalization and Localization.
- * Explaining gettext::             How GNU `gettext' works.
- * Programmer i18n::                Features for the programmer.
- * Translator i18n::                Features for the translator.
- * String Extraction::              Extracting marked strings.
- * Printf Ordering::                Rearranging `printf' arguments.
- * I18N Portability::               `awk'-level portability issues.
- * I18N Example::                   A simple i18n example.
- * Gawk I18N::                      `gawk' is also internationalized.
- * General Arithmetic::             An introduction to computer arithmetic.
- * Floating Point Issues::          Stuff to know about floating-point numbers.
- * String Conversion Precision::    The String Value Can Lie.
- * Unexpected Results::             Floating Point Numbers Are Not Abstract
-                                    Numbers.
- * POSIX Floating Point Problems::  Standards Versus Existing Practice.
- * Integer Programming::            Effective integer programming.
- * Floating-point Programming::     Effective floating-point programming.
- * Floating-point Representation::  Binary floating-point representation.
- * Floating-point Context::         Floating-point context.
- * Rounding Mode::                  Floating-point rounding mode.
- * Gawk and MPFR::                  How `gawk' provides
-                                    aribitrary-precision arithmetic.
- * Arbitrary Precision Floats::     Arbitrary precision floating-point
-                                    arithmetic with `gawk'.
- * Setting Precision::              Setting the working precision.
- * Setting Rounding Mode::          Setting the rounding mode.
- * Floating-point Constants::       Representing floating-point constants.
- * Changing Precision::             Changing the precision of a number.
- * Exact Arithmetic::               Exact arithmetic with floating-point
-                                    numbers.
- * Arbitrary Precision Integers::   Arbitrary precision integer arithmetic with
-                                    `gawk'.
- * Nondecimal Data::                Allowing nondecimal input data.
- * Array Sorting::                  Facilities for controlling array traversal
-                                    and sorting arrays.
- * Controlling Array Traversal::    How to use PROCINFO["sorted_in"].
- * Array Sorting Functions::        How to use `asort()' and
-                                    `asorti()'.
- * Two-way I/O::                    Two-way communications with another
-                                    process.
- * TCP/IP Networking::              Using `gawk' for network
-                                    programming.
- * Profiling::                      Profiling your `awk' programs.
- * Library Names::                  How to best name private global variables
-                                    in library functions.
- * General Functions::              Functions that are of general use.
- * Strtonum Function::              A replacement for the built-in
-                                    `strtonum()' function.
- * Assert Function::                A function for assertions in `awk'
-                                    programs.
- * Round Function::                 A function for rounding if `sprintf()'
-                                    does not do it correctly.
- * Cliff Random Function::          The Cliff Random Number Generator.
- * Ordinal Functions::              Functions for using characters as numbers
-                                    and vice versa.
- * Join Function::                  A function to join an array into a string.
- * Gettimeofday Function::          A function to get formatted times.
- * Data File Management::           Functions for managing command-line data
-                                    files.
- * Filetrans Function::             A function for handling data file
-                                    transitions.
- * Rewind Function::                A function for rereading the current file.
- * File Checking::                  Checking that data files are readable.
- * Empty Files::                    Checking for zero-length files.
- * Ignoring Assigns::               Treating assignments as file names.
- * Getopt Function::                A function for processing command-line
-                                    arguments.
- * Passwd Functions::               Functions for getting user information.
- * Group Functions::                Functions for getting group information.
- * Walking Arrays::                 A function to walk arrays of arrays.
- * Running Examples::               How to run these examples.
- * Clones::                         Clones of common utilities.
- * Cut Program::                    The `cut' utility.
- * Egrep Program::                  The `egrep' utility.
- * Id Program::                     The `id' utility.
- * Split Program::                  The `split' utility.
- * Tee Program::                    The `tee' utility.
- * Uniq Program::                   The `uniq' utility.
- * Wc Program::                     The `wc' utility.
- * Miscellaneous Programs::         Some interesting `awk' programs.
- * Dupword Program::                Finding duplicated words in a document.
- * Alarm Program::                  An alarm clock.
- * Translate Program::              A program similar to the `tr'
-                                    utility.
- * Labels Program::                 Printing mailing labels.
- * Word Sorting::                   A program to produce a word usage count.
- * History Sorting::                Eliminating duplicate entries from a
-                                    history file.
- * Extract Program::                Pulling out programs from Texinfo source
-                                    files.
- * Simple Sed::                     A Simple Stream Editor.
- * Igawk Program::                  A wrapper for `awk' that includes
-                                    files.
- * Anagram Program::                Finding anagrams from a dictionary.
- * Signature Program::              People do amazing things with too much time
-                                    on their hands.
- * Debugging::                      Introduction to `gawk' debugger.
- * Debugging Concepts::             Debugging in General.
- * Debugging Terms::                Additional Debugging Concepts.
- * Awk Debugging::                  Awk Debugging.
- * Sample Debugging Session::       Sample debugging session.
- * Debugger Invocation::            How to Start the Debugger.
- * Finding The Bug::                Finding the Bug.
- * List of Debugger Commands::      Main debugger commands.
- * Breakpoint Control::             Control of Breakpoints.
- * Debugger Execution Control::     Control of Execution.
- * Viewing And Changing Data::      Viewing and Changing Data.
- * Execution Stack::                Dealing with the Stack.
- * Debugger Info::                  Obtaining Information about the Program and
-                                    the Debugger State.
- * Miscellaneous Debugger Commands:: Miscellaneous Commands.
- * Readline Support::               Readline support.
- * Limitations::                    Limitations and future plans.
- * V7/SVR3.1::                      The major changes between V7 and System V
-                                    Release 3.1.
- * SVR4::                           Minor changes between System V Releases 3.1
-                                    and 4.
- * POSIX::                          New features from the POSIX standard.
- * BTL::                            New features from Brian Kernighan's version
-                                    of `awk'.
- * POSIX/GNU::                      The extensions in `gawk' not in
-                                    POSIX `awk'.
- * Common Extensions::              Common Extensions Summary.
- * Ranges and Locales::             How locales used to affect regexp ranges.
- * Contributors::                   The major contributors to `gawk'.
- * Gawk Distribution::              What is in the `gawk' distribution.
- * Getting::                        How to get the distribution.
- * Extracting::                     How to extract the distribution.
- * Distribution contents::          What is in the distribution.
- * Unix Installation::              Installing `gawk' under various
-                                    versions of Unix.
- * Quick Installation::             Compiling `gawk' under Unix.
- * Additional Configuration Options:: Other compile-time options.
- * Configuration Philosophy::       How it's all supposed to work.
- * Non-Unix Installation::          Installation on Other Operating Systems.
- * PC Installation::                Installing and Compiling `gawk' on
-                                    MS-DOS and OS/2.
- * PC Binary Installation::         Installing a prepared distribution.
- * PC Compiling::                   Compiling `gawk' for MS-DOS,
-                                    Windows32, and OS/2.
- * PC Testing::                     Testing `gawk' on PC systems.
- * PC Using::                       Running `gawk' on MS-DOS, Windows32
-                                    and OS/2.
- * Cygwin::                         Building and running `gawk' for
-                                    Cygwin.
- * MSYS::                           Using `gawk' In The MSYS
-                                    Environment.
- * VMS Installation::               Installing `gawk' on VMS.
- * VMS Compilation::                How to compile `gawk' under VMS.
- * VMS Installation Details::       How to install `gawk' under VMS.
- * VMS Running::                    How to run `gawk' under VMS.
- * VMS Old Gawk::                   An old version comes with some VMS systems.
- * Bugs::                           Reporting Problems and Bugs.
- * Other Versions::                 Other freely available `awk'
-                                    implementations.
- * Compatibility Mode::             How to disable certain `gawk'
-                                    extensions.
- * Additions::                      Making Additions To `gawk'.
- * Accessing The Source::           Accessing the Git repository.
- * Adding Code::                    Adding code to the main body of
-                                    `gawk'.
- * New Ports::                      Porting `gawk' to a new operating
-                                    system.
--* Dynamic Extensions::             Adding new built-in functions to
--                                   `gawk'.
- * Internals::                      A brief look at some `gawk'
-                                    internals.
- * Plugin License::                 A note about licensing.
- * Loading Extensions::             How to load dynamic extensions.
- * Sample Library::                 A example of new functions.
- * Internal File Description::      What the new functions will do.
- * Internal File Ops::              The code for internal file operations.
- * Using Internal File Ops::        How to use an external extension.
- * Future Extensions::              New features that may be implemented one
-                                    day.
- * Basic High Level::               The high level view.
- * Basic Data Typing::              A very quick intro to data types.
- 
-                   To Miriam, for making me complete.
- 
-                   To Chana, for the joy you bring us.
- 
-                 To Rivka, for the exponential increase.
- 
-                   To Nachum, for the added dimension.
- 
-                    To Malka, for the new beginning.
- 
- File: gawk.info,  Node: Foreword,  Next: Preface,  Prev: Top,  Up: Top
- 
- Foreword
- ********
- 
- Arnold Robbins and I are good friends. We were introduced in 1990 by
- circumstances--and our favorite programming language, AWK.  The
- circumstances started a couple of years earlier. I was working at a new
- job and noticed an unplugged Unix computer sitting in the corner.  No
- one knew how to use it, and neither did I.  However, a couple of days
- later it was running, and I was `root' and the one-and-only user.  That
- day, I began the transition from statistician to Unix programmer.
- 
-    On one of many trips to the library or bookstore in search of books
- on Unix, I found the gray AWK book, a.k.a. Aho, Kernighan and
- Weinberger, `The AWK Programming Language', Addison-Wesley, 1988.
- AWK's simple programming paradigm--find a pattern in the input and then
- perform an action--often reduced complex or tedious data manipulations
- to few lines of code.  I was excited to try my hand at programming in
- AWK.
- 
-    Alas,  the `awk' on my computer was a limited version of the
- language described in the AWK book.  I discovered that my computer had
- "old `awk'" and the AWK book described "new `awk'."  I learned that
- this was typical; the old version refused to step aside or relinquish
- its name.  If a system had a new `awk', it was invariably called
- `nawk', and few systems had it.  The best way to get a new `awk' was to
- `ftp' the source code for `gawk' from `prep.ai.mit.edu'.  `gawk' was a
- version of new `awk' written by David Trueman and Arnold, and available
- under the GNU General Public License.
- 
-    (Incidentally, it's no longer difficult to find a new `awk'. `gawk'
- ships with GNU/Linux, and you can download binaries or source code for
- almost any system; my wife uses `gawk' on her VMS box.)
- 
-    My Unix system started out unplugged from the wall; it certainly was
- not plugged into a network.  So, oblivious to the existence of `gawk'
- and the Unix community in general, and desiring a new `awk', I wrote my
- own, called `mawk'.  Before I was finished I knew about `gawk', but it
- was too late to stop, so I eventually posted to a `comp.sources'
- newsgroup.
- 
-    A few days after my posting, I got a friendly email from Arnold
- introducing himself.   He suggested we share design and algorithms and
- attached a draft of the POSIX standard so that I could update `mawk' to
- support language extensions added after publication of the AWK book.
- 
-    Frankly, if our roles had been reversed, I would not have been so
- open and we probably would have never met.  I'm glad we did meet.  He
- is an AWK expert's AWK expert and a genuinely nice person.  Arnold
- contributes significant amounts of his expertise and time to the Free
- Software Foundation.
- 
-    This book is the `gawk' reference manual, but at its core it is a
- book about AWK programming that will appeal to a wide audience.  It is
- a definitive reference to the AWK language as defined by the 1987 Bell
- Laboratories release and codified in the 1992 POSIX Utilities standard.
- 
-    On the other hand, the novice AWK programmer can study a wealth of
- practical programs that emphasize the power of AWK's basic idioms: data
- driven control-flow, pattern matching with regular expressions, and
- associative arrays.  Those looking for something new can try out
- `gawk''s interface to network protocols via special `/inet' files.
- 
-    The programs in this book make clear that an AWK program is
- typically much smaller and faster to develop than a counterpart written
- in C.  Consequently, there is often a payoff to prototype an algorithm
- or design in AWK to get it running quickly and expose problems early.
- Often, the interpreted performance is adequate and the AWK prototype
- becomes the product.
- 
-    The new `pgawk' (profiling `gawk'), produces program execution
- counts.  I recently experimented with an algorithm that for n lines of
- input, exhibited ~ C n^2 performance, while theory predicted ~ C n log n
- behavior. A few minutes poring over the `awkprof.out' profile
- pinpointed the problem to a single line of code.  `pgawk' is a welcome
- addition to my programmer's toolbox.
- 
-    Arnold has distilled over a decade of experience writing and using
- AWK programs, and developing `gawk', into this book.  If you use AWK or
- want to learn how, then read this book.
- 
-      Michael Brennan
-      Author of `mawk'
-      March, 2001
- 
- 
- File: gawk.info,  Node: Preface,  Next: Getting Started,  Prev: Foreword,  
Up: Top
- 
- Preface
- *******
- 
- Several kinds of tasks occur repeatedly when working with text files.
- You might want to extract certain lines and discard the rest.  Or you
- may need to make changes wherever certain patterns appear, but leave
- the rest of the file alone.  Writing single-use programs for these
- tasks in languages such as C, C++, or Java is time-consuming and
- inconvenient.  Such jobs are often easier with `awk'.  The `awk'
- utility interprets a special-purpose programming language that makes it
- easy to handle simple data-reformatting jobs.
- 
-    The GNU implementation of `awk' is called `gawk'; if you invoke it
- with the proper options or environment variables (*note Options::), it
- is fully compatible with the POSIX(1) specification of the `awk'
- language and with the Unix version of `awk' maintained by Brian
- Kernighan.  This means that all properly written `awk' programs should
- work with `gawk'.  Thus, we usually don't distinguish between `gawk'
- and other `awk' implementations.
- 
-    Using `awk' allows you to:
- 
-    * Manage small, personal databases
- 
-    * Generate reports
- 
-    * Validate data
- 
-    * Produce indexes and perform other document preparation tasks
- 
-    * Experiment with algorithms that you can adapt later to other
-      computer languages
- 
-    In addition, `gawk' provides facilities that make it easy to:
- 
-    * Extract bits and pieces of data for processing
- 
-    * Sort data
- 
-    * Perform simple network communications
- 
-    This Info file teaches you about the `awk' language and how you can
- use it effectively.  You should already be familiar with basic system
- commands, such as `cat' and `ls',(2) as well as basic shell facilities,
- such as input/output (I/O) redirection and pipes.
- 
-    Implementations of the `awk' language are available for many
- different computing environments.  This Info file, while describing the
- `awk' language in general, also describes the particular implementation
- of `awk' called `gawk' (which stands for "GNU awk").  `gawk' runs on a
- broad range of Unix systems, ranging from Intel(R)-architecture
- PC-based computers up through large-scale systems, such as Crays.
- `gawk' has also been ported to Mac OS X, Microsoft Windows (all
- versions) and OS/2 PCs, and VMS.  (Some other, obsolete systems to
- which `gawk' was once ported are no longer supported and the code for
- those systems has been removed.)
- 
- * Menu:
- 
- * History::                     The history of `gawk' and
-                                 `awk'.
- * Names::                       What name to use to find `awk'.
- * This Manual::                 Using this Info file. Includes sample
-                                 input files that you can use.
- * Conventions::                 Typographical Conventions.
- * Manual History::              Brief history of the GNU project and this
-                                 Info file.
- * How To Contribute::           Helping to save the world.
- * Acknowledgments::             Acknowledgments.
- 
-    ---------- Footnotes ----------
- 
-    (1) The 2008 POSIX standard can be found online at
- `http://www.opengroup.org/onlinepubs/9699919799/'.
- 
-    (2) These commands are available on POSIX-compliant systems, as well
- as on traditional Unix-based systems. If you are using some other
- operating system, you still need to be familiar with the ideas of I/O
- redirection and pipes.
- 
- 
- File: gawk.info,  Node: History,  Next: Names,  Up: Preface
- 
- History of `awk' and `gawk'
- ===========================
- 
-                    Recipe For A Programming Language
- 
-           1 part  `egrep'   1 part  `snobol'
-           2 parts `ed'      3 parts C
- 
-      Blend all parts well using `lex' and `yacc'.  Document minimally
-      and release.
- 
-      After eight years, add another part `egrep' and two more parts C.
-      Document very well and release.
- 
-    The name `awk' comes from the initials of its designers: Alfred V.
- Aho, Peter J. Weinberger and Brian W. Kernighan.  The original version
- of `awk' was written in 1977 at AT&T Bell Laboratories.  In 1985, a new
- version made the programming language more powerful, introducing
- user-defined functions, multiple input streams, and computed regular
- expressions.  This new version became widely available with Unix System
- V Release 3.1 (1987).  The version in System V Release 4 (1989) added
- some new features and cleaned up the behavior in some of the "dark
- corners" of the language.  The specification for `awk' in the POSIX
- Command Language and Utilities standard further clarified the language.
- Both the `gawk' designers and the original Bell Laboratories `awk'
- designers provided feedback for the POSIX specification.
- 
-    Paul Rubin wrote the GNU implementation, `gawk', in 1986.  Jay
- Fenlason completed it, with advice from Richard Stallman.  John Woods
- contributed parts of the code as well.  In 1988 and 1989, David
- Trueman, with help from me, thoroughly reworked `gawk' for compatibility
- with the newer `awk'.  Circa 1994, I became the primary maintainer.
- Current development focuses on bug fixes, performance improvements,
- standards compliance, and occasionally, new features.
- 
-    In May of 1997, Ju"rgen Kahrs felt the need for network access from
- `awk', and with a little help from me, set about adding features to do
- this for `gawk'.  At that time, he also wrote the bulk of `TCP/IP
- Internetworking with `gawk'' (a separate document, available as part of
- the `gawk' distribution).  His code finally became part of the main
- `gawk' distribution with `gawk' version 3.1.
- 
-    John Haque rewrote the `gawk' internals, in the process providing an
- `awk'-level debugger. This version became available as `gawk' version
- 4.0, in 2011.
- 
-    *Note Contributors::, for a complete list of those who made
- important contributions to `gawk'.
- 
- 
- File: gawk.info,  Node: Names,  Next: This Manual,  Prev: History,  Up: 
Preface
- 
- A Rose by Any Other Name
- ========================
- 
- The `awk' language has evolved over the years. Full details are
- provided in *note Language History::.  The language described in this
- Info file is often referred to as "new `awk'" (`nawk').
- 
-    Because of this, there are systems with multiple versions of `awk'.
- Some systems have an `awk' utility that implements the original version
- of the `awk' language and a `nawk' utility for the new version.  Others
- have an `oawk' version for the "old `awk'" language and plain `awk' for
- the new one.  Still others only have one version, which is usually the
- new one.(1)
- 
-    All in all, this makes it difficult for you to know which version of
- `awk' you should run when writing your programs.  The best advice we
- can give here is to check your local documentation. Look for `awk',
- `oawk', and `nawk', as well as for `gawk'.  It is likely that you
- already have some version of new `awk' on your system, which is what
- you should use when running your programs.  (Of course, if you're
- reading this Info file, chances are good that you have `gawk'!)
- 
-    Throughout this Info file, whenever we refer to a language feature
- that should be available in any complete implementation of POSIX `awk',
- we simply use the term `awk'.  When referring to a feature that is
- specific to the GNU implementation, we use the term `gawk'.
- 
-    ---------- Footnotes ----------
- 
-    (1) Often, these systems use `gawk' for their `awk' implementation!
- 
- 
- File: gawk.info,  Node: This Manual,  Next: Conventions,  Prev: Names,  Up: 
Preface
- 
- Using This Book
- ===============
- 
- The term `awk' refers to a particular program as well as to the
- language you use to tell this program what to do.  When we need to be
- careful, we call the language "the `awk' language," and the program
- "the `awk' utility."  This Info file explains both how to write
- programs in the `awk' language and how to run the `awk' utility.  The
- term "`awk' program" refers to a program written by you in the `awk'
- programming language.
- 
-    Primarily, this Info file explains the features of `awk' as defined
- in the POSIX standard.  It does so in the context of the `gawk'
- implementation.  While doing so, it also attempts to describe important
- differences between `gawk' and other `awk' implementations.(1) Finally,
- any `gawk' features that are not in the POSIX standard for `awk' are
- noted.
- 
-    There are subsections labeled as *Advanced Notes* scattered
- throughout the Info file.  They add a more complete explanation of
- points that are relevant, but not likely to be of interest on first
- reading.  All appear in the index, under the heading "advanced
- features."
- 
-    Most of the time, the examples use complete `awk' programs.  Some of
- the more advanced sections show only the part of the `awk' program that
- illustrates the concept currently being described.
- 
-    While this Info file is aimed principally at people who have not been
- exposed to `awk', there is a lot of information here that even the `awk'
- expert should find useful.  In particular, the description of POSIX
- `awk' and the example programs in *note Library Functions::, and in
- *note Sample Programs::, should be of interest.
- 
-    *note Getting Started::, provides the essentials you need to know to
- begin using `awk'.
- 
-    *note Invoking Gawk::, describes how to run `gawk', the meaning of
- its command-line options, and how it finds `awk' program source files.
- 
-    *note Regexp::, introduces regular expressions in general, and in
- particular the flavors supported by POSIX `awk' and `gawk'.
- 
-    *note Reading Files::, describes how `awk' reads your data.  It
- introduces the concepts of records and fields, as well as the `getline'
- command.  I/O redirection is first described here.  Network I/O is also
- briefly introduced here.
- 
-    *note Printing::, describes how `awk' programs can produce output
- with `print' and `printf'.
- 
-    *note Expressions::, describes expressions, which are the basic
- building blocks for getting most things done in a program.
- 
-    *note Patterns and Actions::, describes how to write patterns for
- matching records, actions for doing something when a record is matched,
- and the built-in variables `awk' and `gawk' use.
- 
-    *note Arrays::, covers `awk''s one-and-only data structure:
- associative arrays.  Deleting array elements and whole arrays is also
- described, as well as sorting arrays in `gawk'.  It also describes how
- `gawk' provides arrays of arrays.
- 
-    *note Functions::, describes the built-in functions `awk' and `gawk'
- provide, as well as how to define your own functions.
- 
-    *note Internationalization::, describes special features in `gawk'
- for translating program messages into different languages at runtime.
- 
-    *note Advanced Features::, describes a number of `gawk'-specific
- advanced features.  Of particular note are the abilities to have
- two-way communications with another process, perform TCP/IP networking,
- and profile your `awk' programs.
- 
-    *note Library Functions::, and *note Sample Programs::, provide many
- sample `awk' programs.  Reading them allows you to see `awk' solving
- real problems.
- 
-    *note Debugger::, describes the `awk' debugger.
- 
-    *note Language History::, describes how the `awk' language has
- evolved since its first release to present.  It also describes how
- `gawk' has acquired features over time.
- 
-    *note Installation::, describes how to get `gawk', how to compile it
- on POSIX-compatible systems, and how to compile and use it on different
- non-POSIX systems.  It also describes how to report bugs in `gawk' and
- where to get other freely available `awk' implementations.
- 
-    *note Notes::, describes how to disable `gawk''s extensions, as well
- as how to contribute new code to `gawk', how to write extension
- libraries, and some possible future directions for `gawk' development.
- 
-    *note Basic Concepts::, provides some very cursory background
- material for those who are completely unfamiliar with computer
- programming.  Also centralized there is a discussion of some of the
- issues surrounding floating-point numbers.
- 
-    The *note Glossary::, defines most, if not all, the significant
- terms used throughout the book.  If you find terms that you aren't
- familiar with, try looking them up here.
- 
-    *note Copying::, and *note GNU Free Documentation License::, present
- the licenses that cover the `gawk' source code and this Info file,
- respectively.
- 
-    ---------- Footnotes ----------
- 
-    (1) All such differences appear in the index under the entry
- "differences in `awk' and `gawk'."
- 
- 
- File: gawk.info,  Node: Conventions,  Next: Manual History,  Prev: This 
Manual,  Up: Preface
- 
- Typographical Conventions
- =========================
- 
- This Info file is written in Texinfo (http://texinfo.org), the GNU
- documentation formatting language.  A single Texinfo source file is
- used to produce both the printed and online versions of the
- documentation.  This minor node briefly documents the typographical
- conventions used in Texinfo.
- 
-    Examples you would type at the command-line are preceded by the
- common shell primary and secondary prompts, `$' and `>'.  Input that
- you type is shown `like this'.  Output from the command is preceded by
- the glyph "-|".  This typically represents the command's standard
- output.  Error messages, and other output on the command's standard
- error, are preceded by the glyph "error-->".  For example:
- 
-      $ echo hi on stdout
-      -| hi on stdout
-      $ echo hello on stderr 1>&2
-      error--> hello on stderr
- 
-    Characters that you type at the keyboard look `like this'.  In
- particular, there are special characters called "control characters."
- These are characters that you type by holding down both the `CONTROL'
- key and another key, at the same time.  For example, a `Ctrl-d' is typed
- by first pressing and holding the `CONTROL' key, next pressing the `d'
- key and finally releasing both keys.
- 
- Dark Corners
- ............
- 
-      Dark corners are basically fractal -- no matter how much you
-      illuminate, there's always a smaller but darker one.
-      Brian Kernighan
- 
-    Until the POSIX standard (and `GAWK: Effective AWK Programming'),
- many features of `awk' were either poorly documented or not documented
- at all.  Descriptions of such features (often called "dark corners")
- are noted in this Info file with "(d.c.)".  They also appear in the
- index under the heading "dark corner."
- 
-    As noted by the opening quote, though, any coverage of dark corners
- is, by definition, incomplete.
- 
-    Extensions to the standard `awk' language that are supported by more
- than one `awk' implementation are marked "(c.e.)," and listed in the
- index under "common extensions" and "extensions, common."
- 
- 
- File: gawk.info,  Node: Manual History,  Next: How To Contribute,  Prev: 
Conventions,  Up: Preface
- 
- The GNU Project and This Book
- =============================
- 
- The Free Software Foundation (FSF) is a nonprofit organization dedicated
- to the production and distribution of freely distributable software.
- It was founded by Richard M. Stallman, the author of the original Emacs
- editor.  GNU Emacs is the most widely used version of Emacs today.
- 
-    The GNU(1) Project is an ongoing effort on the part of the Free
- Software Foundation to create a complete, freely distributable,
- POSIX-compliant computing environment.  The FSF uses the "GNU General
- Public License" (GPL) to ensure that their software's source code is
- always available to the end user. A copy of the GPL is included for
- your reference (*note Copying::).  The GPL applies to the C language
- source code for `gawk'.  To find out more about the FSF and the GNU
- Project online, see the GNU Project's home page (http://www.gnu.org).
- This Info file may also be read from their web site
- (http://www.gnu.org/software/gawk/manual/).
- 
-    A shell, an editor (Emacs), highly portable optimizing C, C++, and
- Objective-C compilers, a symbolic debugger and dozens of large and
- small utilities (such as `gawk'), have all been completed and are
- freely available.  The GNU operating system kernel (the HURD), has been
- released but remains in an early stage of development.
- 
-    Until the GNU operating system is more fully developed, you should
- consider using GNU/Linux, a freely distributable, Unix-like operating
- system for Intel(R), Power Architecture, Sun SPARC, IBM S/390, and other
- systems.(2) Many GNU/Linux distributions are available for download
- from the Internet.
- 
-    (There are numerous other freely available, Unix-like operating
- systems based on the Berkeley Software Distribution, and some of them
- use recent versions of `gawk' for their versions of `awk'.  NetBSD
- (http://www.netbsd.org), FreeBSD (http://www.freebsd.org), and OpenBSD
- (http://www.openbsd.org) are three of the most popular ones, but there
- are others.)
- 
-    The Info file itself has gone through a number of previous editions.
- Paul Rubin wrote the very first draft of `The GAWK Manual'; it was
- around 40 pages in size.  Diane Close and Richard Stallman improved it,
- yielding a version that was around 90 pages long and barely described
- the original, "old" version of `awk'.
- 
-    I started working with that version in the fall of 1988.  As work on
- it progressed, the FSF published several preliminary versions (numbered
- 0.X).  In 1996, Edition 1.0 was released with `gawk' 3.0.0.  The FSF
- published the first two editions under the title `The GNU Awk User's
- Guide'.
- 
-    This edition maintains the basic structure of the previous editions.
- For Edition 4.0, the content has been thoroughly reviewed and updated.
- All references to versions prior to 4.0 have been removed.  Of
- significant note for this edition is *note Debugger::.
- 
-    `GAWK: Effective AWK Programming' will undoubtedly continue to
- evolve.  An electronic version comes with the `gawk' distribution from
- the FSF.  If you find an error in this Info file, please report it!
- *Note Bugs::, for information on submitting problem reports
- electronically.
- 
-    ---------- Footnotes ----------
- 
-    (1) GNU stands for "GNU's not Unix."
- 
-    (2) The terminology "GNU/Linux" is explained in the *note Glossary::.
- 
- 
- File: gawk.info,  Node: How To Contribute,  Next: Acknowledgments,  Prev: 
Manual History,  Up: Preface
- 
- How to Contribute
- =================
- 
- As the maintainer of GNU `awk', I once thought that I would be able to
- manage a collection of publicly available `awk' programs and I even
- solicited contributions.  Making things available on the Internet helps
- keep the `gawk' distribution down to manageable size.
- 
-    The initial collection of material, such as it is, is still available
- at `ftp://ftp.freefriends.org/arnold/Awkstuff'.  In the hopes of doing
- something more broad, I acquired the `awk.info' domain.
- 
-    However, I found that I could not dedicate enough time to managing
- contributed code: the archive did not grow and the domain went unused
- for several years.
- 
-    Fortunately, late in 2008, a volunteer took on the task of setting up
- an `awk'-related web site--`http://awk.info'--and did a very nice job.
- 
-    If you have written an interesting `awk' program, or have written a
- `gawk' extension that you would like to share with the rest of the
- world, please see `http://awk.info/?contribute' for how to contribute
- it to the web site.
- 
- 
- File: gawk.info,  Node: Acknowledgments,  Prev: How To Contribute,  Up: 
Preface
- 
- Acknowledgments
- ===============
- 
- The initial draft of `The GAWK Manual' had the following
- acknowledgments:
- 
-      Many people need to be thanked for their assistance in producing
-      this manual.  Jay Fenlason contributed many ideas and sample
-      programs.  Richard Mlynarik and Robert Chassell gave helpful
-      comments on drafts of this manual.  The paper `A Supplemental
-      Document for `awk'' by John W.  Pierce of the Chemistry Department
-      at UC San Diego, pinpointed several issues relevant both to `awk'
-      implementation and to this manual, that would otherwise have
-      escaped us.
- 
-    I would like to acknowledge Richard M. Stallman, for his vision of a
- better world and for his courage in founding the FSF and starting the
- GNU Project.
- 
-    Earlier editions of this Info file had the following
- acknowledgements:
- 
-      The following people (in alphabetical order) provided helpful
-      comments on various versions of this book, Rick Adams, Dr. Nelson
-      H.F. Beebe, Karl Berry, Dr. Michael Brennan, Rich Burridge, Claire
-      Cloutier, Diane Close, Scott Deifik, Christopher ("Topher") Eliot,
-      Jeffrey Friedl, Dr. Darrel Hankerson, Michal Jaegermann, Dr.
-      Richard J. LeBlanc, Michael Lijewski, Pat Rankin, Miriam Robbins,
-      Mary Sheehan, and Chuck Toporek.
- 
-      Robert J. Chassell provided much valuable advice on the use of
-      Texinfo.  He also deserves special thanks for convincing me _not_
-      to title this Info file `How To Gawk Politely'.  Karl Berry helped
-      significantly with the TeX part of Texinfo.
- 
-      I would like to thank Marshall and Elaine Hartholz of Seattle and
-      Dr. Bert and Rita Schreiber of Detroit for large amounts of quiet
-      vacation time in their homes, which allowed me to make significant
-      progress on this Info file and on `gawk' itself.
- 
-      Phil Hughes of SSC contributed in a very important way by loaning
-      me his laptop GNU/Linux system, not once, but twice, which allowed
-      me to do a lot of work while away from home.
- 
-      David Trueman deserves special credit; he has done a yeoman job of
-      evolving `gawk' so that it performs well and without bugs.
-      Although he is no longer involved with `gawk', working with him on
-      this project was a significant pleasure.
- 
-      The intrepid members of the GNITS mailing list, and most notably
-      Ulrich Drepper, provided invaluable help and feedback for the
-      design of the internationalization features.
- 
-      Chuck Toporek, Mary Sheehan, and Claire Coutier of O'Reilly &
-      Associates contributed significant editorial help for this Info
-      file for the 3.1 release of `gawk'.
- 
-    Dr. Nelson Beebe, Andreas Buening, Antonio Colombo, Stephen Davies,
- Scott Deifik, John H. DuBois III, Darrel Hankerson, Michal Jaegermann,
- Ju"rgen Kahrs, Dave Pitts, Stepan Kasal, Pat Rankin, Andrew Schorr,
- Corinna Vinschen, Anders Wallin, and Eli Zaretskii (in alphabetical
- order) make up the current `gawk' "crack portability team."  Without
- their hard work and help, `gawk' would not be nearly the fine program
- it is today.  It has been and continues to be a pleasure working with
- this team of fine people.
- 
-    John Haque contributed the modifications to convert `gawk' into a
- byte-code interpreter, including the debugger, and the additional
- modifications for support of arbitrary precision arithmetic.  Stephen
- Davies contributed to the effort to bring the byte-code changes into
- the mainstream code base.  Efraim Yawitz contributed the initial text
- of *note Debugger::.  John Haque contributed the initial text of *note
- Arbitrary Precision Arithmetic::.
- 
-    I would like to thank Brian Kernighan for invaluable assistance
- during the testing and debugging of `gawk', and for ongoing help and
- advice in clarifying numerous points about the language.   We could not
- have done nearly as good a job on either `gawk' or its documentation
- without his help.
- 
-    I must thank my wonderful wife, Miriam, for her patience through the
- many versions of this project, for her proofreading, and for sharing me
- with the computer.  I would like to thank my parents for their love,
- and for the grace with which they raised and educated me.  Finally, I
- also must acknowledge my gratitude to G-d, for the many opportunities
- He has sent my way, as well as for the gifts He has given me with which
- to take advantage of those opportunities.
- 
- 
- Arnold Robbins
- Nof Ayalon
- ISRAEL
- March, 2011
- 
- 
- File: gawk.info,  Node: Getting Started,  Next: Invoking Gawk,  Prev: 
Preface,  Up: Top
- 
- 1 Getting Started with `awk'
- ****************************
- 
- The basic function of `awk' is to search files for lines (or other
- units of text) that contain certain patterns.  When a line matches one
- of the patterns, `awk' performs specified actions on that line.  `awk'
- keeps processing input lines in this way until it reaches the end of
- the input files.
- 
-    Programs in `awk' are different from programs in most other
- languages, because `awk' programs are "data-driven"; that is, you
- describe the data you want to work with and then what to do when you
- find it.  Most other languages are "procedural"; you have to describe,
- in great detail, every step the program is to take.  When working with
- procedural languages, it is usually much harder to clearly describe the
- data your program will process.  For this reason, `awk' programs are
- often refreshingly easy to read and write.
- 
-    When you run `awk', you specify an `awk' "program" that tells `awk'
- what to do.  The program consists of a series of "rules".  (It may also
- contain "function definitions", an advanced feature that we will ignore
- for now.  *Note User-defined::.)  Each rule specifies one pattern to
- search for and one action to perform upon finding the pattern.
- 
-    Syntactically, a rule consists of a pattern followed by an action.
- The action is enclosed in curly braces to separate it from the pattern.
- Newlines usually separate rules.  Therefore, an `awk' program looks
- like this:
- 
-      PATTERN { ACTION }
-      PATTERN { ACTION }
-      ...
- 
- * Menu:
- 
- * Running gawk::                How to run `gawk' programs; includes
-                                 command-line syntax.
- * Sample Data Files::           Sample data files for use in the `awk'
-                                 programs illustrated in this Info file.
- * Very Simple::                 A very simple example.
- * Two Rules::                   A less simple one-line example using two
-                                 rules.
- * More Complex::                A more complex example.
- * Statements/Lines::            Subdividing or combining statements into
-                                 lines.
- * Other Features::              Other Features of `awk'.
- * When::                        When to use `gawk' and when to use
-                                 other things.
- 
- 
- File: gawk.info,  Node: Running gawk,  Next: Sample Data Files,  Up: Getting 
Started
- 
- 1.1 How to Run `awk' Programs
- =============================
- 
- There are several ways to run an `awk' program.  If the program is
- short, it is easiest to include it in the command that runs `awk', like
- this:
- 
-      awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ...
- 
-    When the program is long, it is usually more convenient to put it in
- a file and run it with a command like this:
- 
-      awk -f PROGRAM-FILE INPUT-FILE1 INPUT-FILE2 ...
- 
-    This minor node discusses both mechanisms, along with several
- variations of each.
- 
- * Menu:
- 
- * One-shot::                    Running a short throwaway `awk'
-                                 program.
- * Read Terminal::               Using no input files (input from terminal
-                                 instead).
- * Long::                        Putting permanent `awk' programs in
-                                 files.
- * Executable Scripts::          Making self-contained `awk' programs.
- * Comments::                    Adding documentation to `gawk'
-                                 programs.
- * Quoting::                     More discussion of shell quoting issues.
- 
- 
- File: gawk.info,  Node: One-shot,  Next: Read Terminal,  Up: Running gawk
- 
- 1.1.1 One-Shot Throwaway `awk' Programs
- ---------------------------------------
- 
- Once you are familiar with `awk', you will often type in simple
- programs the moment you want to use them.  Then you can write the
- program as the first argument of the `awk' command, like this:
- 
-      awk 'PROGRAM' INPUT-FILE1 INPUT-FILE2 ...
- 
- where PROGRAM consists of a series of PATTERNS and ACTIONS, as
- described earlier.
- 
-    This command format instructs the "shell", or command interpreter,
- to start `awk' and use the PROGRAM to process records in the input
- file(s).  There are single quotes around PROGRAM so the shell won't
- interpret any `awk' characters as special shell characters.  The quotes
- also cause the shell to treat all of PROGRAM as a single argument for
- `awk', and allow PROGRAM to be more than one line long.
- 
-    This format is also useful for running short or medium-sized `awk'
- programs from shell scripts, because it avoids the need for a separate
- file for the `awk' program.  A self-contained shell script is more
- reliable because there are no other files to misplace.
- 
-    *note Very Simple::, presents several short, self-contained programs.
- 
- 
- File: gawk.info,  Node: Read Terminal,  Next: Long,  Prev: One-shot,  Up: 
Running gawk
- 
- 1.1.2 Running `awk' Without Input Files
- ---------------------------------------
- 
- You can also run `awk' without any input files.  If you type the
- following command line:
- 
-      awk 'PROGRAM'
- 
- `awk' applies the PROGRAM to the "standard input", which usually means
- whatever you type on the terminal.  This continues until you indicate
- end-of-file by typing `Ctrl-d'.  (On other operating systems, the
- end-of-file character may be different.  For example, on OS/2, it is
- `Ctrl-z'.)
- 
-    As an example, the following program prints a friendly piece of
- advice (from Douglas Adams's `The Hitchhiker's Guide to the Galaxy'),
- to keep you from worrying about the complexities of computer
- programming(1) (`BEGIN' is a feature we haven't discussed yet):
- 
-      $ awk "BEGIN { print \"Don't Panic!\" }"
-      -| Don't Panic!
- 
-    This program does not read any input.  The `\' before each of the
- inner double quotes is necessary because of the shell's quoting
- rules--in particular because it mixes both single quotes and double
- quotes.(2)
- 
-    This next simple `awk' program emulates the `cat' utility; it copies
- whatever you type on the keyboard to its standard output (why this
- works is explained shortly).
- 
-      $ awk '{ print }'
-      Now is the time for all good men
-      -| Now is the time for all good men
-      to come to the aid of their country.
-      -| to come to the aid of their country.
-      Four score and seven years ago, ...
-      -| Four score and seven years ago, ...
-      What, me worry?
-      -| What, me worry?
-      Ctrl-d
- 
-    ---------- Footnotes ----------
- 
-    (1) If you use Bash as your shell, you should execute the command
- `set +H' before running this program interactively, to disable the C
- shell-style command history, which treats `!' as a special character.
- We recommend putting this command into your personal startup file.
- 
-    (2) Although we generally recommend the use of single quotes around
- the program text, double quotes are needed here in order to put the
- single quote into the message.
- 
- 
- File: gawk.info,  Node: Long,  Next: Executable Scripts,  Prev: Read 
Terminal,  Up: Running gawk
- 
- 1.1.3 Running Long Programs
- ---------------------------
- 
- Sometimes your `awk' programs can be very long.  In this case, it is
- more convenient to put the program into a separate file.  In order to
- tell `awk' to use that file for its program, you type:
- 
-      awk -f SOURCE-FILE INPUT-FILE1 INPUT-FILE2 ...
- 
-    The `-f' instructs the `awk' utility to get the `awk' program from
- the file SOURCE-FILE.  Any file name can be used for SOURCE-FILE.  For
- example, you could put the program:
- 
-      BEGIN { print "Don't Panic!" }
- 
- into the file `advice'.  Then this command:
- 
-      awk -f advice
- 
- does the same thing as this one:
- 
-      awk "BEGIN { print \"Don't Panic!\" }"
- 
- This was explained earlier (*note Read Terminal::).  Note that you
- don't usually need single quotes around the file name that you specify
- with `-f', because most file names don't contain any of the shell's
- special characters.  Notice that in `advice', the `awk' program did not
- have single quotes around it.  The quotes are only needed for programs
- that are provided on the `awk' command line.
- 
-    If you want to clearly identify your `awk' program files as such,
- you can add the extension `.awk' to the file name.  This doesn't affect
- the execution of the `awk' program but it does make "housekeeping"
- easier.
- 
- 
- File: gawk.info,  Node: Executable Scripts,  Next: Comments,  Prev: Long,  
Up: Running gawk
- 
- 1.1.4 Executable `awk' Programs
- -------------------------------
- 
- Once you have learned `awk', you may want to write self-contained `awk'
- scripts, using the `#!' script mechanism.  You can do this on many
- systems.(1) For example, you could update the file `advice' to look
- like this:
- 
-      #! /bin/awk -f
- 
-      BEGIN { print "Don't Panic!" }
- 
- After making this file executable (with the `chmod' utility), simply
- type `advice' at the shell and the system arranges to run `awk'(2) as
- if you had typed `awk -f advice':
- 
-      $ chmod +x advice
-      $ advice
-      -| Don't Panic!
- 
- (We assume you have the current directory in your shell's search path
- variable [typically `$PATH'].  If not, you may need to type `./advice'
- at the shell.)
- 
-    Self-contained `awk' scripts are useful when you want to write a
- program that users can invoke without their having to know that the
- program is written in `awk'.
- 
- Advanced Notes: Portability Issues with `#!'
- --------------------------------------------
- 
- Some systems limit the length of the interpreter name to 32 characters.
- Often, this can be dealt with by using a symbolic link.
- 
-    You should not put more than one argument on the `#!' line after the
- path to `awk'. It does not work. The operating system treats the rest
- of the line as a single argument and passes it to `awk'.  Doing this
- leads to confusing behavior--most likely a usage diagnostic of some
- sort from `awk'.
- 
-    Finally, the value of `ARGV[0]' (*note Built-in Variables::) varies
- depending upon your operating system.  Some systems put `awk' there,
- some put the full pathname of `awk' (such as `/bin/awk'), and some put
- the name of your script (`advice').  (d.c.)  Don't rely on the value of
- `ARGV[0]' to provide your script name.
- 
-    ---------- Footnotes ----------
- 
-    (1) The `#!' mechanism works on GNU/Linux systems, BSD-based systems
- and commercial Unix systems.
- 
-    (2) The line beginning with `#!' lists the full file name of an
- interpreter to run and an optional initial command-line argument to
- pass to that interpreter.  The operating system then runs the
- interpreter with the given argument and the full argument list of the
- executed program.  The first argument in the list is the full file name
- of the `awk' program.  The rest of the argument list contains either
- options to `awk', or data files, or both. Note that on many systems
- `awk' may be found in `/usr/bin' instead of in `/bin'. Caveat Emptor.
- 
- 
- File: gawk.info,  Node: Comments,  Next: Quoting,  Prev: Executable Scripts,  
Up: Running gawk
- 
- 1.1.5 Comments in `awk' Programs
- --------------------------------
- 
- A "comment" is some text that is included in a program for the sake of
- human readers; it is not really an executable part of the program.
- Comments can explain what the program does and how it works.  Nearly all
- programming languages have provisions for comments, as programs are
- typically hard to understand without them.
- 
-    In the `awk' language, a comment starts with the sharp sign
- character (`#') and continues to the end of the line.  The `#' does not
- have to be the first character on the line. The `awk' language ignores
- the rest of a line following a sharp sign.  For example, we could have
- put the following into `advice':
- 
-      # This program prints a nice friendly message.  It helps
-      # keep novice users from being afraid of the computer.
-      BEGIN    { print "Don't Panic!" }
- 
-    You can put comment lines into keyboard-composed throwaway `awk'
- programs, but this usually isn't very useful; the purpose of a comment
- is to help you or another person understand the program when reading it
- at a later time.
- 
-      CAUTION: As mentioned in *note One-shot::, you can enclose small
-      to medium programs in single quotes, in order to keep your shell
-      scripts self-contained.  When doing so, _don't_ put an apostrophe
-      (i.e., a single quote) into a comment (or anywhere else in your
-      program). The shell interprets the quote as the closing quote for
-      the entire program. As a result, usually the shell prints a
-      message about mismatched quotes, and if `awk' actually runs, it
-      will probably print strange messages about syntax errors.  For
-      example, look at the following:
- 
-           $ awk '{ print "hello" } # let's be cute'
-           >
- 
-      The shell sees that the first two quotes match, and that a new
-      quoted object begins at the end of the command line.  It therefore
-      prompts with the secondary prompt, waiting for more input.  With
-      Unix `awk', closing the quoted string produces this result:
- 
-           $ awk '{ print "hello" } # let's be cute'
-           > '
-           error--> awk: can't open file be
-           error-->  source line number 1
- 
-      Putting a backslash before the single quote in `let's' wouldn't
-      help, since backslashes are not special inside single quotes.  The
-      next node describes the shell's quoting rules.
- 
- 
- File: gawk.info,  Node: Quoting,  Prev: Comments,  Up: Running gawk
- 
- 1.1.6 Shell-Quoting Issues
- --------------------------
- 
- * Menu:
- 
- * DOS Quoting::                 Quoting in Windows Batch Files.
- 
-    For short to medium length `awk' programs, it is most convenient to
- enter the program on the `awk' command line.  This is best done by
- enclosing the entire program in single quotes.  This is true whether
- you are entering the program interactively at the shell prompt, or
- writing it as part of a larger shell script:
- 
-      awk 'PROGRAM TEXT' INPUT-FILE1 INPUT-FILE2 ...
- 
-    Once you are working with the shell, it is helpful to have a basic
- knowledge of shell quoting rules.  The following rules apply only to
- POSIX-compliant, Bourne-style shells (such as Bash, the GNU Bourne-Again
- Shell).  If you use the C shell, you're on your own.
- 
-    * Quoted items can be concatenated with nonquoted items as well as
-      with other quoted items.  The shell turns everything into one
-      argument for the command.
- 
-    * Preceding any single character with a backslash (`\') quotes that
-      character.  The shell removes the backslash and passes the quoted
-      character on to the command.
- 
-    * Single quotes protect everything between the opening and closing
-      quotes.  The shell does no interpretation of the quoted text,
-      passing it on verbatim to the command.  It is _impossible_ to
-      embed a single quote inside single-quoted text.  Refer back to
-      *note Comments::, for an example of what happens if you try.
- 
-    * Double quotes protect most things between the opening and closing
-      quotes.  The shell does at least variable and command substitution
-      on the quoted text.  Different shells may do additional kinds of
-      processing on double-quoted text.
- 
-      Since certain characters within double-quoted text are processed
-      by the shell, they must be "escaped" within the text.  Of note are
-      the characters `$', ``', `\', and `"', all of which must be
-      preceded by a backslash within double-quoted text if they are to
-      be passed on literally to the program.  (The leading backslash is
-      stripped first.)  Thus, the example seen in *note Read Terminal::,
-      is applicable:
- 
-           $ awk "BEGIN { print \"Don't Panic!\" }"
-           -| Don't Panic!
- 
-      Note that the single quote is not special within double quotes.
- 
-    * Null strings are removed when they occur as part of a non-null
-      command-line argument, while explicit non-null objects are kept.
-      For example, to specify that the field separator `FS' should be
-      set to the null string, use:
- 
-           awk -F "" 'PROGRAM' FILES # correct
- 
-      Don't use this:
- 
-           awk -F"" 'PROGRAM' FILES  # wrong!
- 
-      In the second case, `awk' will attempt to use the text of the
-      program as the value of `FS', and the first file name as the text
-      of the program!  This results in syntax errors at best, and
-      confusing behavior at worst.
- 
-    Mixing single and double quotes is difficult.  You have to resort to
- shell quoting tricks, like this:
- 
-      $ awk 'BEGIN { print "Here is a single quote <'"'"'>" }'
-      -| Here is a single quote <'>
- 
- This program consists of three concatenated quoted strings.  The first
- and the third are single-quoted, the second is double-quoted.
- 
-    This can be "simplified" to:
- 
-      $ awk 'BEGIN { print "Here is a single quote <'\''>" }'
-      -| Here is a single quote <'>
- 
- Judge for yourself which of these two is the more readable.
- 
-    Another option is to use double quotes, escaping the embedded,
- `awk'-level double quotes:
- 
-      $ awk "BEGIN { print \"Here is a single quote <'>\" }"
-      -| Here is a single quote <'>
- 
- This option is also painful, because double quotes, backslashes, and
- dollar signs are very common in more advanced `awk' programs.
- 
-    A third option is to use the octal escape sequence equivalents
- (*note Escape Sequences::) for the single- and double-quote characters,
- like so:
- 
-      $ awk 'BEGIN { print "Here is a single quote <\47>" }'
-      -| Here is a single quote <'>
-      $ awk 'BEGIN { print "Here is a double quote <\42>" }'
-      -| Here is a double quote <">
- 
- This works nicely, except that you should comment clearly what the
- escapes mean.
- 
-    A fourth option is to use command-line variable assignment, like
- this:
- 
-      $ awk -v sq="'" 'BEGIN { print "Here is a single quote <" sq ">" }'
-      -| Here is a single quote <'>
- 
-    If you really need both single and double quotes in your `awk'
- program, it is probably best to move it into a separate file, where the
- shell won't be part of the picture, and you can say what you mean.
- 
- 
- File: gawk.info,  Node: DOS Quoting,  Up: Quoting
- 
- 1.1.6.1 Quoting in MS-Windows Batch Files
- .........................................
- 
- Although this Info file generally only worries about POSIX systems and
- the POSIX shell, the following issue arises often enough for many users
- that it is worth addressing.
- 
-    The "shells" on Microsoft Windows systems use the double-quote
- character for quoting, and make it difficult or impossible to include an
- escaped double-quote character in a command-line script.  The following
- example, courtesy of Jeroen Brink, shows how to print all lines in a
- file surrounded by double quotes:
- 
-      gawk "{ print \"\042\" $0 \"\042\" }" FILE
- 
- 
- File: gawk.info,  Node: Sample Data Files,  Next: Very Simple,  Prev: Running 
gawk,  Up: Getting Started
- 
- 1.2 Data Files for the Examples
- ===============================
- 
- Many of the examples in this Info file take their input from two sample
- data files.  The first, `BBS-list', represents a list of computer
- bulletin board systems together with information about those systems.
- The second data file, called `inventory-shipped', contains information
- about monthly shipments.  In both files, each line is considered to be
- one "record".
- 
-    In the data file `BBS-list', each record contains the name of a
- computer bulletin board, its phone number, the board's baud rate(s),
- and a code for the number of hours it is operational.  An `A' in the
- last column means the board operates 24 hours a day.  A `B' in the last
- column means the board only operates on evening and weekend hours.  A
- `C' means the board operates only on weekends:
- 
-      aardvark     555-5553     1200/300          B
-      alpo-net     555-3412     2400/1200/300     A
-      barfly       555-7685     1200/300          A
-      bites        555-1675     2400/1200/300     A
-      camelot      555-0542     300               C
-      core         555-2912     1200/300          C
-      fooey        555-1234     2400/1200/300     B
-      foot         555-6699     1200/300          B
-      macfoo       555-6480     1200/300          A
-      sdace        555-3430     2400/1200/300     A
-      sabafoo      555-2127     1200/300          C
- 
-    The data file `inventory-shipped' represents information about
- shipments during the year.  Each record contains the month, the number
- of green crates shipped, the number of red boxes shipped, the number of
- orange bags shipped, and the number of blue packages shipped,
- respectively.  There are 16 entries, covering the 12 months of last year
- and the first four months of the current year.
- 
-      Jan  13  25  15 115
-      Feb  15  32  24 226
-      Mar  15  24  34 228
-      Apr  31  52  63 420
-      May  16  34  29 208
-      Jun  31  42  75 492
-      Jul  24  34  67 436
-      Aug  15  34  47 316
-      Sep  13  55  37 277
-      Oct  29  54  68 525
-      Nov  20  87  82 577
-      Dec  17  35  61 401
- 
-      Jan  21  36  64 620
-      Feb  26  58  80 652
-      Mar  24  75  70 495
-      Apr  21  70  74 514
- 
-    If you are reading this in GNU Emacs using Info, you can copy the
- regions of text showing these sample files into your own test files.
- This way you can try out the examples shown in the remainder of this
- document.  You do this by using the command `M-x write-region' to copy
- text from the Info file into a file for use with `awk' (*Note
- Miscellaneous File Operations: (emacs)Misc File Ops, for more
- information).  Using this information, create your own `BBS-list' and
- `inventory-shipped' files and practice what you learn in this Info file.
- 
-    If you are using the stand-alone version of Info, see *note Extract
- Program::, for an `awk' program that extracts these data files from
- `gawk.texi', the Texinfo source file for this Info file.
- 
- 
- File: gawk.info,  Node: Very Simple,  Next: Two Rules,  Prev: Sample Data 
Files,  Up: Getting Started
- 
- 1.3 Some Simple Examples
- ========================
- 
- The following command runs a simple `awk' program that searches the
- input file `BBS-list' for the character string `foo' (a grouping of
- characters is usually called a "string"; the term "string" is based on
- similar usage in English, such as "a string of pearls," or "a string of
- cars in a train"):
- 
-      awk '/foo/ { print $0 }' BBS-list
- 
- When lines containing `foo' are found, they are printed because
- `print $0' means print the current line.  (Just `print' by itself means
- the same thing, so we could have written that instead.)
- 
-    You will notice that slashes (`/') surround the string `foo' in the
- `awk' program.  The slashes indicate that `foo' is the pattern to
- search for.  This type of pattern is called a "regular expression",
- which is covered in more detail later (*note Regexp::).  The pattern is
- allowed to match parts of words.  There are single quotes around the
- `awk' program so that the shell won't interpret any of it as special
- shell characters.
- 
-    Here is what this program prints:
- 
-      $ awk '/foo/ { print $0 }' BBS-list
-      -| fooey        555-1234     2400/1200/300     B
-      -| foot         555-6699     1200/300          B
-      -| macfoo       555-6480     1200/300          A
-      -| sabafoo      555-2127     1200/300          C
- 
-    In an `awk' rule, either the pattern or the action can be omitted,
- but not both.  If the pattern is omitted, then the action is performed
- for _every_ input line.  If the action is omitted, the default action
- is to print all lines that match the pattern.
- 
-    Thus, we could leave out the action (the `print' statement and the
- curly braces) in the previous example and the result would be the same:
- `awk' prints all lines matching the pattern `foo'.  By comparison,
- omitting the `print' statement but retaining the curly braces makes an
- empty action that does nothing (i.e., no lines are printed).
- 
-    Many practical `awk' programs are just a line or two.  Following is a
- collection of useful, short programs to get you started.  Some of these
- programs contain constructs that haven't been covered yet. (The
- description of the program will give you a good idea of what is going
- on, but please read the rest of the Info file to become an `awk'
- expert!)  Most of the examples use a data file named `data'.  This is
- just a placeholder; if you use these programs yourself, substitute your
- own file names for `data'.  For future reference, note that there is
- often more than one way to do things in `awk'.  At some point, you may
- want to look back at these examples and see if you can come up with
- different ways to do the same things shown here:
- 
-    * Print the length of the longest input line:
- 
-           awk '{ if (length($0) > max) max = length($0) }
-                END { print max }' data
- 
-    * Print every line that is longer than 80 characters:
- 
-           awk 'length($0) > 80' data
- 
-      The sole rule has a relational expression as its pattern and it
-      has no action--so the default action, printing the record, is used.
- 
-    * Print the length of the longest line in `data':
- 
-           expand data | awk '{ if (x < length()) x = length() }
-                         END { print "maximum line length is " x }'
- 
-      The input is processed by the `expand' utility to change TABs into
-      spaces, so the widths compared are actually the right-margin
-      columns.
- 
-    * Print every line that has at least one field:
- 
-           awk 'NF > 0' data
- 
-      This is an easy way to delete blank lines from a file (or rather,
-      to create a new file similar to the old file but from which the
-      blank lines have been removed).
- 
-    * Print seven random numbers from 0 to 100, inclusive:
- 
-           awk 'BEGIN { for (i = 1; i <= 7; i++)
-                            print int(101 * rand()) }'
- 
-    * Print the total number of bytes used by FILES:
- 
-           ls -l FILES | awk '{ x += $5 }
-                             END { print "total bytes: " x }'
- 
-    * Print the total number of kilobytes used by FILES:
- 
-           ls -l FILES | awk '{ x += $5 }
-              END { print "total K-bytes:", x / 1024 }'
- 
-    * Print a sorted list of the login names of all users:
- 
-           awk -F: '{ print $1 }' /etc/passwd | sort
- 
-    * Count the lines in a file:
- 
-           awk 'END { print NR }' data
- 
-    * Print the even-numbered lines in the data file:
- 
-           awk 'NR % 2 == 0' data
- 
-      If you use the expression `NR % 2 == 1' instead, the program would
-      print the odd-numbered lines.
- 
- 
- File: gawk.info,  Node: Two Rules,  Next: More Complex,  Prev: Very Simple,  
Up: Getting Started
- 
- 1.4 An Example with Two Rules
- =============================
- 
- The `awk' utility reads the input files one line at a time.  For each
- line, `awk' tries the patterns of each of the rules.  If several
- patterns match, then several actions are run in the order in which they
- appear in the `awk' program.  If no patterns match, then no actions are
- run.
- 
-    After processing all the rules that match the line (and perhaps
- there are none), `awk' reads the next line.  (However, *note Next
- Statement::, and also *note Nextfile Statement::).  This continues
- until the program reaches the end of the file.  For example, the
- following `awk' program contains two rules:
- 
-      /12/  { print $0 }
-      /21/  { print $0 }
- 
- The first rule has the string `12' as the pattern and `print $0' as the
- action.  The second rule has the string `21' as the pattern and also
- has `print $0' as the action.  Each rule's action is enclosed in its
- own pair of braces.
- 
-    This program prints every line that contains the string `12' _or_
- the string `21'.  If a line contains both strings, it is printed twice,
- once by each rule.
- 
-    This is what happens if we run this program on our two sample data
- files, `BBS-list' and `inventory-shipped':
- 
-      $ awk '/12/ { print $0 }
-      >      /21/ { print $0 }' BBS-list inventory-shipped
-      -| aardvark     555-5553     1200/300          B
-      -| alpo-net     555-3412     2400/1200/300     A
-      -| barfly       555-7685     1200/300          A
-      -| bites        555-1675     2400/1200/300     A
-      -| core         555-2912     1200/300          C
-      -| fooey        555-1234     2400/1200/300     B
-      -| foot         555-6699     1200/300          B
-      -| macfoo       555-6480     1200/300          A
-      -| sdace        555-3430     2400/1200/300     A
-      -| sabafoo      555-2127     1200/300          C
-      -| sabafoo      555-2127     1200/300          C
-      -| Jan  21  36  64 620
-      -| Apr  21  70  74 514
- 
- Note how the line beginning with `sabafoo' in `BBS-list' was printed
- twice, once for each rule.
- 
- 
- File: gawk.info,  Node: More Complex,  Next: Statements/Lines,  Prev: Two 
Rules,  Up: Getting Started
- 
- 1.5 A More Complex Example
- ==========================
- 
- Now that we've mastered some simple tasks, let's look at what typical
- `awk' programs do.  This example shows how `awk' can be used to
- summarize, select, and rearrange the output of another utility.  It uses
- features that haven't been covered yet, so don't worry if you don't
- understand all the details:
- 
-      LC_ALL=C ls -l | awk '$6 == "Nov" { sum += $5 }
-                            END { print sum }'
- 
-    This command prints the total number of bytes in all the files in the
- current directory that were last modified in November (of any year).
- The `ls -l' part of this example is a system command that gives you a
- listing of the files in a directory, including each file's size and the
- date the file was last modified. Its output looks like this:
- 
-      -rw-r--r--  1 arnold   user   1933 Nov  7 13:05 Makefile
-      -rw-r--r--  1 arnold   user  10809 Nov  7 13:03 awk.h
-      -rw-r--r--  1 arnold   user    983 Apr 13 12:14 awk.tab.h
-      -rw-r--r--  1 arnold   user  31869 Jun 15 12:20 awkgram.y
-      -rw-r--r--  1 arnold   user  22414 Nov  7 13:03 awk1.c
-      -rw-r--r--  1 arnold   user  37455 Nov  7 13:03 awk2.c
-      -rw-r--r--  1 arnold   user  27511 Dec  9 13:07 awk3.c
-      -rw-r--r--  1 arnold   user   7989 Nov  7 13:03 awk4.c
- 
- The first field contains read-write permissions, the second field
- contains the number of links to the file, and the third field
- identifies the owner of the file. The fourth field identifies the group
- of the file.  The fifth field contains the size of the file in bytes.
- The sixth, seventh, and eighth fields contain the month, day, and time,
- respectively, that the file was last modified.  Finally, the ninth field
- contains the file name.(1)
- 
-    The `$6 == "Nov"' in our `awk' program is an expression that tests
- whether the sixth field of the output from `ls -l' matches the string
- `Nov'.  Each time a line has the string `Nov' for its sixth field, the
- action `sum += $5' is performed.  This adds the fifth field (the file's
- size) to the variable `sum'.  As a result, when `awk' has finished
- reading all the input lines, `sum' is the total of the sizes of the
- files whose lines matched the pattern.  (This works because `awk'
- variables are automatically initialized to zero.)
- 
-    After the last line of output from `ls' has been processed, the
- `END' rule executes and prints the value of `sum'.  In this example,
- the value of `sum' is 80600.
- 
-    These more advanced `awk' techniques are covered in later sections
- (*note Action Overview::).  Before you can move on to more advanced
- `awk' programming, you have to know how `awk' interprets your input and
- displays your output.  By manipulating fields and using `print'
- statements, you can produce some very useful and impressive-looking
- reports.
- 
-    ---------- Footnotes ----------
- 
-    (1) The `LC_ALL=C' is needed to produce this traditional-style
- output from `ls'.
- 
- 
- File: gawk.info,  Node: Statements/Lines,  Next: Other Features,  Prev: More 
Complex,  Up: Getting Started
- 
- 1.6 `awk' Statements Versus Lines
- =================================
- 
- Most often, each line in an `awk' program is a separate statement or
- separate rule, like this:
- 
-      awk '/12/  { print $0 }
-           /21/  { print $0 }' BBS-list inventory-shipped
- 
-    However, `gawk' ignores newlines after any of the following symbols
- and keywords:
- 
-      ,    {    ?    :    ||    &&    do    else
- 
- A newline at any other point is considered the end of the statement.(1)
- 
-    If you would like to split a single statement into two lines at a
- point where a newline would terminate it, you can "continue" it by
- ending the first line with a backslash character (`\').  The backslash
- must be the final character on the line in order to be recognized as a
- continuation character.  A backslash is allowed anywhere in the
- statement, even in the middle of a string or regular expression.  For
- example:
- 
-      awk '/This regular expression is too long, so continue it\
-       on the next line/ { print $1 }'
- 
- We have generally not used backslash continuation in our sample
- programs.  `gawk' places no limit on the length of a line, so backslash
- continuation is never strictly necessary; it just makes programs more
- readable.  For this same reason, as well as for clarity, we have kept
- most statements short in the sample programs presented throughout the
- Info file.  Backslash continuation is most useful when your `awk'
- program is in a separate source file instead of entered from the
- command line.  You should also note that many `awk' implementations are
- more particular about where you may use backslash continuation. For
- example, they may not allow you to split a string constant using
- backslash continuation.  Thus, for maximum portability of your `awk'
- programs, it is best not to split your lines in the middle of a regular
- expression or a string.
- 
-      CAUTION: _Backslash continuation does not work as described with
-      the C shell._  It works for `awk' programs in files and for
-      one-shot programs, _provided_ you are using a POSIX-compliant
-      shell, such as the Unix Bourne shell or Bash.  But the C shell
-      behaves differently!  There, you must use two backslashes in a
-      row, followed by a newline.  Note also that when using the C
-      shell, _every_ newline in your `awk' program must be escaped with
-      a backslash. To illustrate:
- 
-           % awk 'BEGIN { \
-           ?   print \\
-           ?       "hello, world" \
-           ? }'
-           -| hello, world
- 
-      Here, the `%' and `?' are the C shell's primary and secondary
-      prompts, analogous to the standard shell's `$' and `>'.
- 
-      Compare the previous example to how it is done with a
-      POSIX-compliant shell:
- 
-           $ awk 'BEGIN {
-           >   print \
-           >       "hello, world"
-           > }'
-           -| hello, world
- 
-    `awk' is a line-oriented language.  Each rule's action has to begin
- on the same line as the pattern.  To have the pattern and action on
- separate lines, you _must_ use backslash continuation; there is no
- other option.
- 
-    Another thing to keep in mind is that backslash continuation and
- comments do not mix. As soon as `awk' sees the `#' that starts a
- comment, it ignores _everything_ on the rest of the line. For example:
- 
-      $ gawk 'BEGIN { print "dont panic" # a friendly \
-      >                                    BEGIN rule
-      > }'
-      error--> gawk: cmd. line:2:                BEGIN rule
-      error--> gawk: cmd. line:2:                ^ parse error
- 
- In this case, it looks like the backslash would continue the comment
- onto the next line. However, the backslash-newline combination is never
- even noticed because it is "hidden" inside the comment. Thus, the
- `BEGIN' is noted as a syntax error.
- 
-    When `awk' statements within one rule are short, you might want to
- put more than one of them on a line.  This is accomplished by
- separating the statements with a semicolon (`;').  This also applies to
- the rules themselves.  Thus, the program shown at the start of this
- minor node could also be written this way:
- 
-      /12/ { print $0 } ; /21/ { print $0 }
- 
-      NOTE: The requirement that states that rules on the same line must
-      be separated with a semicolon was not in the original `awk'
-      language; it was added for consistency with the treatment of
-      statements within an action.
- 
-    ---------- Footnotes ----------
- 
-    (1) The `?' and `:' referred to here is the three-operand
- conditional expression described in *note Conditional Exp::.  Splitting
- lines after `?' and `:' is a minor `gawk' extension; if `--posix' is
- specified (*note Options::), then this extension is disabled.
- 
- 
- File: gawk.info,  Node: Other Features,  Next: When,  Prev: Statements/Lines, 
 Up: Getting Started
- 
- 1.7 Other Features of `awk'
- ===========================
- 
- The `awk' language provides a number of predefined, or "built-in",
- variables that your programs can use to get information from `awk'.
- There are other variables your program can set as well to control how
- `awk' processes your data.
- 
-    In addition, `awk' provides a number of built-in functions for doing
- common computational and string-related operations.  `gawk' provides
- built-in functions for working with timestamps, performing bit
- manipulation, for runtime string translation (internationalization),
- determining the type of a variable, and array sorting.
- 
-    As we develop our presentation of the `awk' language, we introduce
- most of the variables and many of the functions. They are described
- systematically in *note Built-in Variables::, and *note Built-in::.
- 
- 
- File: gawk.info,  Node: When,  Prev: Other Features,  Up: Getting Started
- 
- 1.8 When to Use `awk'
- =====================
- 
- Now that you've seen some of what `awk' can do, you might wonder how
- `awk' could be useful for you.  By using utility programs, advanced
- patterns, field separators, arithmetic statements, and other selection
- criteria, you can produce much more complex output.  The `awk' language
- is very useful for producing reports from large amounts of raw data,
- such as summarizing information from the output of other utility
- programs like `ls'.  (*Note More Complex::.)
- 
-    Programs written with `awk' are usually much smaller than they would
- be in other languages.  This makes `awk' programs easy to compose and
- use.  Often, `awk' programs can be quickly composed at your keyboard,
- used once, and thrown away.  Because `awk' programs are interpreted, you
- can avoid the (usually lengthy) compilation part of the typical
- edit-compile-test-debug cycle of software development.
- 
-    Complex programs have been written in `awk', including a complete
- retargetable assembler for eight-bit microprocessors (*note Glossary::,
- for more information), and a microcode assembler for a special-purpose
- Prolog computer.  While the original `awk''s capabilities were strained
- by tasks of such complexity, modern versions are more capable.  Even
- Brian Kernighan's version of `awk' has fewer predefined limits, and
- those that it has are much larger than they used to be.
- 
-    If you find yourself writing `awk' scripts of more than, say, a few
- hundred lines, you might consider using a different programming
- language.  Emacs Lisp is a good choice if you need sophisticated string
- or pattern matching capabilities.  The shell is also good at string and
- pattern matching; in addition, it allows powerful use of the system
- utilities.  More conventional languages, such as C, C++, and Java, offer
- better facilities for system programming and for managing the complexity
- of large programs.  Programs in these languages may require more lines
- of source code than the equivalent `awk' programs, but they are easier
- to maintain and usually run more efficiently.
- 
- 
- File: gawk.info,  Node: Invoking Gawk,  Next: Regexp,  Prev: Getting Started, 
 Up: Top
- 
- 2 Running `awk' and `gawk'
- **************************
- 
- This major node covers how to run awk, both POSIX-standard and
- `gawk'-specific command-line options, and what `awk' and `gawk' do with
- non-option arguments.  It then proceeds to cover how `gawk' searches
- for source files, reading standard input along with other files,
- `gawk''s environment variables, `gawk''s exit status, using include
- files, and obsolete and undocumented options and/or features.
- 
-    Many of the options and features described here are discussed in
- more detail later in the Info file; feel free to skip over things in
- this major node that don't interest you right now.
- 
- * Menu:
- 
- * Command Line::                How to run `awk'.
- * Options::                     Command-line options and their meanings.
- * Other Arguments::             Input file names and variable assignments.
- * Naming Standard Input::       How to specify standard input with other
-                                 files.
- * Environment Variables::       The environment variables `gawk' uses.
- * Exit Status::                 `gawk''s exit status.
- * Include Files::               Including other files into your program.
- * Loading Shared Libraries::    Loading shared libraries into your program.
- * Obsolete::                    Obsolete Options and/or features.
- * Undocumented::                Undocumented Options and Features.
- 
- 
- File: gawk.info,  Node: Command Line,  Next: Options,  Up: Invoking Gawk
- 
- 2.1 Invoking `awk'
- ==================
- 
- There are two ways to run `awk'--with an explicit program or with one
- or more program files.  Here are templates for both of them; items
- enclosed in [...] in these templates are optional:
- 
-      awk [OPTIONS] -f progfile [`--'] FILE ...
-      awk [OPTIONS] [`--'] 'PROGRAM' FILE ...
- 
-    Besides traditional one-letter POSIX-style options, `gawk' also
- supports GNU long options.
- 
-    It is possible to invoke `awk' with an empty program:
- 
-      awk '' datafile1 datafile2
- 
- Doing so makes little sense, though; `awk' exits silently when given an
- empty program.  (d.c.)  If `--lint' has been specified on the command
- line, `gawk' issues a warning that the program is empty.
- 
- 
- File: gawk.info,  Node: Options,  Next: Other Arguments,  Prev: Command Line, 
 Up: Invoking Gawk
- 
- 2.2 Command-Line Options
- ========================
- 
- Options begin with a dash and consist of a single character.  GNU-style
- long options consist of two dashes and a keyword.  The keyword can be
- abbreviated, as long as the abbreviation allows the option to be
- uniquely identified.  If the option takes an argument, then the keyword
- is either immediately followed by an equals sign (`=') and the
- argument's value, or the keyword and the argument's value are separated
- by whitespace.  If a particular option with a value is given more than
- once, it is the last value that counts.
- 
-    Each long option for `gawk' has a corresponding POSIX-style short
- option.  The long and short options are interchangeable in all contexts.
- The following list describes options mandated by the POSIX standard:
- 
- `-F FS'
- `--field-separator FS'
-      Set the `FS' variable to FS (*note Field Separators::).
- 
- `-f SOURCE-FILE'
- `--file SOURCE-FILE'
-      Read `awk' program source from SOURCE-FILE instead of in the first
-      non-option argument.  This option may be given multiple times; the
-      `awk' program consists of the concatenation the contents of each
-      specified SOURCE-FILE.
- 
- `-v VAR=VAL'
- `--assign VAR=VAL'
-      Set the variable VAR to the value VAL _before_ execution of the
-      program begins.  Such variable values are available inside the
-      `BEGIN' rule (*note Other Arguments::).
- 
-      The `-v' option can only set one variable, but it can be used more
-      than once, setting another variable each time, like this: `awk
-      -v foo=1 -v bar=2 ...'.
- 
-           CAUTION: Using `-v' to set the values of the built-in
-           variables may lead to surprising results.  `awk' will reset
-           the values of those variables as it needs to, possibly
-           ignoring any predefined value you may have given.
- 
- `-W GAWK-OPT'
-      Provide an implementation-specific option.  This is the POSIX
-      convention for providing implementation-specific options.  These
-      options also have corresponding GNU-style long options.  Note that
-      the long options may be abbreviated, as long as the abbreviations
-      remain unique.  The full list of `gawk'-specific options is
-      provided next.
- 
- `--'
-      Signal the end of the command-line options.  The following
-      arguments are not treated as options even if they begin with `-'.
-      This interpretation of `--' follows the POSIX argument parsing
-      conventions.
- 
-      This is useful if you have file names that start with `-', or in
-      shell scripts, if you have file names that will be specified by
-      the user that could start with `-'.  It is also useful for passing
-      options on to the `awk' program; see *note Getopt Function::.
- 
-    The following list describes `gawk'-specific options:
- 
- `-b'
- `--characters-as-bytes'
-      Cause `gawk' to treat all input data as single-byte characters.
-      In addition, all output written with `print' or `printf' are
-      treated as single-byte characters.
- 
-      Normally, `gawk' follows the POSIX standard and attempts to process
-      its input data according to the current locale. This can often
-      involve converting multibyte characters into wide characters
-      (internally), and can lead to problems or confusion if the input
-      data does not contain valid multibyte characters. This option is
-      an easy way to tell `gawk': "hands off my data!".
- 
- `-c'
- `--traditional'
-      Specify "compatibility mode", in which the GNU extensions to the
-      `awk' language are disabled, so that `gawk' behaves just like
-      Brian Kernighan's version `awk'.  *Note POSIX/GNU::, which
-      summarizes the extensions.  Also see *note Compatibility Mode::.
- 
- `-C'
- `--copyright'
-      Print the short version of the General Public License and then
-      exit.
- 
- `-d[FILE]'
- `--dump-variables[=FILE]'
-      Print a sorted list of global variables, their types, and final
-      values to FILE.  If no FILE is provided, print this list to the
-      file named `awkvars.out' in the current directory.  No space is
-      allowed between the `-d' and FILE, if FILE is supplied.
- 
-      Having a list of all global variables is a good way to look for
-      typographical errors in your programs.  You would also use this
-      option if you have a large program with a lot of functions, and
-      you want to be sure that your functions don't inadvertently use
-      global variables that you meant to be local.  (This is a
-      particularly easy mistake to make with simple variable names like
-      `i', `j', etc.)
- 
- `-D[FILE]'
- `--debug=[FILE]'
-      Enable debugging of `awk' programs (*note Debugging::).  By
-      default, the debugger reads commands interactively from the
-      terminal.  The optional FILE argument allows you to specify a file
-      with a list of commands for the debugger to execute
-      non-interactively.  No space is allowed between the `-D' and FILE,
-      if FILE is supplied.
- 
- `-e PROGRAM-TEXT'
- `--source PROGRAM-TEXT'
-      Provide program source code in the PROGRAM-TEXT.  This option
-      allows you to mix source code in files with source code that you
-      enter on the command line.  This is particularly useful when you
-      have library functions that you want to use from your command-line
-      programs (*note AWKPATH Variable::).
- 
- `-E FILE'
- `--exec FILE'
-      Similar to `-f', read `awk' program text from FILE.  There are two
-      differences from `-f':
- 
-         * This option terminates option processing; anything else on
-           the command line is passed on directly to the `awk' program.
- 
-         * Command-line variable assignments of the form `VAR=VALUE' are
-           disallowed.
- 
-      This option is particularly necessary for World Wide Web CGI
-      applications that pass arguments through the URL; using this
-      option prevents a malicious (or other) user from passing in
-      options, assignments, or `awk' source code (via `--source') to the
-      CGI application.  This option should be used with `#!' scripts
-      (*note Executable Scripts::), like so:
- 
-           #! /usr/local/bin/gawk -E
- 
-           AWK PROGRAM HERE ...
- 
- `-g'
- `--gen-pot'
-      Analyze the source program and generate a GNU `gettext' Portable
-      Object Template file on standard output for all string constants
-      that have been marked for translation.  *Note
-      Internationalization::, for information about this option.
- 
- `-h'
- `--help'
-      Print a "usage" message summarizing the short and long style
-      options that `gawk' accepts and then exit.
- 
- `-l LIB'
- `--load LIB'
-      Load a shared library LIB. This searches for the library using the
-      `AWKLIBPATH' environment variable.  The correct library suffix for
-      your platform will be supplied by default, so it need not be
-      specified in the library name.  The library initialization routine
-      should be named `dlload()'.  An alternative is to use the address@hidden'
-      keyword inside the program to load a shared library.
- 
- `-L [value]'
- `--lint[=value]'
-      Warn about constructs that are dubious or nonportable to other
-      `awk' implementations.  Some warnings are issued when `gawk' first
-      reads your program.  Others are issued at runtime, as your program
-      executes.  With an optional argument of `fatal', lint warnings
-      become fatal errors.  This may be drastic, but its use will
-      certainly encourage the development of cleaner `awk' programs.
-      With an optional argument of `invalid', only warnings about things
-      that are actually invalid are issued. (This is not fully
-      implemented yet.)
- 
-      Some warnings are only printed once, even if the dubious
-      constructs they warn about occur multiple times in your `awk'
-      program.  Thus, when eliminating problems pointed out by `--lint',
-      you should take care to search for all occurrences of each
-      inappropriate construct. As `awk' programs are usually short,
-      doing so is not burdensome.
- 
- `-M'
- `--bignum'
-      Force arbitrary precision arithmetic on numbers. This option has
-      no effect if `gawk' is not compiled to use the GNU MPFR and MP
-      libraries (*note Arbitrary Precision Arithmetic::).
- 
- `-n'
- `--non-decimal-data'
-      Enable automatic interpretation of octal and hexadecimal values in
-      input data (*note Nondecimal Data::).
- 
-           CAUTION: This option can severely break old programs.  Use
-           with care.
- 
- `-N'
- `--use-lc-numeric'
-      Force the use of the locale's decimal point character when parsing
-      numeric input data (*note Locales::).
- 
- `-o[FILE]'
- `--pretty-print[=FILE]'
-      Enable pretty-printing of `awk' programs.  By default, output
-      program is created in a file named `awkprof.out'.  The optional
-      FILE argument allows you to specify a different file name for the
-      output.  No space is allowed between the `-o' and FILE, if FILE is
-      supplied.
- 
- `-O'
- `--optimize'
-      Enable some optimizations on the internal representation of the
-      program.  At the moment this includes just simple constant
-      folding. The `gawk' maintainer hopes to add more optimizations
-      over time.
- 
- `-p[FILE]'
- `--profile[=FILE]'
-      Enable profiling of `awk' programs (*note Profiling::).  By
-      default, profiles are created in a file named `awkprof.out'.  The
-      optional FILE argument allows you to specify a different file name
-      for the profile file.  No space is allowed between the `-p' and
-      FILE, if FILE is supplied.
- 
-      The profile contains execution counts for each statement in the
-      program in the left margin, and function call counts for each
-      function.
- 
- `-P'
- `--posix'
-      Operate in strict POSIX mode.  This disables all `gawk' extensions
-      (just like `--traditional') and disables all extensions not
-      allowed by POSIX.  *Note Common Extensions::, for a summary of the
-      extensions in `gawk' that are disabled by this option.  Also, the
-      following additional restrictions apply:
- 
-         * Newlines do not act as whitespace to separate fields when
-           `FS' is equal to a single space (*note Fields::).
- 
-         * Newlines are not allowed after `?' or `:' (*note Conditional
-           Exp::).
- 
-         * Specifying `-Ft' on the command-line does not set the value
-           of `FS' to be a single TAB character (*note Field
-           Separators::).
- 
-         * The locale's decimal point character is used for parsing input
-           data (*note Locales::).
- 
-      If you supply both `--traditional' and `--posix' on the command
-      line, `--posix' takes precedence. `gawk' also issues a warning if
-      both options are supplied.
- 
- `-r'
- `--re-interval'
-      Allow interval expressions (*note Regexp Operators::) in regexps.
-      This is now `gawk''s default behavior.  Nevertheless, this option
-      remains both for backward compatibility, and for use in
-      combination with the `--traditional' option.
- 
- `-S'
- `--sandbox'
-      Disable the `system()' function, input redirections with `getline',
-      output redirections with `print' and `printf', and dynamic
-      extensions.  This is particularly useful when you want to run
-      `awk' scripts from questionable sources and need to make sure the
-      scripts can't access your system (other than the specified input
-      data file).
- 
- `-t'
- `--lint-old'
-      Warn about constructs that are not available in the original
-      version of `awk' from Version 7 Unix (*note V7/SVR3.1::).
- 
- `-V'
- `--version'
-      Print version information for this particular copy of `gawk'.
-      This allows you to determine if your copy of `gawk' is up to date
-      with respect to whatever the Free Software Foundation is currently
-      distributing.  It is also useful for bug reports (*note Bugs::).
- 
-    As long as program text has been supplied, any other options are
- flagged as invalid with a warning message but are otherwise ignored.
- 
-    In compatibility mode, as a special case, if the value of FS supplied
- to the `-F' option is `t', then `FS' is set to the TAB character
- (`"\t"').  This is true only for `--traditional' and not for `--posix'
- (*note Field Separators::).
- 
-    The `-f' option may be used more than once on the command line.  If
- it is, `awk' reads its program source from all of the named files, as
- if they had been concatenated together into one big file.  This is
- useful for creating libraries of `awk' functions.  These functions can
- be written once and then retrieved from a standard place, instead of
- having to be included into each individual program.  (As mentioned in
- *note Definition Syntax::, function names must be unique.)
- 
-    With standard `awk', library functions can still be used, even if
- the program is entered at the terminal, by specifying `-f /dev/tty'.
- After typing your program, type `Ctrl-d' (the end-of-file character) to
- terminate it.  (You may also use `-f -' to read program source from the
- standard input but then you will not be able to also use the standard
- input as a source of data.)
- 
-    Because it is clumsy using the standard `awk' mechanisms to mix
- source file and command-line `awk' programs, `gawk' provides the
- `--source' option.  This does not require you to pre-empt the standard
- input for your source code; it allows you to easily mix command-line
- and library source code (*note AWKPATH Variable::).  The `--source'
- option may also be used multiple times on the command line.
- 
-    If no `-f' or `--source' option is specified, then `gawk' uses the
- first non-option command-line argument as the text of the program
- source code.
- 
-    If the environment variable `POSIXLY_CORRECT' exists, then `gawk'
- behaves in strict POSIX mode, exactly as if you had supplied the
- `--posix' command-line option.  Many GNU programs look for this
- environment variable to suppress extensions that conflict with POSIX,
- but `gawk' behaves differently: it suppresses all extensions, even
- those that do not conflict with POSIX, and behaves in strict POSIX
- mode. If `--lint' is supplied on the command line and `gawk' turns on
- POSIX mode because of `POSIXLY_CORRECT', then it issues a warning
- message indicating that POSIX mode is in effect.  You would typically
- set this variable in your shell's startup file.  For a
- Bourne-compatible shell (such as Bash), you would add these lines to
- the `.profile' file in your home directory:
- 
-      POSIXLY_CORRECT=true
-      export POSIXLY_CORRECT
- 
-    For a C shell-compatible shell,(1) you would add this line to the
- `.login' file in your home directory:
- 
-      setenv POSIXLY_CORRECT true
- 
-    Having `POSIXLY_CORRECT' set is not recommended for daily use, but
- it is good for testing the portability of your programs to other
- environments.
- 
-    ---------- Footnotes ----------
- 
-    (1) Not recommended.
- 
- 
- File: gawk.info,  Node: Other Arguments,  Next: Naming Standard Input,  Prev: 
Options,  Up: Invoking Gawk
- 
- 2.3 Other Command-Line Arguments
- ================================
- 
- Any additional arguments on the command line are normally treated as
- input files to be processed in the order specified.   However, an
- argument that has the form `VAR=VALUE', assigns the value VALUE to the
- variable VAR--it does not specify a file at all.  (See *note Assignment
- Options::.)
- 
-    All these arguments are made available to your `awk' program in the
- `ARGV' array (*note Built-in Variables::).  Command-line options and
- the program text (if present) are omitted from `ARGV'.  All other
- arguments, including variable assignments, are included.   As each
- element of `ARGV' is processed, `gawk' sets the variable `ARGIND' to
- the index in `ARGV' of the current element.
- 
-    The distinction between file name arguments and variable-assignment
- arguments is made when `awk' is about to open the next input file.  At
- that point in execution, it checks the file name to see whether it is
- really a variable assignment; if so, `awk' sets the variable instead of
- reading a file.
- 
-    Therefore, the variables actually receive the given values after all
- previously specified files have been read.  In particular, the values of
- variables assigned in this fashion are _not_ available inside a `BEGIN'
- rule (*note BEGIN/END::), because such rules are run before `awk'
- begins scanning the argument list.
- 
-    The variable values given on the command line are processed for
- escape sequences (*note Escape Sequences::).  (d.c.)
- 
-    In some earlier implementations of `awk', when a variable assignment
- occurred before any file names, the assignment would happen _before_
- the `BEGIN' rule was executed.  `awk''s behavior was thus inconsistent;
- some command-line assignments were available inside the `BEGIN' rule,
- while others were not.  Unfortunately, some applications came to depend
- upon this "feature."  When `awk' was changed to be more consistent, the
- `-v' option was added to accommodate applications that depended upon
- the old behavior.
- 
-    The variable assignment feature is most useful for assigning to
- variables such as `RS', `OFS', and `ORS', which control input and
- output formats before scanning the data files.  It is also useful for
- controlling state if multiple passes are needed over a data file.  For
- example:
- 
-      awk 'pass == 1  { PASS 1 STUFF }
-           pass == 2  { PASS 2 STUFF }' pass=1 mydata pass=2 mydata
- 
-    Given the variable assignment feature, the `-F' option for setting
- the value of `FS' is not strictly necessary.  It remains for historical
- compatibility.
- 
- 
- File: gawk.info,  Node: Naming Standard Input,  Next: Environment Variables,  
Prev: Other Arguments,  Up: Invoking Gawk
- 
- 2.4 Naming Standard Input
- =========================
- 
- Often, you may wish to read standard input together with other files.
- For example, you may wish to read one file, read standard input coming
- from a pipe, and then read another file.
- 
-    The way to name the standard input, with all versions of `awk', is
- to use a single, standalone minus sign or dash, `-'.  For example:
- 
-      SOME_COMMAND | awk -f myprog.awk file1 - file2
- 
- Here, `awk' first reads `file1', then it reads the output of
- SOME_COMMAND, and finally it reads `file2'.
- 
-    You may also use `"-"' to name standard input when reading files
- with `getline' (*note Getline/File::).
- 
-    In addition, `gawk' allows you to specify the special file name
- `/dev/stdin', both on the command line and with `getline'.  Some other
- versions of `awk' also support this, but it is not standard.  (Some
- operating systems provide a `/dev/stdin' file in the file system,
- however, `gawk' always processes this file name itself.)
- 
- 
- File: gawk.info,  Node: Environment Variables,  Next: Exit Status,  Prev: 
Naming Standard Input,  Up: Invoking Gawk
- 
- 2.5 The Environment Variables `gawk' Uses
- =========================================
- 
- A number of environment variables influence how `gawk' behaves.
- 
- * Menu:
- 
- * AWKPATH Variable::            Searching directories for `awk'
-                                 programs.
- * AWKLIBPATH Variable::         Searching directories for `awk' shared
-                                 libraries.
- * Other Environment Variables:: The environment variables.
- 
- 
- File: gawk.info,  Node: AWKPATH Variable,  Next: AWKLIBPATH Variable,  Up: 
Environment Variables
- 
- 2.5.1 The `AWKPATH' Environment Variable
- ----------------------------------------
- 
- The previous minor node described how `awk' program files can be named
- on the command-line with the `-f' option.  In most `awk'
- implementations, you must supply a precise path name for each program
- file, unless the file is in the current directory.  But in `gawk', if
- the file name supplied to the `-f' option does not contain a `/', then
- `gawk' searches a list of directories (called the "search path"), one
- by one, looking for a file with the specified name.
- 
- The search path is a string consisting of directory names separated by
- colons.  `gawk' gets its search path from the `AWKPATH' environment
- variable.  If that variable does not exist, `gawk' uses a default path,
- `.:/usr/local/share/awk'.(1)
- 
-    The search path feature is particularly useful for building libraries
- of useful `awk' functions.  The library files can be placed in a
- standard directory in the default path and then specified on the
- command line with a short file name.  Otherwise, the full file name
- would have to be typed for each file.
- 
-    By using both the `--source' and `-f' options, your command-line
- `awk' programs can use facilities in `awk' library files (*note Library
- Functions::).  Path searching is not done if `gawk' is in compatibility
- mode.  This is true for both `--traditional' and `--posix'.  *Note
- Options::.
- 
-      NOTE: To include the current directory in the path, either place
-      `.' explicitly in the path or write a null entry in the path.  (A
-      null entry is indicated by starting or ending the path with a
-      colon or by placing two colons next to each other (`::').)  This
-      path search mechanism is similar to the shell's.
- 
-      However, `gawk' always looks in the current directory _before_
-      searching `AWKPATH', so there is no real reason to include the
-      current directory in the search path.
- 
-    If `AWKPATH' is not defined in the environment, `gawk' places its
- default search path into `ENVIRON["AWKPATH"]'. This makes it easy to
- determine the actual search path that `gawk' will use from within an
- `awk' program.
- 
-    While you can change `ENVIRON["AWKPATH"]' within your `awk' program,
- this has no effect on the running program's behavior.  This makes
- sense: the `AWKPATH' environment variable is used to find the program
- source files.  Once your program is running, all the files have been
- found, and `gawk' no longer needs to use `AWKPATH'.
- 
-    ---------- Footnotes ----------
- 
-    (1) Your version of `gawk' may use a different directory; it will
- depend upon how `gawk' was built and installed. The actual directory is
- the value of `$(datadir)' generated when `gawk' was configured.  You
- probably don't need to worry about this, though.
- 
- 
- File: gawk.info,  Node: AWKLIBPATH Variable,  Next: Other Environment 
Variables,  Prev: AWKPATH Variable,  Up: Environment Variables
- 
- 2.5.2 The `AWKLIBPATH' Environment Variable
- -------------------------------------------
- 
- The `AWKLIBPATH' environment variable is similar to the `AWKPATH'
- variable, but it is used to search for shared libraries specified with
- the `-l' option rather than for source files.  If the library is not
- found, the path is searched again after adding the appropriate shared
- library suffix for the platform.  For example, on GNU/Linux systems,
- the suffix `.so' is used.
- 
- 
- File: gawk.info,  Node: Other Environment Variables,  Prev: AWKLIBPATH 
Variable,  Up: Environment Variables
- 
- 2.5.3 Other Environment Variables
- ---------------------------------
- 
- A number of other environment variables affect `gawk''s behavior, but
- they are more specialized. Those in the following list are meant to be
- used by regular users.
- 
- `POSIXLY_CORRECT'
-      Causes `gawk' to switch POSIX compatibility mode, disabling all
-      traditional and GNU extensions.  *Note Options::.
- 
- `GAWK_SOCK_RETRIES'
-      Controls the number of time `gawk' will attempt to retry a two-way
-      TCP/IP (socket) connection before giving up.  *Note TCP/IP
-      Networking::.
- 
- `GAWK_MSEC_SLEEP'
-      Specifies the interval between connection retries, in
-      milliseconds. On systems that do not support the `usleep()' system
-      call, the value is rounded up to an integral number of seconds.
- 
- `GAWK_READ_TIMEOUT'
-      Specifies the time, in milliseconds, for `gawk' to wait for input
-      before returning with an error.  *Note Read Timeout::.
- 
-    The environment variables in the following list are meant for use by
- the `gawk' developers for testing and tuning.  They are subject to
- change. The variables are:
- 
- `AVG_CHAIN_MAX'
-      The average number of items `gawk' will maintain on a hash chain
-      for managing arrays.
- 
- `AWK_HASH'
-      If this variable exists with a value of `gst', `gawk' will switch
-      to using the hash function from GNU Smalltalk for managing arrays.
-      This function may be marginally faster than the standard function.
- 
- `AWKREADFUNC'
-      If this variable exists, `gawk' switches to reading source files
-      one line at a time, instead of reading in blocks. This exists for
-      debugging problems on filesystems on non-POSIX operating systems
-      where I/O is performed in records, not in blocks.
- 
- `GAWK_NO_DFA'
-      If this variable exists, `gawk' does not use the DFA regexp matcher
-      for "does it match" kinds of tests. This can cause `gawk' to be
-      slower. Its purpose is to help isolate differences between the two
-      regexp matchers that `gawk' uses internally. (There aren't
-      supposed to be differences, but occasionally theory and practice
-      don't coordinate with each other.)
- 
- `GAWK_STACKSIZE'
-      This specifies the amount by which `gawk' should grow its internal
-      evaluation stack, when needed.
- 
- `TIDYMEM'
-      If this variable exists, `gawk' uses the `mtrace()' library calls
-      from GNU LIBC to help track down possible memory leaks.
- 
- 
- File: gawk.info,  Node: Exit Status,  Next: Include Files,  Prev: Environment 
Variables,  Up: Invoking Gawk
- 
- 2.6 `gawk''s Exit Status
- ========================
- 
- If the `exit' statement is used with a value (*note Exit Statement::),
- then `gawk' exits with the numeric value given to it.
- 
-    Otherwise, if there were no problems during execution, `gawk' exits
- with the value of the C constant `EXIT_SUCCESS'.  This is usually zero.
- 
-    If an error occurs, `gawk' exits with the value of the C constant
- `EXIT_FAILURE'.  This is usually one.
- 
-    If `gawk' exits because of a fatal error, the exit status is 2.  On
- non-POSIX systems, this value may be mapped to `EXIT_FAILURE'.
- 
- 
- File: gawk.info,  Node: Include Files,  Next: Loading Shared Libraries,  
Prev: Exit Status,  Up: Invoking Gawk
- 
- 2.7 Including Other Files Into Your Program
- ===========================================
- 
- This minor node describes a feature that is specific to `gawk'.
- 
-    The address@hidden' keyword can be used to read external `awk' source
- files.  This gives you the ability to split large `awk' source files
- into smaller, more manageable pieces, and also lets you reuse common
- `awk' code from various `awk' scripts.  In other words, you can group
- together `awk' functions, used to carry out specific tasks, into
- external files. These files can be used just like function libraries,
- using the address@hidden' keyword in conjunction with the `AWKPATH'
- environment variable.
- 
-    Let's see an example.  We'll start with two (trivial) `awk' scripts,
- namely `test1' and `test2'. Here is the `test1' script:
- 
-      BEGIN {
-          print "This is script test1."
-      }
- 
- and here is `test2':
- 
-      @include "test1"
-      BEGIN {
-          print "This is script test2."
-      }
- 
-    Running `gawk' with `test2' produces the following result:
- 
-      $ gawk -f test2
-      -| This is file test1.
-      -| This is file test2.
- 
-    `gawk' runs the `test2' script which includes `test1' using the
- address@hidden' keyword.  So, to include external `awk' source files you just
- use address@hidden' followed by the name of the file to be included,
- enclosed in double quotes.
- 
-      NOTE: Keep in mind that this is a language construct and the file
-      name cannot be a string variable, but rather just a literal string
-      in double quotes.
- 
-    The files to be included may be nested; e.g., given a third script,
- namely `test3':
- 
-      @include "test2"
-      BEGIN {
-          print "This is script test3."
-      }
- 
- Running `gawk' with the `test3' script produces the following results:
- 
-      $ gawk -f test3
-      -| This is file test1.
-      -| This is file test2.
-      -| This is file test3.
- 
-    The file name can, of course, be a pathname. For example:
- 
-      @include "../io_funcs"
- 
- or:
- 
-      @include "/usr/awklib/network"
- 
- are valid. The `AWKPATH' environment variable can be of great value
- when using address@hidden'. The same rules for the use of the `AWKPATH'
- variable in command-line file searches (*note AWKPATH Variable::) apply
- to address@hidden' also.
- 
-    This is very helpful in constructing `gawk' function libraries.  If
- you have a large script with useful, general purpose `awk' functions,
- you can break it down into library files and put those files in a
- special directory.  You can then include those "libraries," using
- either the full pathnames of the files, or by setting the `AWKPATH'
- environment variable accordingly and then using address@hidden' with just
- the file part of the full pathname. Of course you can have more than
- one directory to keep library files; the more complex the working
- environment is, the more directories you may need to organize the files
- to be included.
- 
-    Given the ability to specify multiple `-f' options, the address@hidden'
- mechanism is not strictly necessary.  However, the address@hidden' keyword
- can help you in constructing self-contained `gawk' programs, thus
- reducing the need for writing complex and tedious command lines.  In
- particular, address@hidden' is very useful for writing CGI scripts to be run
- from web pages.
- 
-    As mentioned in *note AWKPATH Variable::, the current directory is
- always searched first for source files, before searching in `AWKPATH',
- and this also applies to files named with address@hidden'.
- 
- 
- File: gawk.info,  Node: Loading Shared Libraries,  Next: Obsolete,  Prev: 
Include Files,  Up: Invoking Gawk
- 
- 2.8 Loading Shared Libraries Into Your Program
- ==============================================
- 
- This minor node describes a feature that is specific to `gawk'.
- 
-    The address@hidden' keyword can be used to read external `awk' shared
- libraries.  This allows you to link in compiled code that may offer
- superior performance and/or give you access to extended capabilities
- not supported by the `awk' language.  The `AWKLIBPATH' variable is used
- to search for the shared library.  Using address@hidden' is completely
- equivalent to using the `-l' command-line option.
- 
-    If the shared library is not initially found in `AWKLIBPATH', another
- search is conducted after appending the platform's default shared
- library suffix to the filename.  For example, on GNU/Linux systems, the
- suffix `.so' is used.
- 
-      $ gawk '@load "ordchr"; BEGIN {print chr(65)}'
-      -| A
- 
- This is equivalent to the following example:
- 
-      $ gawk -lordchr 'BEGIN {print chr(65)}'
-      -| A
- 
- For command-line usage, the `-l' option is more convenient, but 
address@hidden'
- is useful for embedding inside an `awk' source file that requires
- access to a shared library.
- 
- 
- File: gawk.info,  Node: Obsolete,  Next: Undocumented,  Prev: Loading Shared 
Libraries,  Up: Invoking Gawk
- 
- 2.9 Obsolete Options and/or Features
- ====================================
- 
- This minor node describes features and/or command-line options from
- previous releases of `gawk' that are either not available in the
- current version or that are still supported but deprecated (meaning that
- they will _not_ be in the next release).
- 
-    The process-related special files `/dev/pid', `/dev/ppid',
- `/dev/pgrpid', and `/dev/user' were deprecated in `gawk' 3.1, but still
- worked.  As of version 4.0, they are no longer interpreted specially by
- `gawk'.  (Use `PROCINFO' instead; see *note Auto-set::.)
- 
- 
- File: gawk.info,  Node: Undocumented,  Prev: Obsolete,  Up: Invoking Gawk
- 
- 2.10 Undocumented Options and Features
- ======================================
- 
-      Use the Source, Luke!
-      Obi-Wan
- 
-    This minor node intentionally left blank.
- 
- 
- File: gawk.info,  Node: Regexp,  Next: Reading Files,  Prev: Invoking Gawk,  
Up: Top
- 
- 3 Regular Expressions
- *********************
- 
- A "regular expression", or "regexp", is a way of describing a set of
- strings.  Because regular expressions are such a fundamental part of
- `awk' programming, their format and use deserve a separate major node.
- 
-    A regular expression enclosed in slashes (`/') is an `awk' pattern
- that matches every input record whose text belongs to that set.  The
- simplest regular expression is a sequence of letters, numbers, or both.
- Such a regexp matches any string that contains that sequence.  Thus,
- the regexp `foo' matches any string containing `foo'.  Therefore, the
- pattern `/foo/' matches any input record containing the three
- characters `foo' _anywhere_ in the record.  Other kinds of regexps let
- you specify more complicated classes of strings.
- 
- * Menu:
- 
- * Regexp Usage::                How to Use Regular Expressions.
- * Escape Sequences::            How to write nonprinting characters.
- * Regexp Operators::            Regular Expression Operators.
- * Bracket Expressions::         What can go between `[...]'.
- * GNU Regexp Operators::        Operators specific to GNU software.
- * Case-sensitivity::            How to do case-insensitive matching.
- * Leftmost Longest::            How much text matches.
- * Computed Regexps::            Using Dynamic Regexps.
- 
- 
- File: gawk.info,  Node: Regexp Usage,  Next: Escape Sequences,  Up: Regexp
- 
- 3.1 How to Use Regular Expressions
- ==================================
- 
- A regular expression can be used as a pattern by enclosing it in
- slashes.  Then the regular expression is tested against the entire text
- of each record.  (Normally, it only needs to match some part of the
- text in order to succeed.)  For example, the following prints the
- second field of each record that contains the string `foo' anywhere in
- it:
- 
-      $ awk '/foo/ { print $2 }' BBS-list
-      -| 555-1234
-      -| 555-6699
-      -| 555-6480
-      -| 555-2127
- 
-    Regular expressions can also be used in matching expressions.  These
- expressions allow you to specify the string to match against; it need
- not be the entire current input record.  The two operators `~' and `!~'
- perform regular expression comparisons.  Expressions using these
- operators can be used as patterns, or in `if', `while', `for', and `do'
- statements.  (*Note Statements::.)  For example:
- 
-      EXP ~ /REGEXP/
- 
- is true if the expression EXP (taken as a string) matches REGEXP.  The
- following example matches, or selects, all input records with the
- uppercase letter `J' somewhere in the first field:
- 
-      $ awk '$1 ~ /J/' inventory-shipped
-      -| Jan  13  25  15 115
-      -| Jun  31  42  75 492
-      -| Jul  24  34  67 436
-      -| Jan  21  36  64 620
- 
-    So does this:
- 
-      awk '{ if ($1 ~ /J/) print }' inventory-shipped
- 
-    This next example is true if the expression EXP (taken as a
- character string) does _not_ match REGEXP:
- 
-      EXP !~ /REGEXP/
- 
-    The following example matches, or selects, all input records whose
- first field _does not_ contain the uppercase letter `J':
- 
-      $ awk '$1 !~ /J/' inventory-shipped
-      -| Feb  15  32  24 226
-      -| Mar  15  24  34 228
-      -| Apr  31  52  63 420
-      -| May  16  34  29 208
-      ...
- 
-    When a regexp is enclosed in slashes, such as `/foo/', we call it a
- "regexp constant", much like `5.27' is a numeric constant and `"foo"'
- is a string constant.
- 
- 
- File: gawk.info,  Node: Escape Sequences,  Next: Regexp Operators,  Prev: 
Regexp Usage,  Up: Regexp
- 
- 3.2 Escape Sequences
- ====================
- 
- Some characters cannot be included literally in string constants
- (`"foo"') or regexp constants (`/foo/').  Instead, they should be
- represented with "escape sequences", which are character sequences
- beginning with a backslash (`\').  One use of an escape sequence is to
- include a double-quote character in a string constant.  Because a plain
- double quote ends the string, you must use `\"' to represent an actual
- double-quote character as a part of the string.  For example:
- 
-      $ awk 'BEGIN { print "He said \"hi!\" to her." }'
-      -| He said "hi!" to her.
- 
-    The  backslash character itself is another character that cannot be
- included normally; you must write `\\' to put one backslash in the
- string or regexp.  Thus, the string whose contents are the two
- characters `"' and `\' must be written `"\"\\"'.
- 
-    Other escape sequences represent unprintable characters such as TAB
- or newline.  While there is nothing to stop you from entering most
- unprintable characters directly in a string constant or regexp constant,
- they may look ugly.
- 
-    The following table lists all the escape sequences used in `awk' and
- what they represent. Unless noted otherwise, all these escape sequences
- apply to both string constants and regexp constants:
- 
- `\\'
-      A literal backslash, `\'.
- 
- `\a'
-      The "alert" character, `Ctrl-g', ASCII code 7 (BEL).  (This
-      usually makes some sort of audible noise.)
- 
- `\b'
-      Backspace, `Ctrl-h', ASCII code 8 (BS).
- 
- `\f'
-      Formfeed, `Ctrl-l', ASCII code 12 (FF).
- 
- `\n'
-      Newline, `Ctrl-j', ASCII code 10 (LF).
- 
- `\r'
-      Carriage return, `Ctrl-m', ASCII code 13 (CR).
- 
- `\t'
-      Horizontal TAB, `Ctrl-i', ASCII code 9 (HT).
- 
- `\v'
-      Vertical tab, `Ctrl-k', ASCII code 11 (VT).
- 
- `\NNN'
-      The octal value NNN, where NNN stands for 1 to 3 digits between
-      `0' and `7'.  For example, the code for the ASCII ESC (escape)
-      character is `\033'.
- 
- `\xHH...'
-      The hexadecimal value HH, where HH stands for a sequence of
-      hexadecimal digits (`0'-`9', and either `A'-`F' or `a'-`f').  Like
-      the same construct in ISO C, the escape sequence continues until
-      the first nonhexadecimal digit is seen. (c.e.)  However, using
-      more than two hexadecimal digits produces undefined results. (The
-      `\x' escape sequence is not allowed in POSIX `awk'.)
- 
- `\/'
-      A literal slash (necessary for regexp constants only).  This
-      sequence is used when you want to write a regexp constant that
-      contains a slash. Because the regexp is delimited by slashes, you
-      need to escape the slash that is part of the pattern, in order to
-      tell `awk' to keep processing the rest of the regexp.
- 
- `\"'
-      A literal double quote (necessary for string constants only).
-      This sequence is used when you want to write a string constant
-      that contains a double quote. Because the string is delimited by
-      double quotes, you need to escape the quote that is part of the
-      string, in order to tell `awk' to keep processing the rest of the
-      string.
- 
-    In `gawk', a number of additional two-character sequences that begin
- with a backslash have special meaning in regexps.  *Note GNU Regexp
- Operators::.
- 
-    In a regexp, a backslash before any character that is not in the
- previous list and not listed in *note GNU Regexp Operators::, means
- that the next character should be taken literally, even if it would
- normally be a regexp operator.  For example, `/a\+b/' matches the three
- characters `a+b'.
- 
-    For complete portability, do not use a backslash before any
- character not shown in the previous list.
- 
-    To summarize:
- 
-    * The escape sequences in the table above are always processed first,
-      for both string constants and regexp constants. This happens very
-      early, as soon as `awk' reads your program.
- 
-    * `gawk' processes both regexp constants and dynamic regexps (*note
-      Computed Regexps::), for the special operators listed in *note GNU
-      Regexp Operators::.
- 
-    * A backslash before any other character means to treat that
-      character literally.
- 
- Advanced Notes: Backslash Before Regular Characters
- ---------------------------------------------------
- 
- If you place a backslash in a string constant before something that is
- not one of the characters previously listed, POSIX `awk' purposely
- leaves what happens as undefined.  There are two choices:
- 
- Strip the backslash out
-      This is what Brian Kernighan's `awk' and `gawk' both do.  For
-      example, `"a\qc"' is the same as `"aqc"'.  (Because this is such
-      an easy bug both to introduce and to miss, `gawk' warns you about
-      it.)  Consider `FS = "[ \t]+\|[ \t]+"' to use vertical bars
-      surrounded by whitespace as the field separator. There should be
-      two backslashes in the string: `FS = "[ \t]+\\|[ \t]+"'.)
- 
- Leave the backslash alone
-      Some other `awk' implementations do this.  In such
-      implementations, typing `"a\qc"' is the same as typing `"a\\qc"'.
- 
- Advanced Notes: Escape Sequences for Metacharacters
- ---------------------------------------------------
- 
- Suppose you use an octal or hexadecimal escape to represent a regexp
- metacharacter.  (See *note Regexp Operators::.)  Does `awk' treat the
- character as a literal character or as a regexp operator?
- 
-    Historically, such characters were taken literally.  (d.c.)
- However, the POSIX standard indicates that they should be treated as
- real metacharacters, which is what `gawk' does.  In compatibility mode
- (*note Options::), `gawk' treats the characters represented by octal
- and hexadecimal escape sequences literally when used in regexp
- constants. Thus, `/a\52b/' is equivalent to `/a\*b/'.
- 
- 
- File: gawk.info,  Node: Regexp Operators,  Next: Bracket Expressions,  Prev: 
Escape Sequences,  Up: Regexp
- 
- 3.3 Regular Expression Operators
- ================================
- 
- You can combine regular expressions with special characters, called
- "regular expression operators" or "metacharacters", to increase the
- power and versatility of regular expressions.
- 
-    The escape sequences described in *note Escape Sequences::, are
- valid inside a regexp.  They are introduced by a `\' and are recognized
- and converted into corresponding real characters as the very first step
- in processing regexps.
- 
-    Here is a list of metacharacters.  All characters that are not escape
- sequences and that are not listed in the table stand for themselves:
- 
- `\'
-      This is used to suppress the special meaning of a character when
-      matching.  For example, `\$' matches the character `$'.
- 
- `^'
-      This matches the beginning of a string.  For example, address@hidden'
-      matches address@hidden' at the beginning of a string and can be used to
-      identify chapter beginnings in Texinfo source files.  The `^' is
-      known as an "anchor", because it anchors the pattern to match only
-      at the beginning of the string.
- 
-      It is important to realize that `^' does not match the beginning of
-      a line embedded in a string.  The condition is not true in the
-      following example:
- 
-           if ("line1\nLINE 2" ~ /^L/) ...
- 
- `$'
-      This is similar to `^', but it matches only at the end of a string.
-      For example, `p$' matches a record that ends with a `p'.  The `$'
-      is an anchor and does not match the end of a line embedded in a
-      string.  The condition in the following example is not true:
- 
-           if ("line1\nLINE 2" ~ /1$/) ...
- 
- `. (period)'
-      This matches any single character, _including_ the newline
-      character.  For example, `.P' matches any single character
-      followed by a `P' in a string.  Using concatenation, we can make a
-      regular expression such as `U.A', which matches any
-      three-character sequence that begins with `U' and ends with `A'.
- 
-      In strict POSIX mode (*note Options::), `.' does not match the NUL
-      character, which is a character with all bits equal to zero.
-      Otherwise, NUL is just another character. Other versions of `awk'
-      may not be able to match the NUL character.
- 
- `[...]'
-      This is called a "bracket expression".(1) It matches any _one_ of
-      the characters that are enclosed in the square brackets.  For
-      example, `[MVX]' matches any one of the characters `M', `V', or
-      `X' in a string.  A full discussion of what can be inside the
-      square brackets of a bracket expression is given in *note Bracket
-      Expressions::.
- 
- `[^ ...]'
-      This is a "complemented bracket expression".  The first character
-      after the `[' _must_ be a `^'.  It matches any characters _except_
-      those in the square brackets.  For example, `[^awk]' matches any
-      character that is not an `a', `w', or `k'.
- 
- `|'
-      This is the "alternation operator" and it is used to specify
-      alternatives.  The `|' has the lowest precedence of all the regular
-      expression operators.  For example, `^P|[[:digit:]]' matches any
-      string that matches either `^P' or `[[:digit:]]'.  This means it
-      matches any string that starts with `P' or contains a digit.
- 
-      The alternation applies to the largest possible regexps on either
-      side.
- 
- `(...)'
-      Parentheses are used for grouping in regular expressions, as in
-      arithmetic.  They can be used to concatenate regular expressions
-      containing the alternation operator, `|'.  For example,
-      `@(samp|code)\{[^}]+\}' matches both address@hidden' and address@hidden'.
-      (These are Texinfo formatting control sequences. The `+' is
-      explained further on in this list.)
- 
- `*'
-      This symbol means that the preceding regular expression should be
-      repeated as many times as necessary to find a match.  For example,
-      `ph*' applies the `*' symbol to the preceding `h' and looks for
-      matches of one `p' followed by any number of `h's.  This also
-      matches just `p' if no `h's are present.
- 
-      The `*' repeats the _smallest_ possible preceding expression.
-      (Use parentheses if you want to repeat a larger expression.)  It
-      finds as many repetitions as possible.  For example, `awk
-      '/\(c[ad][ad]*r x\)/ { print }' sample' prints every record in
-      `sample' containing a string of the form `(car x)', `(cdr x)',
-      `(cadr x)', and so on.  Notice the escaping of the parentheses by
-      preceding them with backslashes.
- 
- `+'
-      This symbol is similar to `*', except that the preceding
-      expression must be matched at least once.  This means that `wh+y'
-      would match `why' and `whhy', but not `wy', whereas `wh*y' would
-      match all three of these strings.  The following is a simpler way
-      of writing the last `*' example:
- 
-           awk '/\(c[ad]+r x\)/ { print }' sample
- 
- `?'
-      This symbol is similar to `*', except that the preceding
-      expression can be matched either once or not at all.  For example,
-      `fe?d' matches `fed' and `fd', but nothing else.
- 
- `{N}'
- `{N,}'
- `{N,M}'
-      One or two numbers inside braces denote an "interval expression".
-      If there is one number in the braces, the preceding regexp is
-      repeated N times.  If there are two numbers separated by a comma,
-      the preceding regexp is repeated N to M times.  If there is one
-      number followed by a comma, then the preceding regexp is repeated
-      at least N times:
- 
-     `wh{3}y'
-           Matches `whhhy', but not `why' or `whhhhy'.
- 
-     `wh{3,5}y'
-           Matches `whhhy', `whhhhy', or `whhhhhy', only.
- 
-     `wh{2,}y'
-           Matches `whhy' or `whhhy', and so on.
- 
-      Interval expressions were not traditionally available in `awk'.
-      They were added as part of the POSIX standard to make `awk' and
-      `egrep' consistent with each other.
- 
-      Initially, because old programs may use `{' and `}' in regexp
-      constants, `gawk' did _not_ match interval expressions in regexps.
- 
-      However, beginning with version 4.0, `gawk' does match interval
-      expressions by default.  This is because compatibility with POSIX
-      has become more important to most `gawk' users than compatibility
-      with old programs.
- 
-      For programs that use `{' and `}' in regexp constants, it is good
-      practice to always escape them with a backslash.  Then the regexp
-      constants are valid and work the way you want them to, using any
-      version of `awk'.(2)
- 
-      Finally, when `{' and `}' appear in regexp constants in a way that
-      cannot be interpreted as an interval expression (such as
-      `/q{a}/'), then they stand for themselves.
- 
-    In regular expressions, the `*', `+', and `?' operators, as well as
- the braces `{' and `}', have the highest precedence, followed by
- concatenation, and finally by `|'.  As in arithmetic, parentheses can
- change how operators are grouped.
- 
-    In POSIX `awk' and `gawk', the `*', `+', and `?' operators stand for
- themselves when there is nothing in the regexp that precedes them.  For
- example, `/+/' matches a literal plus sign.  However, many other
- versions of `awk' treat such a usage as a syntax error.
- 
-    If `gawk' is in compatibility mode (*note Options::), interval
- expressions are not available in regular expressions.
- 
-    ---------- Footnotes ----------
- 
-    (1) In other literature, you may see a bracket expression referred
- to as either a "character set", a "character class", or a "character
- list".
- 
-    (2) Use two backslashes if you're using a string constant with a
- regexp operator or function.
- 
- 
- File: gawk.info,  Node: Bracket Expressions,  Next: GNU Regexp Operators,  
Prev: Regexp Operators,  Up: Regexp
- 
- 3.4 Using Bracket Expressions
- =============================
- 
- As mentioned earlier, a bracket expression matches any character amongst
- those listed between the opening and closing square brackets.
- 
-    Within a bracket expression, a "range expression" consists of two
- characters separated by a hyphen.  It matches any single character that
- sorts between the two characters, based upon the system's native
- character set.  For example, `[0-9]' is equivalent to `[0123456789]'.
- (See *note Ranges and Locales::, for an explanation of how the POSIX
- standard and `gawk' have changed over time.  This is mainly of
- historical interest.)
- 
-    To include one of the characters `\', `]', `-', or `^' in a bracket
- expression, put a `\' in front of it.  For example:
- 
-      [d\]]
- 
- matches either `d' or `]'.
- 
-    This treatment of `\' in bracket expressions is compatible with
- other `awk' implementations and is also mandated by POSIX.  The regular
- expressions in `awk' are a superset of the POSIX specification for
- Extended Regular Expressions (EREs).  POSIX EREs are based on the
- regular expressions accepted by the traditional `egrep' utility.
- 
-    "Character classes" are a feature introduced in the POSIX standard.
- A character class is a special notation for describing lists of
- characters that have a specific attribute, but the actual characters
- can vary from country to country and/or from character set to character
- set.  For example, the notion of what is an alphabetic character
- differs between the United States and France.
- 
-    A character class is only valid in a regexp _inside_ the brackets of
- a bracket expression.  Character classes consist of `[:', a keyword
- denoting the class, and `:]'.  *note table-char-classes:: lists the
- character classes defined by the POSIX standard.
- 
- Class       Meaning
- -------------------------------------------------------------------------- 
- `[:alnum:]' Alphanumeric characters.
- `[:alpha:]' Alphabetic characters.
- `[:blank:]' Space and TAB characters.
- `[:cntrl:]' Control characters.
- `[:digit:]' Numeric characters.
- `[:graph:]' Characters that are both printable and visible.  (A space is
-             printable but not visible, whereas an `a' is both.)
- `[:lower:]' Lowercase alphabetic characters.
- `[:print:]' Printable characters (characters that are not control
-             characters).
- `[:punct:]' Punctuation characters (characters that are not letters,
-             digits, control characters, or space characters).
- `[:space:]' Space characters (such as space, TAB, and formfeed, to name
-             a few).
- `[:upper:]' Uppercase alphabetic characters.
- `[:xdigit:]'Characters that are hexadecimal digits.
- 
- Table 3.1: POSIX Character Classes
- 
-    For example, before the POSIX standard, you had to write
- `/[A-Za-z0-9]/' to match alphanumeric characters.  If your character
- set had other alphabetic characters in it, this would not match them.
- With the POSIX character classes, you can write `/[[:alnum:]]/' to
- match the alphabetic and numeric characters in your character set.
- 
-    Two additional special sequences can appear in bracket expressions.
- These apply to non-ASCII character sets, which can have single symbols
- (called "collating elements") that are represented with more than one
- character. They can also have several characters that are equivalent for
- "collating", or sorting, purposes.  (For example, in French, a plain "e"
- and a grave-accented "e`" are equivalent.)  These sequences are:
- 
- Collating symbols
-      Multicharacter collating elements enclosed between `[.' and `.]'.
-      For example, if `ch' is a collating element, then `[[.ch.]]' is a
-      regexp that matches this collating element, whereas `[ch]' is a
-      regexp that matches either `c' or `h'.
- 
- Equivalence classes
-      Locale-specific names for a list of characters that are equal. The
-      name is enclosed between `[=' and `=]'.  For example, the name `e'
-      might be used to represent all of "e," "e`," and "e'." In this
-      case, `[[=e=]]' is a regexp that matches any of `e', `e'', or `e`'.
- 
-    These features are very valuable in non-English-speaking locales.
- 
-      CAUTION: The library functions that `gawk' uses for regular
-      expression matching currently recognize only POSIX character
-      classes; they do not recognize collating symbols or equivalence
-      classes.
- 
- 
- File: gawk.info,  Node: GNU Regexp Operators,  Next: Case-sensitivity,  Prev: 
Bracket Expressions,  Up: Regexp
- 
- 3.5 `gawk'-Specific Regexp Operators
- ====================================
- 
- GNU software that deals with regular expressions provides a number of
- additional regexp operators.  These operators are described in this
- minor node and are specific to `gawk'; they are not available in other
- `awk' implementations.  Most of the additional operators deal with word
- matching.  For our purposes, a "word" is a sequence of one or more
- letters, digits, or underscores (`_'):
- 
- `\s'
-      Matches any whitespace character.  Think of it as shorthand for
-      `[[:space:]]'.
- 
- `\S'
-      Matches any character that is not whitespace.  Think of it as
-      shorthand for `[^[:space:]]'.
- 
- `\w'
-      Matches any word-constituent character--that is, it matches any
-      letter, digit, or underscore. Think of it as shorthand for
-      `[[:alnum:]_]'.
- 
- `\W'
-      Matches any character that is not word-constituent.  Think of it
-      as shorthand for `[^[:alnum:]_]'.
- 
- `\<'
-      Matches the empty string at the beginning of a word.  For example,
-      `/\<away/' matches `away' but not `stowaway'.
- 
- `\>'
-      Matches the empty string at the end of a word.  For example,
-      `/stow\>/' matches `stow' but not `stowaway'.
- 
- `\y'
-      Matches the empty string at either the beginning or the end of a
-      word (i.e., the word boundar*y*).  For example, `\yballs?\y'
-      matches either `ball' or `balls', as a separate word.
- 
- `\B'
-      Matches the empty string that occurs between two word-constituent
-      characters. For example, `/\Brat\B/' matches `crate' but it does
-      not match `dirty rat'.  `\B' is essentially the opposite of `\y'.
- 
-    There are two other operators that work on buffers.  In Emacs, a
- "buffer" is, naturally, an Emacs buffer.  For other programs, `gawk''s
- regexp library routines consider the entire string to match as the
- buffer.  The operators are:
- 
- `\`'
-      Matches the empty string at the beginning of a buffer (string).
- 
- `\''
-      Matches the empty string at the end of a buffer (string).
- 
-    Because `^' and `$' always work in terms of the beginning and end of
- strings, these operators don't add any new capabilities for `awk'.
- They are provided for compatibility with other GNU software.
- 
-    In other GNU software, the word-boundary operator is `\b'. However,
- that conflicts with the `awk' language's definition of `\b' as
- backspace, so `gawk' uses a different letter.  An alternative method
- would have been to require two backslashes in the GNU operators, but
- this was deemed too confusing. The current method of using `\y' for the
- GNU `\b' appears to be the lesser of two evils.
- 
-    The various command-line options (*note Options::) control how
- `gawk' interprets characters in regexps:
- 
- No options
-      In the default case, `gawk' provides all the facilities of POSIX
-      regexps and the GNU regexp operators described in *note Regexp
-      Operators::.
- 
- `--posix'
-      Only POSIX regexps are supported; the GNU operators are not special
-      (e.g., `\w' matches a literal `w').  Interval expressions are
-      allowed.
- 
- `--traditional'
-      Traditional Unix `awk' regexps are matched. The GNU operators are
-      not special, and interval expressions are not available.  The
-      POSIX character classes (`[[:alnum:]]', etc.) are supported, as
-      Brian Kernighan's `awk' does support them.  Characters described
-      by octal and hexadecimal escape sequences are treated literally,
-      even if they represent regexp metacharacters.
- 
- `--re-interval'
-      Allow interval expressions in regexps, if `--traditional' has been
-      provided.  Otherwise, interval expressions are available by
-      default.
- 
- 
- File: gawk.info,  Node: Case-sensitivity,  Next: Leftmost Longest,  Prev: GNU 
Regexp Operators,  Up: Regexp
- 
- 3.6 Case Sensitivity in Matching
- ================================
- 
- Case is normally significant in regular expressions, both when matching
- ordinary characters (i.e., not metacharacters) and inside bracket
- expressions.  Thus, a `w' in a regular expression matches only a
- lowercase `w' and not an uppercase `W'.
- 
-    The simplest way to do a case-independent match is to use a bracket
- expression--for example, `[Ww]'.  However, this can be cumbersome if
- you need to use it often, and it can make the regular expressions harder
- to read.  There are two alternatives that you might prefer.
- 
-    One way to perform a case-insensitive match at a particular point in
- the program is to convert the data to a single case, using the
- `tolower()' or `toupper()' built-in string functions (which we haven't
- discussed yet; *note String Functions::).  For example:
- 
-      tolower($1) ~ /foo/  { ... }
- 
- converts the first field to lowercase before matching against it.  This
- works in any POSIX-compliant `awk'.
- 
-    Another method, specific to `gawk', is to set the variable
- `IGNORECASE' to a nonzero value (*note Built-in Variables::).  When
- `IGNORECASE' is not zero, _all_ regexp and string operations ignore
- case.  Changing the value of `IGNORECASE' dynamically controls the
- case-sensitivity of the program as it runs.  Case is significant by
- default because `IGNORECASE' (like most variables) is initialized to
- zero:
- 
-      x = "aB"
-      if (x ~ /ab/) ...   # this test will fail
- 
-      IGNORECASE = 1
-      if (x ~ /ab/) ...   # now it will succeed
- 
-    In general, you cannot use `IGNORECASE' to make certain rules
- case-insensitive and other rules case-sensitive, because there is no
- straightforward way to set `IGNORECASE' just for the pattern of a
- particular rule.(1) To do this, use either bracket expressions or
- `tolower()'.  However, one thing you can do with `IGNORECASE' only is
- dynamically turn case-sensitivity on or off for all the rules at once.
- 
-    `IGNORECASE' can be set on the command line or in a `BEGIN' rule
- (*note Other Arguments::; also *note Using BEGIN/END::).  Setting
- `IGNORECASE' from the command line is a way to make a program
- case-insensitive without having to edit it.
- 
-    Both regexp and string comparison operations are affected by
- `IGNORECASE'.
- 
-    In multibyte locales, the equivalences between upper- and lowercase
- characters are tested based on the wide-character values of the
- locale's character set.  Otherwise, the characters are tested based on
- the ISO-8859-1 (ISO Latin-1) character set. This character set is a
- superset of the traditional 128 ASCII characters, which also provides a
- number of characters suitable for use with European languages.(2)
- 
-    The value of `IGNORECASE' has no effect if `gawk' is in
- compatibility mode (*note Options::).  Case is always significant in
- compatibility mode.
- 
-    ---------- Footnotes ----------
- 
-    (1) Experienced C and C++ programmers will note that it is possible,
- using something like `IGNORECASE = 1 && /foObAr/ { ... }' and
- `IGNORECASE = 0 || /foobar/ { ... }'.  However, this is somewhat
- obscure and we don't recommend it.
- 
-    (2) If you don't understand this, don't worry about it; it just
- means that `gawk' does the right thing.
- 
- 
- File: gawk.info,  Node: Leftmost Longest,  Next: Computed Regexps,  Prev: 
Case-sensitivity,  Up: Regexp
- 
- 3.7 How Much Text Matches?
- ==========================
- 
- Consider the following:
- 
-      echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
- 
-    This example uses the `sub()' function (which we haven't discussed
- yet; *note String Functions::) to make a change to the input record.
- Here, the regexp `/a+/' indicates "one or more `a' characters," and the
- replacement text is `<A>'.
- 
-    The input contains four `a' characters.  `awk' (and POSIX) regular
- expressions always match the leftmost, _longest_ sequence of input
- characters that can match.  Thus, all four `a' characters are replaced
- with `<A>' in this example:
- 
-      $ echo aaaabcd | awk '{ sub(/a+/, "<A>"); print }'
-      -| <A>bcd
- 
-    For simple match/no-match tests, this is not so important. But when
- doing text matching and substitutions with the `match()', `sub()',
- `gsub()', and `gensub()' functions, it is very important.  *Note String
- Functions::, for more information on these functions.  Understanding
- this principle is also important for regexp-based record and field
- splitting (*note Records::, and also *note Field Separators::).
- 
- 
- File: gawk.info,  Node: Computed Regexps,  Prev: Leftmost Longest,  Up: Regexp
- 
- 3.8 Using Dynamic Regexps
- =========================
- 
- The righthand side of a `~' or `!~' operator need not be a regexp
- constant (i.e., a string of characters between slashes).  It may be any
- expression.  The expression is evaluated and converted to a string if
- necessary; the contents of the string are then used as the regexp.  A
- regexp computed in this way is called a "dynamic regexp":
- 
-      BEGIN { digits_regexp = "[[:digit:]]+" }
-      $0 ~ digits_regexp    { print }
- 
- This sets `digits_regexp' to a regexp that describes one or more digits,
- and tests whether the input record matches this regexp.
- 
-      NOTE: When using the `~' and `!~' operators, there is a difference
-      between a regexp constant enclosed in slashes and a string
-      constant enclosed in double quotes.  If you are going to use a
-      string constant, you have to understand that the string is, in
-      essence, scanned _twice_: the first time when `awk' reads your
-      program, and the second time when it goes to match the string on
-      the lefthand side of the operator with the pattern on the right.
-      This is true of any string-valued expression (such as
-      `digits_regexp', shown previously), not just string constants.
- 
-    What difference does it make if the string is scanned twice? The
- answer has to do with escape sequences, and particularly with
- backslashes.  To get a backslash into a regular expression inside a
- string, you have to type two backslashes.
- 
-    For example, `/\*/' is a regexp constant for a literal `*'.  Only
- one backslash is needed.  To do the same thing with a string, you have
- to type `"\\*"'.  The first backslash escapes the second one so that
- the string actually contains the two characters `\' and `*'.
- 
-    Given that you can use both regexp and string constants to describe
- regular expressions, which should you use?  The answer is "regexp
- constants," for several reasons:
- 
-    * String constants are more complicated to write and more difficult
-      to read. Using regexp constants makes your programs less
-      error-prone.  Not understanding the difference between the two
-      kinds of constants is a common source of errors.
- 
-    * It is more efficient to use regexp constants. `awk' can note that
-      you have supplied a regexp and store it internally in a form that
-      makes pattern matching more efficient.  When using a string
-      constant, `awk' must first convert the string into this internal
-      form and then perform the pattern matching.
- 
-    * Using regexp constants is better form; it shows clearly that you
-      intend a regexp match.
- 
- Advanced Notes: Using `\n' in Bracket Expressions of Dynamic Regexps
- --------------------------------------------------------------------
- 
- Some commercial versions of `awk' do not allow the newline character to
- be used inside a bracket expression for a dynamic regexp:
- 
-      $ awk '$0 ~ "[ \t\n]"'
-      error--> awk: newline in character class [
-      error--> ]...
-      error-->  source line number 1
-      error-->  context is
-      error-->          >>>  <<<
- 
-    But a newline in a regexp constant works with no problem:
- 
-      $ awk '$0 ~ /[ \t\n]/'
-      here is a sample line
-      -| here is a sample line
-      Ctrl-d
- 
-    `gawk' does not have this problem, and it isn't likely to occur
- often in practice, but it's worth noting for future reference.
- 
- 
- File: gawk.info,  Node: Reading Files,  Next: Printing,  Prev: Regexp,  Up: 
Top
- 
- 4 Reading Input Files
- *********************
- 
- In the typical `awk' program, `awk' reads all input either from the
- standard input (by default, this is the keyboard, but often it is a
- pipe from another command) or from files whose names you specify on the
- `awk' command line.  If you specify input files, `awk' reads them in
- order, processing all the data from one before going on to the next.
- The name of the current input file can be found in the built-in variable
- `FILENAME' (*note Built-in Variables::).
- 
-    The input is read in units called "records", and is processed by the
- rules of your program one record at a time.  By default, each record is
- one line.  Each record is automatically split into chunks called
- "fields".  This makes it more convenient for programs to work on the
- parts of a record.
- 
-    On rare occasions, you may need to use the `getline' command.  The
- `getline' command is valuable, both because it can do explicit input
- from any number of files, and because the files used with it do not
- have to be named on the `awk' command line (*note Getline::).
- 
- * Menu:
- 
- * Records::                     Controlling how data is split into records.
- * Fields::                      An introduction to fields.
- * Nonconstant Fields::          Nonconstant Field Numbers.
- * Changing Fields::             Changing the Contents of a Field.
- * Field Separators::            The field separator and how to change it.
- * Constant Size::               Reading constant width data.
- * Splitting By Content::        Defining Fields By Content
- * Multiple Line::               Reading multi-line records.
- * Getline::                     Reading files under explicit program control
-                                 using the `getline' function.
- * Read Timeout::                Reading input with a timeout.
- * Command line directories::    What happens if you put a directory on the
-                                 command line.
- 
- 
- File: gawk.info,  Node: Records,  Next: Fields,  Up: Reading Files
- 
- 4.1 How Input Is Split into Records
- ===================================
- 
- The `awk' utility divides the input for your `awk' program into records
- and fields.  `awk' keeps track of the number of records that have been
- read so far from the current input file.  This value is stored in a
- built-in variable called `FNR'.  It is reset to zero when a new file is
- started.  Another built-in variable, `NR', records the total number of
- input records read so far from all data files.  It starts at zero, but
- is never automatically reset to zero.
- 
-    Records are separated by a character called the "record separator".
- By default, the record separator is the newline character.  This is why
- records are, by default, single lines.  A different character can be
- used for the record separator by assigning the character to the
- built-in variable `RS'.
- 
-    Like any other variable, the value of `RS' can be changed in the
- `awk' program with the assignment operator, `=' (*note Assignment
- Ops::).  The new record-separator character should be enclosed in
- quotation marks, which indicate a string constant.  Often the right
- time to do this is at the beginning of execution, before any input is
- processed, so that the very first record is read with the proper
- separator.  To do this, use the special `BEGIN' pattern (*note
- BEGIN/END::).  For example:
- 
-      awk 'BEGIN { RS = "/" }
-           { print $0 }' BBS-list
- 
- changes the value of `RS' to `"/"', before reading any input.  This is
- a string whose first character is a slash; as a result, records are
- separated by slashes.  Then the input file is read, and the second rule
- in the `awk' program (the action with no pattern) prints each record.
- Because each `print' statement adds a newline at the end of its output,
- this `awk' program copies the input with each slash changed to a
- newline.  Here are the results of running the program on `BBS-list':
- 
-      $ awk 'BEGIN { RS = "/" }
-      >      { print $0 }' BBS-list
-      -| aardvark     555-5553     1200
-      -| 300          B
-      -| alpo-net     555-3412     2400
-      -| 1200
-      -| 300     A
-      -| barfly       555-7685     1200
-      -| 300          A
-      -| bites        555-1675     2400
-      -| 1200
-      -| 300     A
-      -| camelot      555-0542     300               C
-      -| core         555-2912     1200
-      -| 300          C
-      -| fooey        555-1234     2400
-      -| 1200
-      -| 300     B
-      -| foot         555-6699     1200
-      -| 300          B
-      -| macfoo       555-6480     1200
-      -| 300          A
-      -| sdace        555-3430     2400
-      -| 1200
-      -| 300     A
-      -| sabafoo      555-2127     1200
-      -| 300          C
-      -|
- 
- Note that the entry for the `camelot' BBS is not split.  In the
- original data file (*note Sample Data Files::), the line looks like
- this:
- 
-      camelot      555-0542     300               C
- 
- It has one baud rate only, so there are no slashes in the record,
- unlike the others which have two or more baud rates.  In fact, this
- record is treated as part of the record for the `core' BBS; the newline
- separating them in the output is the original newline in the data file,
- not the one added by `awk' when it printed the record!
- 
-    Another way to change the record separator is on the command line,
- using the variable-assignment feature (*note Other Arguments::):
- 
-      awk '{ print $0 }' RS="/" BBS-list
- 
- This sets `RS' to `/' before processing `BBS-list'.
- 
-    Using an unusual character such as `/' for the record separator
- produces correct behavior in the vast majority of cases.  However, the
- following (extreme) pipeline prints a surprising `1':
- 
-      $ echo | awk 'BEGIN { RS = "a" } ; { print NF }'
-      -| 1
- 
-    There is one field, consisting of a newline.  The value of the
- built-in variable `NF' is the number of fields in the current record.
- 
-    Reaching the end of an input file terminates the current input
- record, even if the last character in the file is not the character in
- `RS'.  (d.c.)
- 
-    The empty string `""' (a string without any characters) has a
- special meaning as the value of `RS'. It means that records are
- separated by one or more blank lines and nothing else.  *Note Multiple
- Line::, for more details.
- 
-    If you change the value of `RS' in the middle of an `awk' run, the
- new value is used to delimit subsequent records, but the record
- currently being processed, as well as records already processed, are not
- affected.
- 
-    After the end of the record has been determined, `gawk' sets the
- variable `RT' to the text in the input that matched `RS'.
- 
-    When using `gawk', the value of `RS' is not limited to a
- one-character string.  It can be any regular expression (*note
- Regexp::). (c.e.)  In general, each record ends at the next string that
- matches the regular expression; the next record starts at the end of
- the matching string.  This general rule is actually at work in the
- usual case, where `RS' contains just a newline: a record ends at the
- beginning of the next matching string (the next newline in the input),
- and the following record starts just after the end of this string (at
- the first character of the following line).  The newline, because it
- matches `RS', is not part of either record.
- 
-    When `RS' is a single character, `RT' contains the same single
- character. However, when `RS' is a regular expression, `RT' contains
- the actual input text that matched the regular expression.
- 
-    If the input file ended without any text that matches `RS', `gawk'
- sets `RT' to the null string.
- 
-    The following example illustrates both of these features.  It sets
- `RS' equal to a regular expression that matches either a newline or a
- series of one or more uppercase letters with optional leading and/or
- trailing whitespace:
- 
-      $ echo record 1 AAAA record 2 BBBB record 3 |
-      > gawk 'BEGIN { RS = "\n|( *[[:upper:]]+ *)" }
-      >             { print "Record =", $0, "and RT =", RT }'
-      -| Record = record 1 and RT =  AAAA
-      -| Record = record 2 and RT =  BBBB
-      -| Record = record 3 and RT =
-      -|
- 
- The final line of output has an extra blank line. This is because the
- value of `RT' is a newline, and the `print' statement supplies its own
- terminating newline.  *Note Simple Sed::, for a more useful example of
- `RS' as a regexp and `RT'.
- 
-    If you set `RS' to a regular expression that allows optional
- trailing text, such as `RS = "abc(XYZ)?"' it is possible, due to
- implementation constraints, that `gawk' may match the leading part of
- the regular expression, but not the trailing part, particularly if the
- input text that could match the trailing part is fairly long.  `gawk'
- attempts to avoid this problem, but currently, there's no guarantee
- that this will never happen.
- 
-      NOTE: Remember that in `awk', the `^' and `$' anchor
-      metacharacters match the beginning and end of a _string_, and not
-      the beginning and end of a _line_.  As a result, something like
-      `RS = "^[[:upper:]]"' can only match at the beginning of a file.
-      This is because `gawk' views the input file as one long string
-      that happens to contain newline characters in it.  It is thus best
-      to avoid anchor characters in the value of `RS'.
- 
-    The use of `RS' as a regular expression and the `RT' variable are
- `gawk' extensions; they are not available in compatibility mode (*note
- Options::).  In compatibility mode, only the first character of the
- value of `RS' is used to determine the end of the record.
- 
- Advanced Notes: `RS = "\0"' Is Not Portable
- -------------------------------------------
- 
- There are times when you might want to treat an entire data file as a
- single record.  The only way to make this happen is to give `RS' a
- value that you know doesn't occur in the input file.  This is hard to
- do in a general way, such that a program always works for arbitrary
- input files.
- 
-    You might think that for text files, the NUL character, which
- consists of a character with all bits equal to zero, is a good value to
- use for `RS' in this case:
- 
-      BEGIN { RS = "\0" }  # whole file becomes one record?
- 
-    `gawk' in fact accepts this, and uses the NUL character for the
- record separator.  However, this usage is _not_ portable to other `awk'
- implementations.
- 
-    All other `awk' implementations(1) store strings internally as
- C-style strings.  C strings use the NUL character as the string
- terminator.  In effect, this means that `RS = "\0"' is the same as `RS
- = ""'.  (d.c.)
- 
-    The best way to treat a whole file as a single record is to simply
- read the file in, one record at a time, concatenating each record onto
- the end of the previous ones.
- 
-    ---------- Footnotes ----------
- 
-    (1) At least that we know about.
- 
- 
- File: gawk.info,  Node: Fields,  Next: Nonconstant Fields,  Prev: Records,  
Up: Reading Files
- 
- 4.2 Examining Fields
- ====================
- 
- When `awk' reads an input record, the record is automatically "parsed"
- or separated by the `awk' utility into chunks called "fields".  By
- default, fields are separated by "whitespace", like words in a line.
- Whitespace in `awk' means any string of one or more spaces, TABs, or
- newlines;(1) other characters, such as formfeed, vertical tab, etc.,
- that are considered whitespace by other languages, are _not_ considered
- whitespace by `awk'.
- 
-    The purpose of fields is to make it more convenient for you to refer
- to these pieces of the record.  You don't have to use them--you can
- operate on the whole record if you want--but fields are what make
- simple `awk' programs so powerful.
- 
-    A dollar-sign (`$') is used to refer to a field in an `awk' program,
- followed by the number of the field you want.  Thus, `$1' refers to the
- first field, `$2' to the second, and so on.  (Unlike the Unix shells,
- the field numbers are not limited to single digits.  `$127' is the one
- hundred twenty-seventh field in the record.)  For example, suppose the
- following is a line of input:
- 
-      This seems like a pretty nice example.
- 
- Here the first field, or `$1', is `This', the second field, or `$2', is
- `seems', and so on.  Note that the last field, `$7', is `example.'.
- Because there is no space between the `e' and the `.', the period is
- considered part of the seventh field.
- 
-    `NF' is a built-in variable whose value is the number of fields in
- the current record.  `awk' automatically updates the value of `NF' each
- time it reads a record.  No matter how many fields there are, the last
- field in a record can be represented by `$NF'.  So, `$NF' is the same
- as `$7', which is `example.'.  If you try to reference a field beyond
- the last one (such as `$8' when the record has only seven fields), you
- get the empty string.  (If used in a numeric operation, you get zero.)
- 
-    The use of `$0', which looks like a reference to the "zero-th"
- field, is a special case: it represents the whole input record when you
- are not interested in specific fields.  Here are some more examples:
- 
-      $ awk '$1 ~ /foo/ { print $0 }' BBS-list
-      -| fooey        555-1234     2400/1200/300     B
-      -| foot         555-6699     1200/300          B
-      -| macfoo       555-6480     1200/300          A
-      -| sabafoo      555-2127     1200/300          C
- 
- This example prints each record in the file `BBS-list' whose first
- field contains the string `foo'.  The operator `~' is called a
- "matching operator" (*note Regexp Usage::); it tests whether a string
- (here, the field `$1') matches a given regular expression.
- 
-    By contrast, the following example looks for `foo' in _the entire
- record_ and prints the first field and the last field for each matching
- input record:
- 
-      $ awk '/foo/ { print $1, $NF }' BBS-list
-      -| fooey B
-      -| foot B
-      -| macfoo A
-      -| sabafoo C
- 
-    ---------- Footnotes ----------
- 
-    (1) In POSIX `awk', newlines are not considered whitespace for
- separating fields.
- 
- 
- File: gawk.info,  Node: Nonconstant Fields,  Next: Changing Fields,  Prev: 
Fields,  Up: Reading Files
- 
- 4.3 Nonconstant Field Numbers
- =============================
- 
- The number of a field does not need to be a constant.  Any expression in
- the `awk' language can be used after a `$' to refer to a field.  The
- value of the expression specifies the field number.  If the value is a
- string, rather than a number, it is converted to a number.  Consider
- this example:
- 
-      awk '{ print $NR }'
- 
- Recall that `NR' is the number of records read so far: one in the first
- record, two in the second, etc.  So this example prints the first field
- of the first record, the second field of the second record, and so on.
- For the twentieth record, field number 20 is printed; most likely, the
- record has fewer than 20 fields, so this prints a blank line.  Here is
- another example of using expressions as field numbers:
- 
-      awk '{ print $(2*2) }' BBS-list
- 
-    `awk' evaluates the expression `(2*2)' and uses its value as the
- number of the field to print.  The `*' sign represents multiplication,
- so the expression `2*2' evaluates to four.  The parentheses are used so
- that the multiplication is done before the `$' operation; they are
- necessary whenever there is a binary operator in the field-number
- expression.  This example, then, prints the hours of operation (the
- fourth field) for every line of the file `BBS-list'.  (All of the `awk'
- operators are listed, in order of decreasing precedence, in *note
- Precedence::.)
- 
-    If the field number you compute is zero, you get the entire record.
- Thus, `$(2-2)' has the same value as `$0'.  Negative field numbers are
- not allowed; trying to reference one usually terminates the program.
- (The POSIX standard does not define what happens when you reference a
- negative field number.  `gawk' notices this and terminates your
- program.  Other `awk' implementations may behave differently.)
- 
-    As mentioned in *note Fields::, `awk' stores the current record's
- number of fields in the built-in variable `NF' (also *note Built-in
- Variables::).  The expression `$NF' is not a special feature--it is the
- direct consequence of evaluating `NF' and using its value as a field
- number.
- 
- 
- File: gawk.info,  Node: Changing Fields,  Next: Field Separators,  Prev: 
Nonconstant Fields,  Up: Reading Files
- 
- 4.4 Changing the Contents of a Field
- ====================================
- 
- The contents of a field, as seen by `awk', can be changed within an
- `awk' program; this changes what `awk' perceives as the current input
- record.  (The actual input is untouched; `awk' _never_ modifies the
- input file.)  Consider the following example and its output:
- 
-      $ awk '{ nboxes = $3 ; $3 = $3 - 10
-      >        print nboxes, $3 }' inventory-shipped
-      -| 25 15
-      -| 32 22
-      -| 24 14
-      ...
- 
- The program first saves the original value of field three in the
- variable `nboxes'.  The `-' sign represents subtraction, so this
- program reassigns field three, `$3', as the original value of field
- three minus ten: `$3 - 10'.  (*Note Arithmetic Ops::.)  Then it prints
- the original and new values for field three.  (Someone in the warehouse
- made a consistent mistake while inventorying the red boxes.)
- 
-    For this to work, the text in field `$3' must make sense as a
- number; the string of characters must be converted to a number for the
- computer to do arithmetic on it.  The number resulting from the
- subtraction is converted back to a string of characters that then
- becomes field three.  *Note Conversion::.
- 
-    When the value of a field is changed (as perceived by `awk'), the
- text of the input record is recalculated to contain the new field where
- the old one was.  In other words, `$0' changes to reflect the altered
- field.  Thus, this program prints a copy of the input file, with 10
- subtracted from the second field of each line:
- 
-      $ awk '{ $2 = $2 - 10; print $0 }' inventory-shipped
-      -| Jan 3 25 15 115
-      -| Feb 5 32 24 226
-      -| Mar 5 24 34 228
-      ...
- 
-    It is also possible to also assign contents to fields that are out
- of range.  For example:
- 
-      $ awk '{ $6 = ($5 + $4 + $3 + $2)
-      >        print $6 }' inventory-shipped
-      -| 168
-      -| 297
-      -| 301
-      ...
- 
- We've just created `$6', whose value is the sum of fields `$2', `$3',
- `$4', and `$5'.  The `+' sign represents addition.  For the file
- `inventory-shipped', `$6' represents the total number of parcels
- shipped for a particular month.
- 
-    Creating a new field changes `awk''s internal copy of the current
- input record, which is the value of `$0'.  Thus, if you do `print $0'
- after adding a field, the record printed includes the new field, with
- the appropriate number of field separators between it and the previously
- existing fields.
- 
-    This recomputation affects and is affected by `NF' (the number of
- fields; *note Fields::).  For example, the value of `NF' is set to the
- number of the highest field you create.  The exact format of `$0' is
- also affected by a feature that has not been discussed yet: the "output
- field separator", `OFS', used to separate the fields (*note Output
- Separators::).
- 
-    Note, however, that merely _referencing_ an out-of-range field does
- _not_ change the value of either `$0' or `NF'.  Referencing an
- out-of-range field only produces an empty string.  For example:
- 
-      if ($(NF+1) != "")
-          print "can't happen"
-      else
-          print "everything is normal"
- 
- should print `everything is normal', because `NF+1' is certain to be
- out of range.  (*Note If Statement::, for more information about
- `awk''s `if-else' statements.  *Note Typing and Comparison::, for more
- information about the `!=' operator.)
- 
-    It is important to note that making an assignment to an existing
- field changes the value of `$0' but does not change the value of `NF',
- even when you assign the empty string to a field.  For example:
- 
-      $ echo a b c d | awk '{ OFS = ":"; $2 = ""
-      >                       print $0; print NF }'
-      -| a::c:d
-      -| 4
- 
- The field is still there; it just has an empty value, denoted by the
- two colons between `a' and `c'.  This example shows what happens if you
- create a new field:
- 
-      $ echo a b c d | awk '{ OFS = ":"; $2 = ""; $6 = "new"
-      >                       print $0; print NF }'
-      -| a::c:d::new
-      -| 6
- 
- The intervening field, `$5', is created with an empty value (indicated
- by the second pair of adjacent colons), and `NF' is updated with the
- value six.
- 
-    Decrementing `NF' throws away the values of the fields after the new
- value of `NF' and recomputes `$0'.  (d.c.)  Here is an example:
- 
-      $ echo a b c d e f | awk '{ print "NF =", NF;
-      >                            NF = 3; print $0 }'
-      -| NF = 6
-      -| a b c
- 
-      CAUTION: Some versions of `awk' don't rebuild `$0' when `NF' is
-      decremented. Caveat emptor.
- 
-    Finally, there are times when it is convenient to force `awk' to
- rebuild the entire record, using the current value of the fields and
- `OFS'.  To do this, use the seemingly innocuous assignment:
- 
-      $1 = $1   # force record to be reconstituted
-      print $0  # or whatever else with $0
- 
- This forces `awk' to rebuild the record.  It does help to add a
- comment, as we've shown here.
- 
-    There is a flip side to the relationship between `$0' and the
- fields.  Any assignment to `$0' causes the record to be reparsed into
- fields using the _current_ value of `FS'.  This also applies to any
- built-in function that updates `$0', such as `sub()' and `gsub()'
- (*note String Functions::).
- 
- Advanced Notes: Understanding `$0'
- ----------------------------------
- 
- It is important to remember that `$0' is the _full_ record, exactly as
- it was read from the input.  This includes any leading or trailing
- whitespace, and the exact whitespace (or other characters) that
- separate the fields.
- 
-    It is a not-uncommon error to try to change the field separators in
- a record simply by setting `FS' and `OFS', and then expecting a plain
- `print' or `print $0' to print the modified record.
- 
-    But this does not work, since nothing was done to change the record
- itself.  Instead, you must force the record to be rebuilt, typically
- with a statement such as `$1 = $1', as described earlier.
- 
- 
- File: gawk.info,  Node: Field Separators,  Next: Constant Size,  Prev: 
Changing Fields,  Up: Reading Files
- 
- 4.5 Specifying How Fields Are Separated
- =======================================
- 
- * Menu:
- 
- * Default Field Splitting::      How fields are normally separated.
- * Regexp Field Splitting::       Using regexps as the field separator.
- * Single Character Fields::      Making each character a separate field.
- * Command Line Field Separator:: Setting `FS' from the command-line.
- * Field Splitting Summary::      Some final points and a summary table.
- 
-    The "field separator", which is either a single character or a
- regular expression, controls the way `awk' splits an input record into
- fields.  `awk' scans the input record for character sequences that
- match the separator; the fields themselves are the text between the
- matches.
- 
-    In the examples that follow, we use the bullet symbol (*) to
- represent spaces in the output.  If the field separator is `oo', then
- the following line:
- 
-      moo goo gai pan
- 
- is split into three fields: `m', `*g', and `*gai*pan'.  Note the
- leading spaces in the values of the second and third fields.
- 
-    The field separator is represented by the built-in variable `FS'.
- Shell programmers take note:  `awk' does _not_ use the name `IFS' that
- is used by the POSIX-compliant shells (such as the Unix Bourne shell,
- `sh', or Bash).
- 
-    The value of `FS' can be changed in the `awk' program with the
- assignment operator, `=' (*note Assignment Ops::).  Often the right
- time to do this is at the beginning of execution before any input has
- been processed, so that the very first record is read with the proper
- separator.  To do this, use the special `BEGIN' pattern (*note
- BEGIN/END::).  For example, here we set the value of `FS' to the string
- `","':
- 
-      awk 'BEGIN { FS = "," } ; { print $2 }'
- 
- Given the input line:
- 
-      John Q. Smith, 29 Oak St., Walamazoo, MI 42139
- 
- this `awk' program extracts and prints the string `*29*Oak*St.'.
- 
-    Sometimes the input data contains separator characters that don't
- separate fields the way you thought they would.  For instance, the
- person's name in the example we just used might have a title or suffix
- attached, such as:
- 
-      John Q. Smith, LXIX, 29 Oak St., Walamazoo, MI 42139
- 
- The same program would extract `*LXIX', instead of `*29*Oak*St.'.  If
- you were expecting the program to print the address, you would be
- surprised.  The moral is to choose your data layout and separator
- characters carefully to prevent such problems.  (If the data is not in
- a form that is easy to process, perhaps you can massage it first with a
- separate `awk' program.)
- 
- 
- File: gawk.info,  Node: Default Field Splitting,  Next: Regexp Field 
Splitting,  Up: Field Separators
- 
- 4.5.1 Whitespace Normally Separates Fields
- ------------------------------------------
- 
- Fields are normally separated by whitespace sequences (spaces, TABs,
- and newlines), not by single spaces.  Two spaces in a row do not
- delimit an empty field.  The default value of the field separator `FS'
- is a string containing a single space, `" "'.  If `awk' interpreted
- this value in the usual way, each space character would separate
- fields, so two spaces in a row would make an empty field between them.
- The reason this does not happen is that a single space as the value of
- `FS' is a special case--it is taken to specify the default manner of
- delimiting fields.
- 
-    If `FS' is any other single character, such as `","', then each
- occurrence of that character separates two fields.  Two consecutive
- occurrences delimit an empty field.  If the character occurs at the
- beginning or the end of the line, that too delimits an empty field.  The
- space character is the only single character that does not follow these
- rules.
- 
- 
- File: gawk.info,  Node: Regexp Field Splitting,  Next: Single Character 
Fields,  Prev: Default Field Splitting,  Up: Field Separators
- 
- 4.5.2 Using Regular Expressions to Separate Fields
- --------------------------------------------------
- 
- The previous node discussed the use of single characters or simple
- strings as the value of `FS'.  More generally, the value of `FS' may be
- a string containing any regular expression.  In this case, each match
- in the record for the regular expression separates fields.  For
- example, the assignment:
- 
-      FS = ", \t"
- 
- makes every area of an input line that consists of a comma followed by a
- space and a TAB into a field separator.  (`\t' is an "escape sequence"
- that stands for a TAB; *note Escape Sequences::, for the complete list
- of similar escape sequences.)
- 
-    For a less trivial example of a regular expression, try using single
- spaces to separate fields the way single commas are used.  `FS' can be
- set to `"[ ]"' (left bracket, space, right bracket).  This regular
- expression matches a single space and nothing else (*note Regexp::).
- 
-    There is an important difference between the two cases of `FS = " "'
- (a single space) and `FS = "[ \t\n]+"' (a regular expression matching
- one or more spaces, TABs, or newlines).  For both values of `FS',
- fields are separated by "runs" (multiple adjacent occurrences) of
- spaces, TABs, and/or newlines.  However, when the value of `FS' is
- `" "', `awk' first strips leading and trailing whitespace from the
- record and then decides where the fields are.  For example, the
- following pipeline prints `b':
- 
-      $ echo ' a b c d ' | awk '{ print $2 }'
-      -| b
- 
- However, this pipeline prints `a' (note the extra spaces around each
- letter):
- 
-      $ echo ' a  b  c  d ' | awk 'BEGIN { FS = "[ \t\n]+" }
-      >                                  { print $2 }'
-      -| a
- 
- In this case, the first field is "null" or empty.
- 
-    The stripping of leading and trailing whitespace also comes into
- play whenever `$0' is recomputed.  For instance, study this pipeline:
- 
-      $ echo '   a b c d' | awk '{ print; $2 = $2; print }'
-      -|    a b c d
-      -| a b c d
- 
- The first `print' statement prints the record as it was read, with
- leading whitespace intact.  The assignment to `$2' rebuilds `$0' by
- concatenating `$1' through `$NF' together, separated by the value of
- `OFS'.  Because the leading whitespace was ignored when finding `$1',
- it is not part of the new `$0'.  Finally, the last `print' statement
- prints the new `$0'.
- 
-    There is an additional subtlety to be aware of when using regular
- expressions for field splitting.  It is not well-specified in the POSIX
- standard, or anywhere else, what `^' means when splitting fields.  Does
- the `^'  match only at the beginning of the entire record? Or is each
- field separator a new string?  It turns out that different `awk'
- versions answer this question differently, and you should not rely on
- any specific behavior in your programs.  (d.c.)
- 
-    As a point of information, Brian Kernighan's `awk' allows `^' to
- match only at the beginning of the record. `gawk' also works this way.
- For example:
- 
-      $ echo 'xxAA  xxBxx  C' |
-      > gawk -F '(^x+)|( +)' '{ for (i = 1; i <= NF; i++)
-      >                                   printf "-->%s<--\n", $i }'
-      -| --><--
-      -| -->AA<--
-      -| -->xxBxx<--
-      -| -->C<--
- 
- 
- File: gawk.info,  Node: Single Character Fields,  Next: Command Line Field 
Separator,  Prev: Regexp Field Splitting,  Up: Field Separators
- 
- 4.5.3 Making Each Character a Separate Field
- --------------------------------------------
- 
- There are times when you may want to examine each character of a record
- separately.  This can be done in `gawk' by simply assigning the null
- string (`""') to `FS'. (c.e.)  In this case, each individual character
- in the record becomes a separate field.  For example:
- 
-      $ echo a b | gawk 'BEGIN { FS = "" }
-      >                  {
-      >                      for (i = 1; i <= NF; i = i + 1)
-      >                          print "Field", i, "is", $i
-      >                  }'
-      -| Field 1 is a
-      -| Field 2 is
-      -| Field 3 is b
- 
-    Traditionally, the behavior of `FS' equal to `""' was not defined.
- In this case, most versions of Unix `awk' simply treat the entire record
- as only having one field.  (d.c.)  In compatibility mode (*note
- Options::), if `FS' is the null string, then `gawk' also behaves this
- way.
- 
- 
- File: gawk.info,  Node: Command Line Field Separator,  Next: Field Splitting 
Summary,  Prev: Single Character Fields,  Up: Field Separators
- 
- 4.5.4 Setting `FS' from the Command Line
- ----------------------------------------
- 
- `FS' can be set on the command line.  Use the `-F' option to do so.
- For example:
- 
-      awk -F, 'PROGRAM' INPUT-FILES
- 
- sets `FS' to the `,' character.  Notice that the option uses an
- uppercase `F' instead of a lowercase `f'. The latter option (`-f')
- specifies a file containing an `awk' program.  Case is significant in
- command-line options: the `-F' and `-f' options have nothing to do with
- each other.  You can use both options at the same time to set the `FS'
- variable _and_ get an `awk' program from a file.
- 
-    The value used for the argument to `-F' is processed in exactly the
- same way as assignments to the built-in variable `FS'.  Any special
- characters in the field separator must be escaped appropriately.  For
- example, to use a `\' as the field separator on the command line, you
- would have to type:
- 
-      # same as FS = "\\"
-      awk -F\\\\ '...' files ...
- 
- Because `\' is used for quoting in the shell, `awk' sees `-F\\'.  Then
- `awk' processes the `\\' for escape characters (*note Escape
- Sequences::), finally yielding a single `\' to use for the field
- separator.
- 
-    As a special case, in compatibility mode (*note Options::), if the
- argument to `-F' is `t', then `FS' is set to the TAB character.  If you
- type `-F\t' at the shell, without any quotes, the `\' gets deleted, so
- `awk' figures that you really want your fields to be separated with
- TABs and not `t's.  Use `-v FS="t"' or `-F"[t]"' on the command line if
- you really do want to separate your fields with `t's.
- 
-    As an example, let's use an `awk' program file called `baud.awk'
- that contains the pattern `/300/' and the action `print $1':
- 
-      /300/   { print $1 }
- 
-    Let's also set `FS' to be the `-' character and run the program on
- the file `BBS-list'.  The following command prints a list of the names
- of the bulletin boards that operate at 300 baud and the first three
- digits of their phone numbers:
- 
-      $ awk -F- -f baud.awk BBS-list
-      -| aardvark     555
-      -| alpo
-      -| barfly       555
-      -| bites        555
-      -| camelot      555
-      -| core         555
-      -| fooey        555
-      -| foot         555
-      -| macfoo       555
-      -| sdace        555
-      -| sabafoo      555
- 
- Note the second line of output.  The second line in the original file
- looked like this:
- 
-      alpo-net     555-3412     2400/1200/300     A
- 
-    The `-' as part of the system's name was used as the field
- separator, instead of the `-' in the phone number that was originally
- intended.  This demonstrates why you have to be careful in choosing
- your field and record separators.
- 
-    Perhaps the most common use of a single character as the field
- separator occurs when processing the Unix system password file.  On
- many Unix systems, each user has a separate entry in the system password
- file, one line per user.  The information in these lines is separated
- by colons.  The first field is the user's login name and the second is
- the user's (encrypted or shadow) password.  A password file entry might
- look like this:
- 
-      arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/bash
- 
-    The following program searches the system password file and prints
- the entries for users who have no password:
- 
-      awk -F: '$2 == ""' /etc/passwd
- 
- 
- File: gawk.info,  Node: Field Splitting Summary,  Prev: Command Line Field 
Separator,  Up: Field Separators
- 
- 4.5.5 Field-Splitting Summary
- -----------------------------
- 
- It is important to remember that when you assign a string constant as
- the value of `FS', it undergoes normal `awk' string processing.  For
- example, with Unix `awk' and `gawk', the assignment `FS = "\.."'
- assigns the character string `".."' to `FS' (the backslash is
- stripped).  This creates a regexp meaning "fields are separated by
- occurrences of any two characters."  If instead you want fields to be
- separated by a literal period followed by any single character, use `FS
- = "\\.."'.
- 
-    The following table summarizes how fields are split, based on the
- value of `FS' (`==' means "is equal to"):
- 
- `FS == " "'
-      Fields are separated by runs of whitespace.  Leading and trailing
-      whitespace are ignored.  This is the default.
- 
- `FS == ANY OTHER SINGLE CHARACTER'
-      Fields are separated by each occurrence of the character.  Multiple
-      successive occurrences delimit empty fields, as do leading and
-      trailing occurrences.  The character can even be a regexp
-      metacharacter; it does not need to be escaped.
- 
- `FS == REGEXP'
-      Fields are separated by occurrences of characters that match
-      REGEXP.  Leading and trailing matches of REGEXP delimit empty
-      fields.
- 
- `FS == ""'
-      Each individual character in the record becomes a separate field.
-      (This is a `gawk' extension; it is not specified by the POSIX
-      standard.)
- 
- Advanced Notes: Changing `FS' Does Not Affect the Fields
- --------------------------------------------------------
- 
- According to the POSIX standard, `awk' is supposed to behave as if each
- record is split into fields at the time it is read.  In particular,
- this means that if you change the value of `FS' after a record is read,
- the value of the fields (i.e., how they were split) should reflect the
- old value of `FS', not the new one.
- 
-    However, many older implementations of `awk' do not work this way.
- Instead, they defer splitting the fields until a field is actually
- referenced.  The fields are split using the _current_ value of `FS'!
- (d.c.)  This behavior can be difficult to diagnose. The following
- example illustrates the difference between the two methods.  (The
- `sed'(1) command prints just the first line of `/etc/passwd'.)
- 
-      sed 1q /etc/passwd | awk '{ FS = ":" ; print $1 }'
- 
- which usually prints:
- 
-      root
- 
- on an incorrect implementation of `awk', while `gawk' prints something
- like:
- 
-      root:nSijPlPhZZwgE:0:0:Root:/:
- 
- Advanced Notes: `FS' and `IGNORECASE'
- -------------------------------------
- 
- The `IGNORECASE' variable (*note User-modified::) affects field
- splitting _only_ when the value of `FS' is a regexp.  It has no effect
- when `FS' is a single character, even if that character is a letter.
- Thus, in the following code:
- 
-      FS = "c"
-      IGNORECASE = 1
-      $0 = "aCa"
-      print $1
- 
- The output is `aCa'.  If you really want to split fields on an
- alphabetic character while ignoring case, use a regexp that will do it
- for you.  E.g., `FS = "[c]"'.  In this case, `IGNORECASE' will take
- effect.
- 
-    ---------- Footnotes ----------
- 
-    (1) The `sed' utility is a "stream editor."  Its behavior is also
- defined by the POSIX standard.
- 
- 
- File: gawk.info,  Node: Constant Size,  Next: Splitting By Content,  Prev: 
Field Separators,  Up: Reading Files
- 
- 4.6 Reading Fixed-Width Data
- ============================
- 
- (This minor node discusses an advanced feature of `awk'.  If you are a
- novice `awk' user, you might want to skip it on the first reading.)
- 
- `gawk' provides a facility for dealing with fixed-width fields with no
- distinctive field separator.  For example, data of this nature arises
- in the input for old Fortran programs where numbers are run together,
- or in the output of programs that did not anticipate the use of their
- output as input for other programs.
- 
-    An example of the latter is a table where all the columns are lined
- up by the use of a variable number of spaces and _empty fields are just
- spaces_.  Clearly, `awk''s normal field splitting based on `FS' does
- not work well in this case.  Although a portable `awk' program can use
- a series of `substr()' calls on `$0' (*note String Functions::), this
- is awkward and inefficient for a large number of fields.
- 
-    The splitting of an input record into fixed-width fields is
- specified by assigning a string containing space-separated numbers to
- the built-in variable `FIELDWIDTHS'.  Each number specifies the width
- of the field, _including_ columns between fields.  If you want to
- ignore the columns between fields, you can specify the width as a
- separate field that is subsequently ignored.  It is a fatal error to
- supply a field width that is not a positive number.  The following data
- is the output of the Unix `w' utility.  It is useful to illustrate the
- use of `FIELDWIDTHS':
- 
-       10:06pm  up 21 days, 14:04,  23 users
-      User     tty       login  idle   JCPU   PCPU  what
-      hzuo     ttyV0     8:58pm            9      5  vi p24.tex
-      hzang    ttyV3     6:37pm    50                -csh
-      eklye    ttyV5     9:53pm            7      1  em thes.tex
-      dportein ttyV6     8:17pm  1:47                -csh
-      gierd    ttyD3    10:00pm     1                elm
-      dave     ttyD4     9:47pm            4      4  w
-      brent    ttyp0    26Jun91  4:46  26:46   4:41  bash
-      dave     ttyq4    26Jun9115days     46     46  wnewmail
- 
-    The following program takes the above input, converts the idle time
- to number of seconds, and prints out the first two fields and the
- calculated idle time:
- 
-      NOTE: This program uses a number of `awk' features that haven't
-      been introduced yet.
- 
-      BEGIN  { FIELDWIDTHS = "9 6 10 6 7 7 35" }
-      NR > 2 {
-          idle = $4
-          sub(/^  */, "", idle)   # strip leading spaces
-          if (idle == "")
-              idle = 0
-          if (idle ~ /:/) {
-              split(idle, t, ":")
-              idle = t[1] * 60 + t[2]
-          }
-          if (idle ~ /days/)
-              idle *= 24 * 60 * 60
- 
-          print $1, $2, idle
-      }
- 
-    Running the program on the data produces the following results:
- 
-      hzuo      ttyV0  0
-      hzang     ttyV3  50
-      eklye     ttyV5  0
-      dportein  ttyV6  107
-      gierd     ttyD3  1
-      dave      ttyD4  0
-      brent     ttyp0  286
-      dave      ttyq4  1296000
- 
-    Another (possibly more practical) example of fixed-width input data
- is the input from a deck of balloting cards.  In some parts of the
- United States, voters mark their choices by punching holes in computer
- cards.  These cards are then processed to count the votes for any
- particular candidate or on any particular issue.  Because a voter may
- choose not to vote on some issue, any column on the card may be empty.
- An `awk' program for processing such data could use the `FIELDWIDTHS'
- feature to simplify reading the data.  (Of course, getting `gawk' to
- run on a system with card readers is another story!)
- 
-    Assigning a value to `FS' causes `gawk' to use `FS' for field
- splitting again.  Use `FS = FS' to make this happen, without having to
- know the current value of `FS'.  In order to tell which kind of field
- splitting is in effect, use `PROCINFO["FS"]' (*note Auto-set::).  The
- value is `"FS"' if regular field splitting is being used, or it is
- `"FIELDWIDTHS"' if fixed-width field splitting is being used:
- 
-      if (PROCINFO["FS"] == "FS")
-          REGULAR FIELD SPLITTING ...
-      else if  (PROCINFO["FS"] == "FIELDWIDTHS")
-          FIXED-WIDTH FIELD SPLITTING ...
-      else
-          CONTENT-BASED FIELD SPLITTING ... (see next minor node)
- 
-    This information is useful when writing a function that needs to
- temporarily change `FS' or `FIELDWIDTHS', read some records, and then
- restore the original settings (*note Passwd Functions::, for an example
- of such a function).
- 
- 
- File: gawk.info,  Node: Splitting By Content,  Next: Multiple Line,  Prev: 
Constant Size,  Up: Reading Files
- 
- 4.7 Defining Fields By Content
- ==============================
- 
- (This minor node discusses an advanced feature of `awk'.  If you are a
- novice `awk' user, you might want to skip it on the first reading.)
- 
- Normally, when using `FS', `gawk' defines the fields as the parts of
- the record that occur in between each field separator. In other words,
- `FS' defines what a field _is not_, instead of what a field _is_.
- However, there are times when you really want to define the fields by
- what they are, and not by what they are not.
- 
-    The most notorious such case is so-called "comma separated value"
- (CSV) data. Many spreadsheet programs, for example, can export their
- data into text files, where each record is terminated with a newline,
- and fields are separated by commas. If only commas separated the data,
- there wouldn't be an issue. The problem comes when one of the fields
- contains an _embedded_ comma. While there is no formal standard
- specification for CSV data(1), in such cases, most programs embed the
- field in double quotes. So we might have data like this:
- 
-      Robbins,Arnold,"1234 A Pretty Street, NE",MyTown,MyState,12345-6789,USA
- 
-    The `FPAT' variable offers a solution for cases like this.  The
- value of `FPAT' should be a string that provides a regular expression.
- This regular expression describes the contents of each field.
- 
-    In the case of CSV data as presented above, each field is either
- "anything that is not a comma," or "a double quote, anything that is
- not a double quote, and a closing double quote."  If written as a
- regular expression constant (*note Regexp::), we would have
- `/([^,]+)|("[^"]+")/'.  Writing this as a string requires us to escape
- the double quotes, leading to:
- 
-      FPAT = "([^,]+)|(\"[^\"]+\")"
- 
-    Putting this to use, here is a simple program to parse the data:
- 
-      BEGIN {
-          FPAT = "([^,]+)|(\"[^\"]+\")"
-      }
- 
-      {
-          print "NF = ", NF
-          for (i = 1; i <= NF; i++) {
-              printf("$%d = <%s>\n", i, $i)
-          }
-      }
- 
-    When run, we get the following:
- 
-      $ gawk -f simple-csv.awk addresses.csv
-      NF =  7
-      $1 = <Robbins>
-      $2 = <Arnold>
-      $3 = <"1234 A Pretty Street, NE">
-      $4 = <MyTown>
-      $5 = <MyState>
-      $6 = <12345-6789>
-      $7 = <USA>
- 
-    Note the embedded comma in the value of `$3'.
- 
-    A straightforward improvement when processing CSV data of this sort
- would be to remove the quotes when they occur, with something like this:
- 
-      if (substr($i, 1, 1) == "\"") {
-          len = length($i)
-          $i = substr($i, 2, len - 2)    # Get text within the two quotes
-      }
- 
-    As with `FS', the `IGNORECASE' variable (*note User-modified::)
- affects field splitting with `FPAT'.
- 
-    Similar to `FIELDWIDTHS', the value of `PROCINFO["FS"]' will be
- `"FPAT"' if content-based field splitting is being used.
- 
-      NOTE: Some programs export CSV data that contains embedded
-      newlines between the double quotes.  `gawk' provides no way to
-      deal with this.  Since there is no formal specification for CSV
-      data, there isn't much more to be done; the `FPAT' mechanism
-      provides an elegant solution for the majority of cases, and the
-      `gawk' maintainer is satisfied with that.
- 
-    As written, the regexp used for `FPAT' requires that each field have
- a least one character.  A straightforward modification (changing
- changed the first `+' to `*') allows fields to be empty:
- 
-      FPAT = "([^,]*)|(\"[^\"]+\")"
- 
-    Finally, the `patsplit()' function makes the same functionality
- available for splitting regular strings (*note String Functions::).
- 
-    ---------- Footnotes ----------
- 
-    (1) At least, we don't know of one.
- 
- 
- File: gawk.info,  Node: Multiple Line,  Next: Getline,  Prev: Splitting By 
Content,  Up: Reading Files
- 
- 4.8 Multiple-Line Records
- =========================
- 
- In some databases, a single line cannot conveniently hold all the
- information in one entry.  In such cases, you can use multiline
- records.  The first step in doing this is to choose your data format.
- 
-    One technique is to use an unusual character or string to separate
- records.  For example, you could use the formfeed character (written
- `\f' in `awk', as in C) to separate them, making each record a page of
- the file.  To do this, just set the variable `RS' to `"\f"' (a string
- containing the formfeed character).  Any other character could equally
- well be used, as long as it won't be part of the data in a record.
- 
-    Another technique is to have blank lines separate records.  By a
- special dispensation, an empty string as the value of `RS' indicates
- that records are separated by one or more blank lines.  When `RS' is set
- to the empty string, each record always ends at the first blank line
- encountered.  The next record doesn't start until the first nonblank
- line that follows.  No matter how many blank lines appear in a row, they
- all act as one record separator.  (Blank lines must be completely
- empty; lines that contain only whitespace do not count.)
- 
-    You can achieve the same effect as `RS = ""' by assigning the string
- `"\n\n+"' to `RS'. This regexp matches the newline at the end of the
- record and one or more blank lines after the record.  In addition, a
- regular expression always matches the longest possible sequence when
- there is a choice (*note Leftmost Longest::).  So the next record
- doesn't start until the first nonblank line that follows--no matter how
- many blank lines appear in a row, they are considered one record
- separator.
- 
-    There is an important difference between `RS = ""' and `RS =
- "\n\n+"'. In the first case, leading newlines in the input data file
- are ignored, and if a file ends without extra blank lines after the
- last record, the final newline is removed from the record.  In the
- second case, this special processing is not done.  (d.c.)
- 
-    Now that the input is separated into records, the second step is to
- separate the fields in the record.  One way to do this is to divide each
- of the lines into fields in the normal manner.  This happens by default
- as the result of a special feature.  When `RS' is set to the empty
- string, _and_ `FS' is set to a single character, the newline character
- _always_ acts as a field separator.  This is in addition to whatever
- field separations result from `FS'.(1)
- 
-    The original motivation for this special exception was probably to
- provide useful behavior in the default case (i.e., `FS' is equal to
- `" "').  This feature can be a problem if you really don't want the
- newline character to separate fields, because there is no way to
- prevent it.  However, you can work around this by using the `split()'
- function to break up the record manually (*note String Functions::).
- If you have a single character field separator, you can work around the
- special feature in a different way, by making `FS' into a regexp for
- that single character.  For example, if the field separator is a
- percent character, instead of `FS = "%"', use `FS = "[%]"'.
- 
-    Another way to separate fields is to put each field on a separate
- line: to do this, just set the variable `FS' to the string `"\n"'.
- (This single character separator matches a single newline.)  A
- practical example of a data file organized this way might be a mailing
- list, where each entry is separated by blank lines.  Consider a mailing
- list in a file named `addresses', which looks like this:
- 
-      Jane Doe
-      123 Main Street
-      Anywhere, SE 12345-6789
- 
-      John Smith
-      456 Tree-lined Avenue
-      Smallville, MW 98765-4321
-      ...
- 
- A simple program to process this file is as follows:
- 
-      # addrs.awk --- simple mailing list program
- 
-      # Records are separated by blank lines.
-      # Each line is one field.
-      BEGIN { RS = "" ; FS = "\n" }
- 
-      {
-            print "Name is:", $1
-            print "Address is:", $2
-            print "City and State are:", $3
-            print ""
-      }
- 
-    Running the program produces the following output:
- 
-      $ awk -f addrs.awk addresses
-      -| Name is: Jane Doe
-      -| Address is: 123 Main Street
-      -| City and State are: Anywhere, SE 12345-6789
-      -|
-      -| Name is: John Smith
-      -| Address is: 456 Tree-lined Avenue
-      -| City and State are: Smallville, MW 98765-4321
-      -|
-      ...
- 
-    *Note Labels Program::, for a more realistic program that deals with
- address lists.  The following table summarizes how records are split,
- based on the value of `RS'.  (`==' means "is equal to.")
- 
- `RS == "\n"'
-      Records are separated by the newline character (`\n').  In effect,
-      every line in the data file is a separate record, including blank
-      lines.  This is the default.
- 
- `RS == ANY SINGLE CHARACTER'
-      Records are separated by each occurrence of the character.
-      Multiple successive occurrences delimit empty records.
- 
- `RS == ""'
-      Records are separated by runs of blank lines.  When `FS' is a
-      single character, then the newline character always serves as a
-      field separator, in addition to whatever value `FS' may have.
-      Leading and trailing newlines in a file are ignored.
- 
- `RS == REGEXP'
-      Records are separated by occurrences of characters that match
-      REGEXP.  Leading and trailing matches of REGEXP delimit empty
-      records.  (This is a `gawk' extension; it is not specified by the
-      POSIX standard.)
- 
-    In all cases, `gawk' sets `RT' to the input text that matched the
- value specified by `RS'.  But if the input file ended without any text
- that matches `RS', then `gawk' sets `RT' to the null string.
- 
-    ---------- Footnotes ----------
- 
-    (1) When `FS' is the null string (`""') or a regexp, this special
- feature of `RS' does not apply.  It does apply to the default field
- separator of a single space: `FS = " "'.
- 
- 
- File: gawk.info,  Node: Getline,  Next: Read Timeout,  Prev: Multiple Line,  
Up: Reading Files
- 
- 4.9 Explicit Input with `getline'
- =================================
- 
- So far we have been getting our input data from `awk''s main input
- stream--either the standard input (usually your terminal, sometimes the
- output from another program) or from the files specified on the command
- line.  The `awk' language has a special built-in command called
- `getline' that can be used to read input under your explicit control.
- 
-    The `getline' command is used in several different ways and should
- _not_ be used by beginners.  The examples that follow the explanation
- of the `getline' command include material that has not been covered
- yet.  Therefore, come back and study the `getline' command _after_ you
- have reviewed the rest of this Info file and have a good knowledge of
- how `awk' works.
- 
-    The `getline' command returns one if it finds a record and zero if
- it encounters the end of the file.  If there is some error in getting a
- record, such as a file that cannot be opened, then `getline' returns
- -1.  In this case, `gawk' sets the variable `ERRNO' to a string
- describing the error that occurred.
- 
-    In the following examples, COMMAND stands for a string value that
- represents a shell command.
- 
-      NOTE: When `--sandbox' is specified (*note Options::), reading
-      lines from files, pipes and coprocesses is disabled.
- 
- * Menu:
- 
- * Plain Getline::               Using `getline' with no arguments.
- * Getline/Variable::            Using `getline' into a variable.
- * Getline/File::                Using `getline' from a file.
- * Getline/Variable/File::       Using `getline' into a variable from a
-                                 file.
- * Getline/Pipe::                Using `getline' from a pipe.
- * Getline/Variable/Pipe::       Using `getline' into a variable from a
-                                 pipe.
- * Getline/Coprocess::           Using `getline' from a coprocess.
- * Getline/Variable/Coprocess::  Using `getline' into a variable from a
-                                 coprocess.
- * Getline Notes::               Important things to know about `getline'.
- * Getline Summary::             Summary of `getline' Variants.
- 
- 
- File: gawk.info,  Node: Plain Getline,  Next: Getline/Variable,  Up: Getline
- 
- 4.9.1 Using `getline' with No Arguments
- ---------------------------------------
- 
- The `getline' command can be used without arguments to read input from
- the current input file.  All it does in this case is read the next
- input record and split it up into fields.  This is useful if you've
- finished processing the current record, but want to do some special
- processing on the next record _right now_.  For example:
- 
-      {
-           if ((t = index($0, "/*")) != 0) {
-                # value of `tmp' will be "" if t is 1
-                tmp = substr($0, 1, t - 1)
-                u = index(substr($0, t + 2), "*/")
-                offset = t + 2
-                while (u == 0) {
-                     if (getline <= 0) {
-                          m = "unexpected EOF or error"
-                          m = (m ": " ERRNO)
-                          print m > "/dev/stderr"
-                          exit
-                     }
-                     u = index($0, "*/")
-                     offset = 0
-                }
-                # substr() expression will be "" if */
-                # occurred at end of line
-                $0 = tmp substr($0, offset + u + 2)
-           }
-           print $0
-      }
- 
-    This `awk' program deletes C-style comments (`/* ...  */') from the
- input.  By replacing the `print $0' with other statements, you could
- perform more complicated processing on the decommented input, such as
- searching for matches of a regular expression.  (This program has a
- subtle problem--it does not work if one comment ends and another begins
- on the same line.)
- 
-    This form of the `getline' command sets `NF', `NR', `FNR', and the
- value of `$0'.
- 
-      NOTE: The new value of `$0' is used to test the patterns of any
-      subsequent rules.  The original value of `$0' that triggered the
-      rule that executed `getline' is lost.  By contrast, the `next'
-      statement reads a new record but immediately begins processing it
-      normally, starting with the first rule in the program.  *Note Next
-      Statement::.
- 
- 
- File: gawk.info,  Node: Getline/Variable,  Next: Getline/File,  Prev: Plain 
Getline,  Up: Getline
- 
- 4.9.2 Using `getline' into a Variable
- -------------------------------------
- 
- You can use `getline VAR' to read the next record from `awk''s input
- into the variable VAR.  No other processing is done.  For example,
- suppose the next line is a comment or a special string, and you want to
- read it without triggering any rules.  This form of `getline' allows
- you to read that line and store it in a variable so that the main
- read-a-line-and-check-each-rule loop of `awk' never sees it.  The
- following example swaps every two lines of input:
- 
-      {
-           if ((getline tmp) > 0) {
-                print tmp
-                print $0
-           } else
-                print $0
-      }
- 
- It takes the following list:
- 
-      wan
-      tew
-      free
-      phore
- 
- and produces these results:
- 
-      tew
-      wan
-      phore
-      free
- 
-    The `getline' command used in this way sets only the variables `NR'
- and `FNR' (and of course, VAR).  The record is not split into fields,
- so the values of the fields (including `$0') and the value of `NF' do
- not change.
- 
- 
- File: gawk.info,  Node: Getline/File,  Next: Getline/Variable/File,  Prev: 
Getline/Variable,  Up: Getline
- 
- 4.9.3 Using `getline' from a File
- ---------------------------------
- 
- Use `getline < FILE' to read the next record from FILE.  Here FILE is a
- string-valued expression that specifies the file name.  `< FILE' is
- called a "redirection" because it directs input to come from a
- different place.  For example, the following program reads its input
- record from the file `secondary.input' when it encounters a first field
- with a value equal to 10 in the current input file:
- 
-      {
-          if ($1 == 10) {
-               getline < "secondary.input"
-               print
-          } else
-               print
-      }
- 
-    Because the main input stream is not used, the values of `NR' and
- `FNR' are not changed. However, the record it reads is split into
- fields in the normal manner, so the values of `$0' and the other fields
- are changed, resulting in a new value of `NF'.
- 
-    According to POSIX, `getline < EXPRESSION' is ambiguous if
- EXPRESSION contains unparenthesized operators other than `$'; for
- example, `getline < dir "/" file' is ambiguous because the
- concatenation operator is not parenthesized.  You should write it as
- `getline < (dir "/" file)' if you want your program to be portable to
- all `awk' implementations.
- 
- 
- File: gawk.info,  Node: Getline/Variable/File,  Next: Getline/Pipe,  Prev: 
Getline/File,  Up: Getline
- 
- 4.9.4 Using `getline' into a Variable from a File
- -------------------------------------------------
- 
- Use `getline VAR < FILE' to read input from the file FILE, and put it
- in the variable VAR.  As above, FILE is a string-valued expression that
- specifies the file from which to read.
- 
-    In this version of `getline', none of the built-in variables are
- changed and the record is not split into fields.  The only variable
- changed is VAR.(1) For example, the following program copies all the
- input files to the output, except for records that say
- address@hidden FILENAME'.  Such a record is replaced by the contents of the
- file FILENAME:
- 
-      {
-           if (NF == 2 && $1 == "@include") {
-                while ((getline line < $2) > 0)
-                     print line
-                close($2)
-           } else
-                print
-      }
- 
-    Note here how the name of the extra input file is not built into the
- program; it is taken directly from the data, specifically from the
- second field on the address@hidden' line.
- 
-    The `close()' function is called to ensure that if two identical
- address@hidden' lines appear in the input, the entire specified file is
- included twice.  *Note Close Files And Pipes::.
- 
-    One deficiency of this program is that it does not process nested
- address@hidden' statements (i.e., address@hidden' statements in included 
files)
- the way a true macro preprocessor would.  *Note Igawk Program::, for a
- program that does handle nested address@hidden' statements.
- 
-    ---------- Footnotes ----------
- 
-    (1) This is not quite true. `RT' could be changed if `RS' is a
- regular expression.
- 
- 
- File: gawk.info,  Node: Getline/Pipe,  Next: Getline/Variable/Pipe,  Prev: 
Getline/Variable/File,  Up: Getline
- 
- 4.9.5 Using `getline' from a Pipe
- ---------------------------------
- 
- The output of a command can also be piped into `getline', using
- `COMMAND | getline'.  In this case, the string COMMAND is run as a
- shell command and its output is piped into `awk' to be used as input.
- This form of `getline' reads one record at a time from the pipe.  For
- example, the following program copies its input to its output, except
- for lines that begin with address@hidden', which are replaced by the output
- produced by running the rest of the line as a shell command:
- 
-      {
-           if ($1 == "@execute") {
-                tmp = substr($0, 10)        # Remove "@execute"
-                while ((tmp | getline) > 0)
-                     print
-                close(tmp)
-           } else
-                print
-      }
- 
- The `close()' function is called to ensure that if two identical
- address@hidden' lines appear in the input, the command is run for each one.
- *Note Close Files And Pipes::.  Given the input:
- 
-      foo
-      bar
-      baz
-      @execute who
-      bletch
- 
- the program might produce:
- 
-      foo
-      bar
-      baz
-      arnold     ttyv0   Jul 13 14:22
-      miriam     ttyp0   Jul 13 14:23     (murphy:0)
-      bill       ttyp1   Jul 13 14:23     (murphy:0)
-      bletch
- 
- Notice that this program ran the command `who' and printed the previous
- result.  (If you try this program yourself, you will of course get
- different results, depending upon who is logged in on your system.)
- 
-    This variation of `getline' splits the record into fields, sets the
- value of `NF', and recomputes the value of `$0'.  The values of `NR'
- and `FNR' are not changed.
- 
-    According to POSIX, `EXPRESSION | getline' is ambiguous if
- EXPRESSION contains unparenthesized operators other than `$'--for
- example, `"echo " "date" | getline' is ambiguous because the
- concatenation operator is not parenthesized.  You should write it as
- `("echo " "date") | getline' if you want your program to be portable to
- all `awk' implementations.
- 
-      NOTE: Unfortunately, `gawk' has not been consistent in its
-      treatment of a construct like `"echo " "date" | getline'.  Most
-      versions, including the current version, treat it at as `("echo "
-      "date") | getline'.  (This how Brian Kernighan's `awk' behaves.)
-      Some versions changed and treated it as `"echo " ("date" |
-      getline)'.  (This is how `mawk' behaves.)  In short, _always_ use
-      explicit parentheses, and then you won't have to worry.
- 
- 
- File: gawk.info,  Node: Getline/Variable/Pipe,  Next: Getline/Coprocess,  
Prev: Getline/Pipe,  Up: Getline
- 
- 4.9.6 Using `getline' into a Variable from a Pipe
- -------------------------------------------------
- 
- When you use `COMMAND | getline VAR', the output of COMMAND is sent
- through a pipe to `getline' and into the variable VAR.  For example, the
- following program reads the current date and time into the variable
- `current_time', using the `date' utility, and then prints it:
- 
-      BEGIN {
-           "date" | getline current_time
-           close("date")
-           print "Report printed on " current_time
-      }
- 
-    In this version of `getline', none of the built-in variables are
- changed and the record is not split into fields.
- 
-    According to POSIX, `EXPRESSION | getline VAR' is ambiguous if
- EXPRESSION contains unparenthesized operators other than `$'; for
- example, `"echo " "date" | getline VAR' is ambiguous because the
- concatenation operator is not parenthesized. You should write it as
- `("echo " "date") | getline VAR' if you want your program to be
- portable to other `awk' implementations.
- 
- 
- File: gawk.info,  Node: Getline/Coprocess,  Next: Getline/Variable/Coprocess, 
 Prev: Getline/Variable/Pipe,  Up: Getline
- 
- 4.9.7 Using `getline' from a Coprocess
- --------------------------------------
- 
- Input into `getline' from a pipe is a one-way operation.  The command
- that is started with `COMMAND | getline' only sends data _to_ your
- `awk' program.
- 
-    On occasion, you might want to send data to another program for
- processing and then read the results back.  `gawk' allows you to start
- a "coprocess", with which two-way communications are possible.  This is
- done with the `|&' operator.  Typically, you write data to the
- coprocess first and then read results back, as shown in the following:
- 
-      print "SOME QUERY" |& "db_server"
-      "db_server" |& getline
- 
- which sends a query to `db_server' and then reads the results.
- 
-    The values of `NR' and `FNR' are not changed, because the main input
- stream is not used.  However, the record is split into fields in the
- normal manner, thus changing the values of `$0', of the other fields,
- and of `NF'.
- 
-    Coprocesses are an advanced feature. They are discussed here only
- because this is the minor node on `getline'.  *Note Two-way I/O::,
- where coprocesses are discussed in more detail.
- 
- 
- File: gawk.info,  Node: Getline/Variable/Coprocess,  Next: Getline Notes,  
Prev: Getline/Coprocess,  Up: Getline
- 
- 4.9.8 Using `getline' into a Variable from a Coprocess
- ------------------------------------------------------
- 
- When you use `COMMAND |& getline VAR', the output from the coprocess
- COMMAND is sent through a two-way pipe to `getline' and into the
- variable VAR.
- 
-    In this version of `getline', none of the built-in variables are
- changed and the record is not split into fields.  The only variable
- changed is VAR.
- 
-    Coprocesses are an advanced feature. They are discussed here only
- because this is the minor node on `getline'.  *Note Two-way I/O::,
- where coprocesses are discussed in more detail.
- 
- 
- File: gawk.info,  Node: Getline Notes,  Next: Getline Summary,  Prev: 
Getline/Variable/Coprocess,  Up: Getline
- 
- 4.9.9 Points to Remember About `getline'
- ----------------------------------------
- 
- Here are some miscellaneous points about `getline' that you should bear
- in mind:
- 
-    * When `getline' changes the value of `$0' and `NF', `awk' does
-      _not_ automatically jump to the start of the program and start
-      testing the new record against every pattern.  However, the new
-      record is tested against any subsequent rules.
- 
-    * Many `awk' implementations limit the number of pipelines that an
-      `awk' program may have open to just one.  In `gawk', there is no
-      such limit.  You can open as many pipelines (and coprocesses) as
-      the underlying operating system permits.
- 
-    * An interesting side effect occurs if you use `getline' without a
-      redirection inside a `BEGIN' rule. Because an unredirected
-      `getline' reads from the command-line data files, the first
-      `getline' command causes `awk' to set the value of `FILENAME'.
-      Normally, `FILENAME' does not have a value inside `BEGIN' rules,
-      because you have not yet started to process the command-line data
-      files.  (d.c.)  (*Note BEGIN/END::, also *note Auto-set::.)
- 
-    * Using `FILENAME' with `getline' (`getline < FILENAME') is likely
-      to be a source for confusion.  `awk' opens a separate input stream
-      from the current input file.  However, by not using a variable,
-      `$0' and `NR' are still updated.  If you're doing this, it's
-      probably by accident, and you should reconsider what it is you're
-      trying to accomplish.
- 
-    * *note Getline Summary::, presents a table summarizing the
-      `getline' variants and which variables they can affect.  It is
-      worth noting that those variants which do not use redirection can
-      cause `FILENAME' to be updated if they cause `awk' to start
-      reading a new input file.
- 
- 
- File: gawk.info,  Node: Getline Summary,  Prev: Getline Notes,  Up: Getline
- 
- 4.9.10 Summary of `getline' Variants
- ------------------------------------
- 
- *note table-getline-variants:: summarizes the eight variants of
- `getline', listing which built-in variables are set by each one, and
- whether the variant is standard or a `gawk' extension.
- 
- Variant                  Effect                      Standard /
-                                                      Extension
- ------------------------------------------------------------------------- 
- `getline'                Sets `$0', `NF', `FNR',     Standard
-                          and `NR'                    
- `getline' VAR            Sets VAR, `FNR', and `NR'   Standard
- `getline <' FILE         Sets `$0' and `NF'          Standard
- `getline VAR < FILE'     Sets VAR                    Standard
- COMMAND `| getline'      Sets `$0' and `NF'          Standard
- COMMAND `| getline' VAR  Sets VAR                    Standard
- COMMAND `|& getline'     Sets `$0' and `NF'          Extension
- COMMAND `|& getline'     Sets VAR                    Extension
- VAR                                                  
- 
- Table 4.1: getline Variants and What They Set
- 
- 
- File: gawk.info,  Node: Read Timeout,  Next: Command line directories,  Prev: 
Getline,  Up: Reading Files
- 
- 4.10 Reading Input With A Timeout
- =================================
- 
- You may specify a timeout in milliseconds for reading input from a
- terminal, pipe or two-way communication including, TCP/IP sockets. This
- can be done on a per input, command or connection basis, by setting a
- special element in the `PROCINFO' array:
- 
-      PROCINFO["input_name", "READ_TIMEOUT"] = TIMEOUT IN MILLISECONDS
- 
-    When set, this will cause `gawk' to time out and return failure if
- no data is available to read within the specified timeout period.  For
- example, a TCP client can decide to give up on receiving any response
- from the server after a certain amount of time:
- 
-      Service = "/inet/tcp/0/localhost/daytime"
-      PROCINFO[Service, "READ_TIMEOUT"] = 100
-      if ((Service |& getline) > 0)
-          print $0
-      else if (ERRNO != "")
-          print ERRNO
- 
-    Here is how to read interactively from the terminal(1) without
- waiting for more than five seconds:
- 
-      PROCINFO["/dev/stdin", "READ_TIMEOUT"] = 5000
-      while ((getline < "/dev/stdin") > 0)
-          print $0
- 
-    `gawk' will terminate the read operation if input does not arrive
- after waiting for the timeout period, return failure and set the
- `ERRNO' variable to an appropriate string value.  A negative or zero
- value for the timeout is the same as specifying no timeout at all.
- 
-    A timeout can also be set for reading from the terminal in the
- implicit loop that reads input records and matches them against
- patterns, like so:
- 
-      $  gawk 'BEGIN { PROCINFO["-", "READ_TIMEOUT"] = 5000 }
-      > { print "You entered: " $0 }'
-      gawk
-      -| You entered: gawk
- 
-    In this case, failure to respond within five seconds results in the
- following error message:
- 
-      error--> gawk: cmd. line:2: (FILENAME=- FNR=1) fatal: error reading 
input file `-': Connection timed out
- 
-    The timeout can be set or changed at any time, and will take effect
- on the next attempt to read from the input device. In the following
- example, we start with a timeout value of one second, and progressively
- reduce it by one-tenth of a second until we wait indefinitely for the
- input to arrive:
- 
-      PROCINFO[Service, "READ_TIMEOUT"] = 1000
-      while ((Service |& getline) > 0) {
-          print $0
-          PROCINFO[S, "READ_TIMEOUT"] -= 100
-      }
- 
-      NOTE: You should not assume that the read operation will block
-      exactly after the tenth record has been printed. It is possible
-      that `gawk' will read and buffer more than one record's worth of
-      data the first time. Because of this, changing the value of
-      timeout like in the above example is not very useful.
- 
-    If the `PROCINFO' element is not present and the environment
- variable `GAWK_READ_TIMEOUT' exists, `gawk' uses its value to
- initialize the timeout value.  The exclusive use of the environment
- variable to specify timeout has the disadvantage of not being able to
- control it on a per command or connection basis.
- 
-    `gawk' considers a timeout event to be an error even though the
- attempt to read from the underlying device may succeed in a later
- attempt. This is a limitation, and it also means that you cannot use
- this to multiplex input from two or more sources.
- 
-    Assigning a timeout value prevents read operations from blocking
- indefinitely. But bear in mind that there are other ways `gawk' can
- stall waiting for an input device to be ready.  A network client can
- sometimes take a long time to establish a connection before it can
- start reading any data, or the attempt to open a FIFO special file for
- reading can block indefinitely until some other process opens it for
- writing.
- 
-    ---------- Footnotes ----------
- 
-    (1) This assumes that standard input is the keyboard
- 
- 
- File: gawk.info,  Node: Command line directories,  Prev: Read Timeout,  Up: 
Reading Files
- 
- 4.11 Directories On The Command Line
- ====================================
- 
- According to the POSIX standard, files named on the `awk' command line
- must be text files.  It is a fatal error if they are not.  Most
- versions of `awk' treat a directory on the command line as a fatal
- error.
- 
-    By default, `gawk' produces a warning for a directory on the command
- line, but otherwise ignores it.  If either of the `--posix' or
- `--traditional' options is given, then `gawk' reverts to treating a
- directory on the command line as a fatal error.
- 
- 
- File: gawk.info,  Node: Printing,  Next: Expressions,  Prev: Reading Files,  
Up: Top
- 
- 5 Printing Output
- *****************
- 
- One of the most common programming actions is to "print", or output,
- some or all of the input.  Use the `print' statement for simple output,
- and the `printf' statement for fancier formatting.  The `print'
- statement is not limited when computing _which_ values to print.
- However, with two exceptions, you cannot specify _how_ to print
- them--how many columns, whether to use exponential notation or not, and
- so on.  (For the exceptions, *note Output Separators::, and *note
- OFMT::.)  For printing with specifications, you need the `printf'
- statement (*note Printf::).
- 
-    Besides basic and formatted printing, this major node also covers
- I/O redirections to files and pipes, introduces the special file names
- that `gawk' processes internally, and discusses the `close()' built-in
- function.
- 
- * Menu:
- 
- * Print::                       The `print' statement.
- * Print Examples::              Simple examples of `print' statements.
- * Output Separators::           The output separators and how to change them.
- * OFMT::                        Controlling Numeric Output With `print'.
- * Printf::                      The `printf' statement.
- * Redirection::                 How to redirect output to multiple files and
-                                 pipes.
- * Special Files::               File name interpretation in `gawk'.
-                                 `gawk' allows access to inherited file
-                                 descriptors.
- * Close Files And Pipes::       Closing Input and Output Files and Pipes.
- 
- 
- File: gawk.info,  Node: Print,  Next: Print Examples,  Up: Printing
- 
- 5.1 The `print' Statement
- =========================
- 
- The `print' statement is used for producing output with simple,
- standardized formatting.  Specify only the strings or numbers to print,
- in a list separated by commas.  They are output, separated by single
- spaces, followed by a newline.  The statement looks like this:
- 
-      print ITEM1, ITEM2, ...
- 
- The entire list of items may be optionally enclosed in parentheses.  The
- parentheses are necessary if any of the item expressions uses the `>'
- relational operator; otherwise it could be confused with an output
- redirection (*note Redirection::).
- 
-    The items to print can be constant strings or numbers, fields of the
- current record (such as `$1'), variables, or any `awk' expression.
- Numeric values are converted to strings and then printed.
- 
-    The simple statement `print' with no items is equivalent to `print
- $0': it prints the entire current record.  To print a blank line, use
- `print ""', where `""' is the empty string.  To print a fixed piece of
- text, use a string constant, such as `"Don't Panic"', as one item.  If
- you forget to use the double-quote characters, your text is taken as an
- `awk' expression, and you will probably get an error.  Keep in mind
- that a space is printed between any two items.
- 
- 
- File: gawk.info,  Node: Print Examples,  Next: Output Separators,  Prev: 
Print,  Up: Printing
- 
- 5.2 `print' Statement Examples
- ==============================
- 
- Each `print' statement makes at least one line of output.  However, it
- isn't limited to only one line.  If an item value is a string
- containing a newline, the newline is output along with the rest of the
- string.  A single `print' statement can make any number of lines this
- way.
- 
-    The following is an example of printing a string that contains
- embedded newlines (the `\n' is an escape sequence, used to represent
- the newline character; *note Escape Sequences::):
- 
-      $ awk 'BEGIN { print "line one\nline two\nline three" }'
-      -| line one
-      -| line two
-      -| line three
- 
-    The next example, which is run on the `inventory-shipped' file,
- prints the first two fields of each input record, with a space between
- them:
- 
-      $ awk '{ print $1, $2 }' inventory-shipped
-      -| Jan 13
-      -| Feb 15
-      -| Mar 15
-      ...
- 
-    A common mistake in using the `print' statement is to omit the comma
- between two items.  This often has the effect of making the items run
- together in the output, with no space.  The reason for this is that
- juxtaposing two string expressions in `awk' means to concatenate them.
- Here is the same program, without the comma:
- 
-      $ awk '{ print $1 $2 }' inventory-shipped
-      -| Jan13
-      -| Feb15
-      -| Mar15
-      ...
- 
-    To someone unfamiliar with the `inventory-shipped' file, neither
- example's output makes much sense.  A heading line at the beginning
- would make it clearer.  Let's add some headings to our table of months
- (`$1') and green crates shipped (`$2').  We do this using the `BEGIN'
- pattern (*note BEGIN/END::) so that the headings are only printed once:
- 
-      awk 'BEGIN {  print "Month Crates"
-                    print "----- ------" }
-                 {  print $1, $2 }' inventory-shipped
- 
- When run, the program prints the following:
- 
-      Month Crates
-      ----- ------
-      Jan 13
-      Feb 15
-      Mar 15
-      ...
- 
- The only problem, however, is that the headings and the table data
- don't line up!  We can fix this by printing some spaces between the two
- fields:
- 
-      awk 'BEGIN { print "Month Crates"
-                   print "----- ------" }
-                 { print $1, "     ", $2 }' inventory-shipped
- 
-    Lining up columns this way can get pretty complicated when there are
- many columns to fix.  Counting spaces for two or three columns is
- simple, but any more than this can take up a lot of time. This is why
- the `printf' statement was created (*note Printf::); one of its
- specialties is lining up columns of data.
- 
-      NOTE: You can continue either a `print' or `printf' statement
-      simply by putting a newline after any comma (*note
-      Statements/Lines::).
- 
- 
- File: gawk.info,  Node: Output Separators,  Next: OFMT,  Prev: Print 
Examples,  Up: Printing
- 
- 5.3 Output Separators
- =====================
- 
- As mentioned previously, a `print' statement contains a list of items
- separated by commas.  In the output, the items are normally separated
- by single spaces.  However, this doesn't need to be the case; a single
- space is simply the default.  Any string of characters may be used as
- the "output field separator" by setting the built-in variable `OFS'.
- The initial value of this variable is the string `" "'--that is, a
- single space.
- 
-    The output from an entire `print' statement is called an "output
- record".  Each `print' statement outputs one output record, and then
- outputs a string called the "output record separator" (or `ORS').  The
- initial value of `ORS' is the string `"\n"'; i.e., a newline character.
- Thus, each `print' statement normally makes a separate line.
- 
-    In order to change how output fields and records are separated,
- assign new values to the variables `OFS' and `ORS'.  The usual place to
- do this is in the `BEGIN' rule (*note BEGIN/END::), so that it happens
- before any input is processed.  It can also be done with assignments on
- the command line, before the names of the input files, or using the
- `-v' command-line option (*note Options::).  The following example
- prints the first and second fields of each input record, separated by a
- semicolon, with a blank line added after each newline:
- 
-      $ awk 'BEGIN { OFS = ";"; ORS = "\n\n" }
-      >            { print $1, $2 }' BBS-list
-      -| aardvark;555-5553
-      -|
-      -| alpo-net;555-3412
-      -|
-      -| barfly;555-7685
-      ...
- 
-    If the value of `ORS' does not contain a newline, the program's
- output runs together on a single line.
- 
- 
- File: gawk.info,  Node: OFMT,  Next: Printf,  Prev: Output Separators,  Up: 
Printing
- 
- 5.4 Controlling Numeric Output with `print'
- ===========================================
- 
- When printing numeric values with the `print' statement, `awk'
- internally converts the number to a string of characters and prints
- that string.  `awk' uses the `sprintf()' function to do this conversion
- (*note String Functions::).  For now, it suffices to say that the
- `sprintf()' function accepts a "format specification" that tells it how
- to format numbers (or strings), and that there are a number of
- different ways in which numbers can be formatted.  The different format
- specifications are discussed more fully in *note Control Letters::.
- 
-    The built-in variable `OFMT' contains the default format
- specification that `print' uses with `sprintf()' when it wants to
- convert a number to a string for printing.  The default value of `OFMT'
- is `"%.6g"'.  The way `print' prints numbers can be changed by
- supplying different format specifications as the value of `OFMT', as
- shown in the following example:
- 
-      $ awk 'BEGIN {
-      >   OFMT = "%.0f"  # print numbers as integers (rounds)
-      >   print 17.23, 17.54 }'
-      -| 17 18
- 
- According to the POSIX standard, `awk''s behavior is undefined if
- `OFMT' contains anything but a floating-point conversion specification.
- (d.c.)
- 
- 
- File: gawk.info,  Node: Printf,  Next: Redirection,  Prev: OFMT,  Up: Printing
- 
- 5.5 Using `printf' Statements for Fancier Printing
- ==================================================
- 
- For more precise control over the output format than what is provided
- by `print', use `printf'.  With `printf' you can specify the width to
- use for each item, as well as various formatting choices for numbers
- (such as what output base to use, whether to print an exponent, whether
- to print a sign, and how many digits to print after the decimal point).
- You do this by supplying a string, called the "format string", that
- controls how and where to print the other arguments.
- 
- * Menu:
- 
- * Basic Printf::                Syntax of the `printf' statement.
- * Control Letters::             Format-control letters.
- * Format Modifiers::            Format-specification modifiers.
- * Printf Examples::             Several examples.
- 
- 
- File: gawk.info,  Node: Basic Printf,  Next: Control Letters,  Up: Printf
- 
- 5.5.1 Introduction to the `printf' Statement
- --------------------------------------------
- 
- A simple `printf' statement looks like this:
- 
-      printf FORMAT, ITEM1, ITEM2, ...
- 
- The entire list of arguments may optionally be enclosed in parentheses.
- The parentheses are necessary if any of the item expressions use the `>'
- relational operator; otherwise, it can be confused with an output
- redirection (*note Redirection::).
- 
-    The difference between `printf' and `print' is the FORMAT argument.
- This is an expression whose value is taken as a string; it specifies
- how to output each of the other arguments.  It is called the "format
- string".
- 
-    The format string is very similar to that in the ISO C library
- function `printf()'.  Most of FORMAT is text to output verbatim.
- Scattered among this text are "format specifiers"--one per item.  Each
- format specifier says to output the next item in the argument list at
- that place in the format.
- 
-    The `printf' statement does not automatically append a newline to
- its output.  It outputs only what the format string specifies.  So if a
- newline is needed, you must include one in the format string.  The
- output separator variables `OFS' and `ORS' have no effect on `printf'
- statements. For example:
- 
-      $ awk 'BEGIN {
-      >    ORS = "\nOUCH!\n"; OFS = "+"
-      >    msg = "Dont Panic!"
-      >    printf "%s\n", msg
-      > }'
-      -| Dont Panic!
- 
- Here, neither the `+' nor the `OUCH' appear in the output message.
- 
- 
- File: gawk.info,  Node: Control Letters,  Next: Format Modifiers,  Prev: 
Basic Printf,  Up: Printf
- 
- 5.5.2 Format-Control Letters
- ----------------------------
- 
- A format specifier starts with the character `%' and ends with a
- "format-control letter"--it tells the `printf' statement how to output
- one item.  The format-control letter specifies what _kind_ of value to
- print.  The rest of the format specifier is made up of optional
- "modifiers" that control _how_ to print the value, such as the field
- width.  Here is a list of the format-control letters:
- 
- `%c'
-      Print a number as an ASCII character; thus, `printf "%c", 65'
-      outputs the letter `A'. The output for a string value is the first
-      character of the string.
- 
-           NOTE: The POSIX standard says the first character of a string
-           is printed.  In locales with multibyte characters, `gawk'
-           attempts to convert the leading bytes of the string into a
-           valid wide character and then to print the multibyte encoding
-           of that character.  Similarly, when printing a numeric value,
-           `gawk' allows the value to be within the numeric range of
-           values that can be held in a wide character.
- 
-           Other `awk' versions generally restrict themselves to printing
-           the first byte of a string or to numeric values within the
-           range of a single byte (0-255).
- 
- `%d, %i'
-      Print a decimal integer.  The two control letters are equivalent.
-      (The `%i' specification is for compatibility with ISO C.)
- 
- `%e, %E'
-      Print a number in scientific (exponential) notation; for example:
- 
-           printf "%4.3e\n", 1950
- 
-      prints `1.950e+03', with a total of four significant figures,
-      three of which follow the decimal point.  (The `4.3' represents
-      two modifiers, discussed in the next node.)  `%E' uses `E' instead
-      of `e' in the output.
- 
- `%f'
-      Print a number in floating-point notation.  For example:
- 
-           printf "%4.3f", 1950
- 
-      prints `1950.000', with a total of four significant figures, three
-      of which follow the decimal point.  (The `4.3' represents two
-      modifiers, discussed in the next node.)
- 
-      On systems supporting IEEE 754 floating point format, values
-      representing negative infinity are formatted as `-inf' or
-      `-infinity', and positive infinity as `inf' and `infinity'.  The
-      special "not a number" value formats as `-nan' or `nan'.
- 
- `%F'
-      Like `%f' but the infinity and "not a number" values are spelled
-      using uppercase letters.
- 
-      The `%F' format is a POSIX extension to ISO C; not all systems
-      support it.  On those that don't, `gawk' uses `%f' instead.
- 
- `%g, %G'
-      Print a number in either scientific notation or in floating-point
-      notation, whichever uses fewer characters; if the result is
-      printed in scientific notation, `%G' uses `E' instead of `e'.
- 
- `%o'
-      Print an unsigned octal integer (*note Nondecimal-numbers::).
- 
- `%s'
-      Print a string.
- 
- `%u'
-      Print an unsigned decimal integer.  (This format is of marginal
-      use, because all numbers in `awk' are floating-point; it is
-      provided primarily for compatibility with C.)
- 
- `%x, %X'
-      Print an unsigned hexadecimal integer; `%X' uses the letters `A'
-      through `F' instead of `a' through `f' (*note
-      Nondecimal-numbers::).
- 
- `%%'
-      Print a single `%'.  This does not consume an argument and it
-      ignores any modifiers.
- 
-      NOTE: When using the integer format-control letters for values
-      that are outside the range of the widest C integer type, `gawk'
-      switches to the `%g' format specifier. If `--lint' is provided on
-      the command line (*note Options::), `gawk' warns about this.
-      Other versions of `awk' may print invalid values or do something
-      else entirely.  (d.c.)
- 
- 
- File: gawk.info,  Node: Format Modifiers,  Next: Printf Examples,  Prev: 
Control Letters,  Up: Printf
- 
- 5.5.3 Modifiers for `printf' Formats
- ------------------------------------
- 
- A format specification can also include "modifiers" that can control
- how much of the item's value is printed, as well as how much space it
- gets.  The modifiers come between the `%' and the format-control letter.
- We will use the bullet symbol "*" in the following examples to represent
- spaces in the output. Here are the possible modifiers, in the order in
- which they may appear:
- 
- `N$'
-      An integer constant followed by a `$' is a "positional specifier".
-      Normally, format specifications are applied to arguments in the
-      order given in the format string.  With a positional specifier,
-      the format specification is applied to a specific argument,
-      instead of what would be the next argument in the list.
-      Positional specifiers begin counting with one. Thus:
- 
-           printf "%s %s\n", "don't", "panic"
-           printf "%2$s %1$s\n", "panic", "don't"
- 
-      prints the famous friendly message twice.
- 
-      At first glance, this feature doesn't seem to be of much use.  It
-      is in fact a `gawk' extension, intended for use in translating
-      messages at runtime.  *Note Printf Ordering::, which describes how
-      and why to use positional specifiers.  For now, we will not use
-      them.
- 
- `-'
-      The minus sign, used before the width modifier (see later on in
-      this list), says to left-justify the argument within its specified
-      width.  Normally, the argument is printed right-justified in the
-      specified width.  Thus:
- 
-           printf "%-4s", "foo"
- 
-      prints `foo*'.
- 
- `SPACE'
-      For numeric conversions, prefix positive values with a space and
-      negative values with a minus sign.
- 
- `+'
-      The plus sign, used before the width modifier (see later on in
-      this list), says to always supply a sign for numeric conversions,
-      even if the data to format is positive. The `+' overrides the
-      space modifier.
- 
- `#'
-      Use an "alternate form" for certain control letters.  For `%o',
-      supply a leading zero.  For `%x' and `%X', supply a leading `0x'
-      or `0X' for a nonzero result.  For `%e', `%E', `%f', and `%F', the
-      result always contains a decimal point.  For `%g' and `%G',
-      trailing zeros are not removed from the result.
- 
- `0'
-      A leading `0' (zero) acts as a flag that indicates that output
-      should be padded with zeros instead of spaces.  This applies only
-      to the numeric output formats.  This flag only has an effect when
-      the field width is wider than the value to print.
- 
- `''
-      A single quote or apostrophe character is a POSIX extension to ISO
-      C.  It indicates that the integer part of a floating point value,
-      or the entire part of an integer decimal value, should have a
-      thousands-separator character in it.  This only works in locales
-      that support such characters.  For example:
- 
-           $ cat thousands.awk          Show source program
-           -| BEGIN { printf "%'d\n", 1234567 }
-           $ LC_ALL=C gawk -f thousands.awk
-           -| 1234567                   Results in "C" locale
-           $ LC_ALL=en_US.UTF-8 gawk -f thousands.awk
-           -| 1,234,567                 Results in US English UTF locale
- 
-      For more information about locales and internationalization issues,
-      see *note Locales::.
- 
-           NOTE: The `'' flag is a nice feature, but its use complicates
-           things: it becomes difficult to use it in command-line
-           programs.  For information on appropriate quoting tricks, see
-           *note Quoting::.
- 
- `WIDTH'
-      This is a number specifying the desired minimum width of a field.
-      Inserting any number between the `%' sign and the format-control
-      character forces the field to expand to this width.  The default
-      way to do this is to pad with spaces on the left.  For example:
- 
-           printf "%4s", "foo"
- 
-      prints `*foo'.
- 
-      The value of WIDTH is a minimum width, not a maximum.  If the item
-      value requires more than WIDTH characters, it can be as wide as
-      necessary.  Thus, the following:
- 
-           printf "%4s", "foobar"
- 
-      prints `foobar'.
- 
-      Preceding the WIDTH with a minus sign causes the output to be
-      padded with spaces on the right, instead of on the left.
- 
- `.PREC'
-      A period followed by an integer constant specifies the precision
-      to use when printing.  The meaning of the precision varies by
-      control letter:
- 
-     `%d', `%i', `%o', `%u', `%x', `%X'
-           Minimum number of digits to print.
- 
-     `%e', `%E', `%f', `%F'
-           Number of digits to the right of the decimal point.
- 
-     `%g', `%G'
-           Maximum number of significant digits.
- 
-     `%s'
-           Maximum number of characters from the string that should
-           print.
- 
-      Thus, the following:
- 
-           printf "%.4s", "foobar"
- 
-      prints `foob'.
- 
-    The C library `printf''s dynamic WIDTH and PREC capability (for
- example, `"%*.*s"') is supported.  Instead of supplying explicit WIDTH
- and/or PREC values in the format string, they are passed in the
- argument list.  For example:
- 
-      w = 5
-      p = 3
-      s = "abcdefg"
-      printf "%*.*s\n", w, p, s
- 
- is exactly equivalent to:
- 
-      s = "abcdefg"
-      printf "%5.3s\n", s
- 
- Both programs output `**abc'.  Earlier versions of `awk' did not
- support this capability.  If you must use such a version, you may
- simulate this feature by using concatenation to build up the format
- string, like so:
- 
-      w = 5
-      p = 3
-      s = "abcdefg"
-      printf "%" w "." p "s\n", s
- 
- This is not particularly easy to read but it does work.
- 
-    C programmers may be used to supplying additional `l', `L', and `h'
- modifiers in `printf' format strings. These are not valid in `awk'.
- Most `awk' implementations silently ignore them.  If `--lint' is
- provided on the command line (*note Options::), `gawk' warns about
- their use. If `--posix' is supplied, their use is a fatal error.
- 
- 
- File: gawk.info,  Node: Printf Examples,  Prev: Format Modifiers,  Up: Printf
- 
- 5.5.4 Examples Using `printf'
- -----------------------------
- 
- The following simple example shows how to use `printf' to make an
- aligned table:
- 
-      awk '{ printf "%-10s %s\n", $1, $2 }' BBS-list
- 
- This command prints the names of the bulletin boards (`$1') in the file
- `BBS-list' as a string of 10 characters that are left-justified.  It
- also prints the phone numbers (`$2') next on the line.  This produces
- an aligned two-column table of names and phone numbers, as shown here:
- 
-      $ awk '{ printf "%-10s %s\n", $1, $2 }' BBS-list
-      -| aardvark   555-5553
-      -| alpo-net   555-3412
-      -| barfly     555-7685
-      -| bites      555-1675
-      -| camelot    555-0542
-      -| core       555-2912
-      -| fooey      555-1234
-      -| foot       555-6699
-      -| macfoo     555-6480
-      -| sdace      555-3430
-      -| sabafoo    555-2127
- 
-    In this case, the phone numbers had to be printed as strings because
- the numbers are separated by a dash.  Printing the phone numbers as
- numbers would have produced just the first three digits: `555'.  This
- would have been pretty confusing.
- 
-    It wasn't necessary to specify a width for the phone numbers because
- they are last on their lines.  They don't need to have spaces after
- them.
- 
-    The table could be made to look even nicer by adding headings to the
- tops of the columns.  This is done using the `BEGIN' pattern (*note
- BEGIN/END::) so that the headers are only printed once, at the
- beginning of the `awk' program:
- 
-      awk 'BEGIN { print "Name      Number"
-                   print "----      ------" }
-           { printf "%-10s %s\n", $1, $2 }' BBS-list
- 
-    The above example mixes `print' and `printf' statements in the same
- program.  Using just `printf' statements can produce the same results:
- 
-      awk 'BEGIN { printf "%-10s %s\n", "Name", "Number"
-                   printf "%-10s %s\n", "----", "------" }
-           { printf "%-10s %s\n", $1, $2 }' BBS-list
- 
- Printing each column heading with the same format specification used
- for the column elements ensures that the headings are aligned just like
- the columns.
- 
-    The fact that the same format specification is used three times can
- be emphasized by storing it in a variable, like this:
- 
-      awk 'BEGIN { format = "%-10s %s\n"
-                   printf format, "Name", "Number"
-                   printf format, "----", "------" }
-           { printf format, $1, $2 }' BBS-list
- 
-    At this point, it would be a worthwhile exercise to use the `printf'
- statement to line up the headings and table data for the
- `inventory-shipped' example that was covered earlier in the minor node
- on the `print' statement (*note Print::).
- 
- 
- File: gawk.info,  Node: Redirection,  Next: Special Files,  Prev: Printf,  
Up: Printing
- 
- 5.6 Redirecting Output of `print' and `printf'
- ==============================================
- 
- So far, the output from `print' and `printf' has gone to the standard
- output, usually the screen.  Both `print' and `printf' can also send
- their output to other places.  This is called "redirection".
- 
-      NOTE: When `--sandbox' is specified (*note Options::), redirecting
-      output to files and pipes is disabled.
- 
-    A redirection appears after the `print' or `printf' statement.
- Redirections in `awk' are written just like redirections in shell
- commands, except that they are written inside the `awk' program.
- 
-    There are four forms of output redirection: output to a file, output
- appended to a file, output through a pipe to another command, and output
- to a coprocess.  They are all shown for the `print' statement, but they
- work identically for `printf':
- 
- `print ITEMS > OUTPUT-FILE'
-      This redirection prints the items into the output file named
-      OUTPUT-FILE.  The file name OUTPUT-FILE can be any expression.
-      Its value is changed to a string and then used as a file name
-      (*note Expressions::).
- 
-      When this type of redirection is used, the OUTPUT-FILE is erased
-      before the first output is written to it.  Subsequent writes to
-      the same OUTPUT-FILE do not erase OUTPUT-FILE, but append to it.
-      (This is different from how you use redirections in shell scripts.)
-      If OUTPUT-FILE does not exist, it is created.  For example, here
-      is how an `awk' program can write a list of BBS names to one file
-      named `name-list', and a list of phone numbers to another file
-      named `phone-list':
- 
-           $ awk '{ print $2 > "phone-list"
-           >        print $1 > "name-list" }' BBS-list
-           $ cat phone-list
-           -| 555-5553
-           -| 555-3412
-           ...
-           $ cat name-list
-           -| aardvark
-           -| alpo-net
-           ...
- 
-      Each output file contains one name or number per line.
- 
- `print ITEMS >> OUTPUT-FILE'
-      This redirection prints the items into the pre-existing output file
-      named OUTPUT-FILE.  The difference between this and the single-`>'
-      redirection is that the old contents (if any) of OUTPUT-FILE are
-      not erased.  Instead, the `awk' output is appended to the file.
-      If OUTPUT-FILE does not exist, then it is created.
- 
- `print ITEMS | COMMAND'
-      It is possible to send output to another program through a pipe
-      instead of into a file.   This redirection opens a pipe to
-      COMMAND, and writes the values of ITEMS through this pipe to
-      another process created to execute COMMAND.
- 
-      The redirection argument COMMAND is actually an `awk' expression.
-      Its value is converted to a string whose contents give the shell
-      command to be run.  For example, the following produces two files,
-      one unsorted list of BBS names, and one list sorted in reverse
-      alphabetical order:
- 
-           awk '{ print $1 > "names.unsorted"
-                  command = "sort -r > names.sorted"
-                  print $1 | command }' BBS-list
- 
-      The unsorted list is written with an ordinary redirection, while
-      the sorted list is written by piping through the `sort' utility.
- 
-      The next example uses redirection to mail a message to the mailing
-      list `bug-system'.  This might be useful when trouble is
-      encountered in an `awk' script run periodically for system
-      maintenance:
- 
-           report = "mail bug-system"
-           print "Awk script failed:", $0 | report
-           m = ("at record number " FNR " of " FILENAME)
-           print m | report
-           close(report)
- 
-      The message is built using string concatenation and saved in the
-      variable `m'.  It's then sent down the pipeline to the `mail'
-      program.  (The parentheses group the items to concatenate--see
-      *note Concatenation::.)
- 
-      The `close()' function is called here because it's a good idea to
-      close the pipe as soon as all the intended output has been sent to
-      it.  *Note Close Files And Pipes::, for more information.
- 
-      This example also illustrates the use of a variable to represent a
-      FILE or COMMAND--it is not necessary to always use a string
-      constant.  Using a variable is generally a good idea, because (if
-      you mean to refer to that same file or command) `awk' requires
-      that the string value be spelled identically every time.
- 
- `print ITEMS |& COMMAND'
-      This redirection prints the items to the input of COMMAND.  The
-      difference between this and the single-`|' redirection is that the
-      output from COMMAND can be read with `getline'.  Thus COMMAND is a
-      "coprocess", which works together with, but subsidiary to, the
-      `awk' program.
- 
-      This feature is a `gawk' extension, and is not available in POSIX
-      `awk'.  *Note Getline/Coprocess::, for a brief discussion.  *Note
-      Two-way I/O::, for a more complete discussion.
- 
-    Redirecting output using `>', `>>', `|', or `|&' asks the system to
- open a file, pipe, or coprocess only if the particular FILE or COMMAND
- you specify has not already been written to by your program or if it
- has been closed since it was last written to.
- 
-    It is a common error to use `>' redirection for the first `print' to
- a file, and then to use `>>' for subsequent output:
- 
-      # clear the file
-      print "Don't panic" > "guide.txt"
-      ...
-      # append
-      print "Avoid improbability generators" >> "guide.txt"
- 
- This is indeed how redirections must be used from the shell.  But in
- `awk', it isn't necessary.  In this kind of case, a program should use
- `>' for all the `print' statements, since the output file is only
- opened once. (It happens that if you mix `>' and `>>' that output is
- produced in the expected order. However, mixing the operators for the
- same file is definitely poor style, and is confusing to readers of your
- program.)
- 
-    Many older `awk' implementations limit the number of pipelines that
- an `awk' program may have open to just one!  In `gawk', there is no
- such limit.  `gawk' allows a program to open as many pipelines as the
- underlying operating system permits.
- 
- Advanced Notes: Piping into `sh'
- --------------------------------
- 
- A particularly powerful way to use redirection is to build command lines
- and pipe them into the shell, `sh'.  For example, suppose you have a
- list of files brought over from a system where all the file names are
- stored in uppercase, and you wish to rename them to have names in all
- lowercase.  The following program is both simple and efficient:
- 
-      { printf("mv %s %s\n", $0, tolower($0)) | "sh" }
- 
-      END { close("sh") }
- 
-    The `tolower()' function returns its argument string with all
- uppercase characters converted to lowercase (*note String Functions::).
- The program builds up a list of command lines, using the `mv' utility
- to rename the files.  It then sends the list to the shell for execution.
- 
- 
- File: gawk.info,  Node: Special Files,  Next: Close Files And Pipes,  Prev: 
Redirection,  Up: Printing
- 
- 5.7 Special File Names in `gawk'
- ================================
- 
- `gawk' provides a number of special file names that it interprets
- internally.  These file names provide access to standard file
- descriptors and TCP/IP networking.
- 
- * Menu:
- 
- * Special FD::                  Special files for I/O.
- * Special Network::             Special files for network communications.
- * Special Caveats::             Things to watch out for.
- 
- 
- File: gawk.info,  Node: Special FD,  Next: Special Network,  Up: Special Files
- 
- 5.7.1 Special Files for Standard Descriptors
- --------------------------------------------
- 
- Running programs conventionally have three input and output streams
- already available to them for reading and writing.  These are known as
- the "standard input", "standard output", and "standard error output".
- These streams are, by default, connected to your keyboard and screen,
- but they are often redirected with the shell, via the `<', `<<', `>',
- `>>', `>&', and `|' operators.  Standard error is typically used for
- writing error messages; the reason there are two separate streams,
- standard output and standard error, is so that they can be redirected
- separately.
- 
-    In other implementations of `awk', the only way to write an error
- message to standard error in an `awk' program is as follows:
- 
-      print "Serious error detected!" | "cat 1>&2"
- 
- This works by opening a pipeline to a shell command that can access the
- standard error stream that it inherits from the `awk' process.  This is
- far from elegant, and it is also inefficient, because it requires a
- separate process.  So people writing `awk' programs often don't do
- this.  Instead, they send the error messages to the screen, like this:
- 
-      print "Serious error detected!" > "/dev/tty"
- 
- (`/dev/tty' is a special file supplied by the operating system that is
- connected to your keyboard and screen. It represents the "terminal,"(1)
- which on modern systems is a keyboard and screen, not a serial console.)
- This usually has the same effect but not always: although the standard
- error stream is usually the screen, it can be redirected; when that
- happens, writing to the screen is not correct.  In fact, if `awk' is
- run from a background job, it may not have a terminal at all.  Then
- opening `/dev/tty' fails.
- 
-    `gawk' provides special file names for accessing the three standard
- streams. (c.e.). It also provides syntax for accessing any other
- inherited open files.  If the file name matches one of these special
- names when `gawk' redirects input or output, then it directly uses the
- stream that the file name stands for.  These special file names work
- for all operating systems that `gawk' has been ported to, not just
- those that are POSIX-compliant:
- 
- `/dev/stdin'
-      The standard input (file descriptor 0).
- 
- `/dev/stdout'
-      The standard output (file descriptor 1).
- 
- `/dev/stderr'
-      The standard error output (file descriptor 2).
- 
- `/dev/fd/N'
-      The file associated with file descriptor N.  Such a file must be
-      opened by the program initiating the `awk' execution (typically
-      the shell).  Unless special pains are taken in the shell from which
-      `gawk' is invoked, only descriptors 0, 1, and 2 are available.
- 
-    The file names `/dev/stdin', `/dev/stdout', and `/dev/stderr' are
- aliases for `/dev/fd/0', `/dev/fd/1', and `/dev/fd/2', respectively.
- However, they are more self-explanatory.  The proper way to write an
- error message in a `gawk' program is to use `/dev/stderr', like this:
- 
-      print "Serious error detected!" > "/dev/stderr"
- 
-    Note the use of quotes around the file name.  Like any other
- redirection, the value must be a string.  It is a common error to omit
- the quotes, which leads to confusing results.
- 
-    Finally, using the `close()' function on a file name of the form
- `"/dev/fd/N"', for file descriptor numbers above two, will actually
- close the given file descriptor.
- 
-    The `/dev/stdin', `/dev/stdout', and `/dev/stderr' special files are
- also recognized internally by several other versions of `awk'.
- 
-    ---------- Footnotes ----------
- 
-    (1) The "tty" in `/dev/tty' stands for "Teletype," a serial terminal.
- 
- 
- File: gawk.info,  Node: Special Network,  Next: Special Caveats,  Prev: 
Special FD,  Up: Special Files
- 
- 5.7.2 Special Files for Network Communications
- ----------------------------------------------
- 
- `gawk' programs can open a two-way TCP/IP connection, acting as either
- a client or a server.  This is done using a special file name of the
- form:
- 
-      `/NET-TYPE/PROTOCOL/LOCAL-PORT/REMOTE-HOST/REMOTE-PORT'
- 
-    The NET-TYPE is one of `inet', `inet4' or `inet6'.  The PROTOCOL is
- one of `tcp' or `udp', and the other fields represent the other
- essential pieces of information for making a networking connection.
- These file names are used with the `|&' operator for communicating with
- a coprocess (*note Two-way I/O::).  This is an advanced feature,
- mentioned here only for completeness.  Full discussion is delayed until
- *note TCP/IP Networking::.
- 
- 
- File: gawk.info,  Node: Special Caveats,  Prev: Special Network,  Up: Special 
Files
- 
- 5.7.3 Special File Name Caveats
- -------------------------------
- 
- Here is a list of things to bear in mind when using the special file
- names that `gawk' provides:
- 
-    * Recognition of these special file names is disabled if `gawk' is in
-      compatibility mode (*note Options::).
- 
-    * `gawk' _always_ interprets these special file names.  For example,
-      using `/dev/fd/4' for output actually writes on file descriptor 4,
-      and not on a new file descriptor that is `dup()''ed from file
-      descriptor 4.  Most of the time this does not matter; however, it
-      is important to _not_ close any of the files related to file
-      descriptors 0, 1, and 2.  Doing so results in unpredictable
-      behavior.
- 
- 
- File: gawk.info,  Node: Close Files And Pipes,  Prev: Special Files,  Up: 
Printing
- 
- 5.8 Closing Input and Output Redirections
- =========================================
- 
- If the same file name or the same shell command is used with `getline'
- more than once during the execution of an `awk' program (*note
- Getline::), the file is opened (or the command is executed) the first
- time only.  At that time, the first record of input is read from that
- file or command.  The next time the same file or command is used with
- `getline', another record is read from it, and so on.
- 
-    Similarly, when a file or pipe is opened for output, `awk' remembers
- the file name or command associated with it, and subsequent writes to
- the same file or command are appended to the previous writes.  The file
- or pipe stays open until `awk' exits.
- 
-    This implies that special steps are necessary in order to read the
- same file again from the beginning, or to rerun a shell command (rather
- than reading more output from the same command).  The `close()' function
- makes these things possible:
- 
-      close(FILENAME)
- 
- or:
- 
-      close(COMMAND)
- 
-    The argument FILENAME or COMMAND can be any expression.  Its value
- must _exactly_ match the string that was used to open the file or start
- the command (spaces and other "irrelevant" characters included). For
- example, if you open a pipe with this:
- 
-      "sort -r names" | getline foo
- 
- then you must close it with this:
- 
-      close("sort -r names")
- 
-    Once this function call is executed, the next `getline' from that
- file or command, or the next `print' or `printf' to that file or
- command, reopens the file or reruns the command.  Because the
- expression that you use to close a file or pipeline must exactly match
- the expression used to open the file or run the command, it is good
- practice to use a variable to store the file name or command.  The
- previous example becomes the following:
- 
-      sortcom = "sort -r names"
-      sortcom | getline foo
-      ...
-      close(sortcom)
- 
- This helps avoid hard-to-find typographical errors in your `awk'
- programs.  Here are some of the reasons for closing an output file:
- 
-    * To write a file and read it back later on in the same `awk'
-      program.  Close the file after writing it, then begin reading it
-      with `getline'.
- 
-    * To write numerous files, successively, in the same `awk' program.
-      If the files aren't closed, eventually `awk' may exceed a system
-      limit on the number of open files in one process.  It is best to
-      close each one when the program has finished writing it.
- 
-    * To make a command finish.  When output is redirected through a
-      pipe, the command reading the pipe normally continues to try to
-      read input as long as the pipe is open.  Often this means the
-      command cannot really do its work until the pipe is closed.  For
-      example, if output is redirected to the `mail' program, the
-      message is not actually sent until the pipe is closed.
- 
-    * To run the same program a second time, with the same arguments.
-      This is not the same thing as giving more input to the first run!
- 
-      For example, suppose a program pipes output to the `mail' program.
-      If it outputs several lines redirected to this pipe without closing
-      it, they make a single message of several lines.  By contrast, if
-      the program closes the pipe after each line of output, then each
-      line makes a separate message.
- 
-    If you use more files than the system allows you to have open,
- `gawk' attempts to multiplex the available open files among your data
- files.  `gawk''s ability to do this depends upon the facilities of your
- operating system, so it may not always work.  It is therefore both good
- practice and good portability advice to always use `close()' on your
- files when you are done with them.  In fact, if you are using a lot of
- pipes, it is essential that you close commands when done. For example,
- consider something like this:
- 
-      {
-          ...
-          command = ("grep " $1 " /some/file | my_prog -q " $3)
-          while ((command | getline) > 0) {
-              PROCESS OUTPUT OF command
-          }
-          # need close(command) here
-      }
- 
-    This example creates a new pipeline based on data in _each_ record.
- Without the call to `close()' indicated in the comment, `awk' creates
- child processes to run the commands, until it eventually runs out of
- file descriptors for more pipelines.
- 
-    Even though each command has finished (as indicated by the
- end-of-file return status from `getline'), the child process is not
- terminated;(1) more importantly, the file descriptor for the pipe is
- not closed and released until `close()' is called or `awk' exits.
- 
-    `close()' will silently do nothing if given an argument that does
- not represent a file, pipe or coprocess that was opened with a
- redirection.
- 
-    Note also that `close(FILENAME)' has no "magic" effects on the
- implicit loop that reads through the files named on the command line.
- It is, more likely, a close of a file that was never opened, so `awk'
- silently does nothing.
- 
-    When using the `|&' operator to communicate with a coprocess, it is
- occasionally useful to be able to close one end of the two-way pipe
- without closing the other.  This is done by supplying a second argument
- to `close()'.  As in any other call to `close()', the first argument is
- the name of the command or special file used to start the coprocess.
- The second argument should be a string, with either of the values
- `"to"' or `"from"'.  Case does not matter.  As this is an advanced
- feature, a more complete discussion is delayed until *note Two-way
- I/O::, which discusses it in more detail and gives an example.
- 
- Advanced Notes: Using `close()''s Return Value
- ----------------------------------------------
- 
- In many versions of Unix `awk', the `close()' function is actually a
- statement.  It is a syntax error to try and use the return value from
- `close()': (d.c.)
- 
-      command = "..."
-      command | getline info
-      retval = close(command)  # syntax error in many Unix awks
- 
-    `gawk' treats `close()' as a function.  The return value is -1 if
- the argument names something that was never opened with a redirection,
- or if there is a system problem closing the file or process.  In these
- cases, `gawk' sets the built-in variable `ERRNO' to a string describing
- the problem.
- 
-    In `gawk', when closing a pipe or coprocess (input or output), the
- return value is the exit status of the command.(2) Otherwise, it is the
- return value from the system's `close()' or `fclose()' C functions when
- closing input or output files, respectively.  This value is zero if the
- close succeeds, or -1 if it fails.
- 
-    The POSIX standard is very vague; it says that `close()' returns
- zero on success and nonzero otherwise.  In general, different
- implementations vary in what they report when closing pipes; thus the
- return value cannot be used portably.  (d.c.)  In POSIX mode (*note
- Options::), `gawk' just returns zero when closing a pipe.
- 
-    ---------- Footnotes ----------
- 
-    (1) The technical terminology is rather morbid.  The finished child
- is called a "zombie," and cleaning up after it is referred to as
- "reaping."
- 
-    (2) This is a full 16-bit value as returned by the `wait()' system
- call. See the system manual pages for information on how to decode this
- value.
- 
- 
- File: gawk.info,  Node: Expressions,  Next: Patterns and Actions,  Prev: 
Printing,  Up: Top
- 
- 6 Expressions
- *************
- 
- Expressions are the basic building blocks of `awk' patterns and
- actions.  An expression evaluates to a value that you can print, test,
- or pass to a function.  Additionally, an expression can assign a new
- value to a variable or a field by using an assignment operator.
- 
-    An expression can serve as a pattern or action statement on its own.
- Most other kinds of statements contain one or more expressions that
- specify the data on which to operate.  As in other languages,
- expressions in `awk' include variables, array references, constants,
- and function calls, as well as combinations of these with various
- operators.
- 
- * Menu:
- 
- * Values::                      Constants, Variables, and Regular Expressions.
- * All Operators::               `gawk''s operators.
- * Truth Values and Conditions:: Testing for true and false.
- * Function Calls::              A function call is an expression.
- * Precedence::                  How various operators nest.
- * Locales::                     How the locale affects things.
- 
- 
- File: gawk.info,  Node: Values,  Next: All Operators,  Up: Expressions
- 
- 6.1 Constants, Variables and Conversions
- ========================================
- 
- Expressions are built up from values and the operations performed upon
- them. This minor node describes the elementary objects which provide
- the values used in expressions.
- 
- * Menu:
- 
- * Constants::                   String, numeric and regexp constants.
- * Using Constant Regexps::      When and how to use a regexp constant.
- * Variables::                   Variables give names to values for later use.
- * Conversion::                  The conversion of strings to numbers and vice
-                                 versa.
- 
- 
- File: gawk.info,  Node: Constants,  Next: Using Constant Regexps,  Up: Values
- 
- 6.1.1 Constant Expressions
- --------------------------
- 
- The simplest type of expression is the "constant", which always has the
- same value.  There are three types of constants: numeric, string, and
- regular expression.
- 
-    Each is used in the appropriate context when you need a data value
- that isn't going to change.  Numeric constants can have different
- forms, but are stored identically internally.
- 
- * Menu:
- 
- * Scalar Constants::            Numeric and string constants.
- * Nondecimal-numbers::          What are octal and hex numbers.
- * Regexp Constants::            Regular Expression constants.
- 
- 
- File: gawk.info,  Node: Scalar Constants,  Next: Nondecimal-numbers,  Up: 
Constants
- 
- 6.1.1.1 Numeric and String Constants
- ....................................
- 
- A "numeric constant" stands for a number.  This number can be an
- integer, a decimal fraction, or a number in scientific (exponential)
- notation.(1) Here are some examples of numeric constants that all have
- the same value:
- 
-      105
-      1.05e+2
-      1050e-1
- 
-    A string constant consists of a sequence of characters enclosed in
- double-quotation marks.  For example:
- 
-      "parrot"
- 
- represents the string whose contents are `parrot'.  Strings in `gawk'
- can be of any length, and they can contain any of the possible
- eight-bit ASCII characters including ASCII NUL (character code zero).
- Other `awk' implementations may have difficulty with some character
- codes.
- 
-    ---------- Footnotes ----------
- 
-    (1) The internal representation of all numbers, including integers,
- uses double precision floating-point numbers.  On most modern systems,
- these are in IEEE 754 standard format.
- 
- 
- File: gawk.info,  Node: Nondecimal-numbers,  Next: Regexp Constants,  Prev: 
Scalar Constants,  Up: Constants
- 
- 6.1.1.2 Octal and Hexadecimal Numbers
- .....................................
- 
- In `awk', all numbers are in decimal; i.e., base 10.  Many other
- programming languages allow you to specify numbers in other bases, often
- octal (base 8) and hexadecimal (base 16).  In octal, the numbers go 0,
- 1, 2, 3, 4, 5, 6, 7, 10, 11, 12, etc.  Just as `11', in decimal, is 1
- times 10 plus 1, so `11', in octal, is 1 times 8, plus 1. This equals 9
- in decimal.  In hexadecimal, there are 16 digits. Since the everyday
- decimal number system only has ten digits (`0'-`9'), the letters `a'
- through `f' are used to represent the rest.  (Case in the letters is
- usually irrelevant; hexadecimal `a' and `A' have the same value.)
- Thus, `11', in hexadecimal, is 1 times 16 plus 1, which equals 17 in
- decimal.
- 
-    Just by looking at plain `11', you can't tell what base it's in.
- So, in C, C++, and other languages derived from C, there is a special
- notation to signify the base.  Octal numbers start with a leading `0',
- and hexadecimal numbers start with a leading `0x' or `0X':
- 
- `11'
-      Decimal value 11.
- 
- `011'
-      Octal 11, decimal value 9.
- 
- `0x11'
-      Hexadecimal 11, decimal value 17.
- 
-    This example shows the difference:
- 
-      $ gawk 'BEGIN { printf "%d, %d, %d\n", 011, 11, 0x11 }'
-      -| 9, 11, 17
- 
-    Being able to use octal and hexadecimal constants in your programs
- is most useful when working with data that cannot be represented
- conveniently as characters or as regular numbers, such as binary data
- of various sorts.
- 
-    `gawk' allows the use of octal and hexadecimal constants in your
- program text.  However, such numbers in the input data are not treated
- differently; doing so by default would break old programs.  (If you
- really need to do this, use the `--non-decimal-data' command-line
- option; *note Nondecimal Data::.)  If you have octal or hexadecimal
- data, you can use the `strtonum()' function (*note String Functions::)
- to convert the data into a number.  Most of the time, you will want to
- use octal or hexadecimal constants when working with the built-in bit
- manipulation functions; see *note Bitwise Functions::, for more
- information.
- 
-    Unlike some early C implementations, `8' and `9' are not valid in
- octal constants; e.g., `gawk' treats `018' as decimal 18:
- 
-      $ gawk 'BEGIN { print "021 is", 021 ; print 018 }'
-      -| 021 is 17
-      -| 18
- 
-    Octal and hexadecimal source code constants are a `gawk' extension.
- If `gawk' is in compatibility mode (*note Options::), they are not
- available.
- 
- Advanced Notes: A Constant's Base Does Not Affect Its Value
- -----------------------------------------------------------
- 
- Once a numeric constant has been converted internally into a number,
- `gawk' no longer remembers what the original form of the constant was;
- the internal value is always used.  This has particular consequences
- for conversion of numbers to strings:
- 
-      $ gawk 'BEGIN { printf "0x11 is <%s>\n", 0x11 }'
-      -| 0x11 is <17>
- 
- 
- File: gawk.info,  Node: Regexp Constants,  Prev: Nondecimal-numbers,  Up: 
Constants
- 
- 6.1.1.3 Regular Expression Constants
- ....................................
- 
- A regexp constant is a regular expression description enclosed in
- slashes, such as `/^beginning and end$/'.  Most regexps used in `awk'
- programs are constant, but the `~' and `!~' matching operators can also
- match computed or dynamic regexps (which are just ordinary strings or
- variables that contain a regexp).
- 
- 
- File: gawk.info,  Node: Using Constant Regexps,  Next: Variables,  Prev: 
Constants,  Up: Values
- 
- 6.1.2 Using Regular Expression Constants
- ----------------------------------------
- 
- When used on the righthand side of the `~' or `!~' operators, a regexp
- constant merely stands for the regexp that is to be matched.  However,
- regexp constants (such as `/foo/') may be used like simple expressions.
- When a regexp constant appears by itself, it has the same meaning as if
- it appeared in a pattern, i.e., `($0 ~ /foo/)' (d.c.)  *Note Expression
- Patterns::.  This means that the following two code segments:
- 
-      if ($0 ~ /barfly/ || $0 ~ /camelot/)
-          print "found"
- 
- and:
- 
-      if (/barfly/ || /camelot/)
-          print "found"
- 
- are exactly equivalent.  One rather bizarre consequence of this rule is
- that the following Boolean expression is valid, but does not do what
- the user probably intended:
- 
-      # Note that /foo/ is on the left of the ~
-      if (/foo/ ~ $1) print "found foo"
- 
- This code is "obviously" testing `$1' for a match against the regexp
- `/foo/'.  But in fact, the expression `/foo/ ~ $1' really means `($0 ~
- /foo/) ~ $1'.  In other words, first match the input record against the
- regexp `/foo/'.  The result is either zero or one, depending upon the
- success or failure of the match.  That result is then matched against
- the first field in the record.  Because it is unlikely that you would
- ever really want to make this kind of test, `gawk' issues a warning
- when it sees this construct in a program.  Another consequence of this
- rule is that the assignment statement:
- 
-      matches = /foo/
- 
- assigns either zero or one to the variable `matches', depending upon
- the contents of the current input record.
- 
-    Constant regular expressions are also used as the first argument for
- the `gensub()', `sub()', and `gsub()' functions, as the second argument
- of the `match()' function, and as the third argument of the
- `patsplit()' function (*note String Functions::).  Modern
- implementations of `awk', including `gawk', allow the third argument of
- `split()' to be a regexp constant, but some older implementations do
- not.  (d.c.)  This can lead to confusion when attempting to use regexp
- constants as arguments to user-defined functions (*note User-defined::).
- For example:
- 
-      function mysub(pat, repl, str, global)
-      {
-          if (global)
-              gsub(pat, repl, str)
-          else
-              sub(pat, repl, str)
-          return str
-      }
- 
-      {
-          ...
-          text = "hi! hi yourself!"
-          mysub(/hi/, "howdy", text, 1)
-          ...
-      }
- 
-    In this example, the programmer wants to pass a regexp constant to
- the user-defined function `mysub', which in turn passes it on to either
- `sub()' or `gsub()'.  However, what really happens is that the `pat'
- parameter is either one or zero, depending upon whether or not `$0'
- matches `/hi/'.  `gawk' issues a warning when it sees a regexp constant
- used as a parameter to a user-defined function, since passing a truth
- value in this way is probably not what was intended.
- 
- 
- File: gawk.info,  Node: Variables,  Next: Conversion,  Prev: Using Constant 
Regexps,  Up: Values
- 
- 6.1.3 Variables
- ---------------
- 
- Variables are ways of storing values at one point in your program for
- use later in another part of your program.  They can be manipulated
- entirely within the program text, and they can also be assigned values
- on the `awk' command line.
- 
- * Menu:
- 
- * Using Variables::             Using variables in your programs.
- * Assignment Options::          Setting variables on the command-line and a
-                                 summary of command-line syntax. This is an
-                                 advanced method of input.
- 
- 
- File: gawk.info,  Node: Using Variables,  Next: Assignment Options,  Up: 
Variables
- 
- 6.1.3.1 Using Variables in a Program
- ....................................
- 
- Variables let you give names to values and refer to them later.
- Variables have already been used in many of the examples.  The name of
- a variable must be a sequence of letters, digits, or underscores, and
- it may not begin with a digit.  Case is significant in variable names;
- `a' and `A' are distinct variables.
- 
-    A variable name is a valid expression by itself; it represents the
- variable's current value.  Variables are given new values with
- "assignment operators", "increment operators", and "decrement
- operators".  *Note Assignment Ops::.  In addition, the `sub()' and
- `gsub()' functions can change a variable's value, and the `match()',
- `patsplit()' and `split()' functions can change the contents of their
- array parameters. *Note String Functions::.
- 
-    A few variables have special built-in meanings, such as `FS' (the
- field separator), and `NF' (the number of fields in the current input
- record).  *Note Built-in Variables::, for a list of the built-in
- variables.  These built-in variables can be used and assigned just like
- all other variables, but their values are also used or changed
- automatically by `awk'.  All built-in variables' names are entirely
- uppercase.
- 
-    Variables in `awk' can be assigned either numeric or string values.
- The kind of value a variable holds can change over the life of a
- program.  By default, variables are initialized to the empty string,
- which is zero if converted to a number.  There is no need to explicitly
- "initialize" a variable in `awk', which is what you would do in C and
- in most other traditional languages.
- 
- 
- File: gawk.info,  Node: Assignment Options,  Prev: Using Variables,  Up: 
Variables
- 
- 6.1.3.2 Assigning Variables on the Command Line
- ...............................................
- 
- Any `awk' variable can be set by including a "variable assignment"
- among the arguments on the command line when `awk' is invoked (*note
- Other Arguments::).  Such an assignment has the following form:
- 
-      VARIABLE=TEXT
- 
- With it, a variable is set either at the beginning of the `awk' run or
- in between input files.  When the assignment is preceded with the `-v'
- option, as in the following:
- 
-      -v VARIABLE=TEXT
- 
- the variable is set at the very beginning, even before the `BEGIN'
- rules execute.  The `-v' option and its assignment must precede all the
- file name arguments, as well as the program text.  (*Note Options::,
- for more information about the `-v' option.)  Otherwise, the variable
- assignment is performed at a time determined by its position among the
- input file arguments--after the processing of the preceding input file
- argument.  For example:
- 
-      awk '{ print $n }' n=4 inventory-shipped n=2 BBS-list
- 
- prints the value of field number `n' for all input records.  Before the
- first file is read, the command line sets the variable `n' equal to
- four.  This causes the fourth field to be printed in lines from
- `inventory-shipped'.  After the first file has finished, but before the
- second file is started, `n' is set to two, so that the second field is
- printed in lines from `BBS-list':
- 
-      $ awk '{ print $n }' n=4 inventory-shipped n=2 BBS-list
-      -| 15
-      -| 24
-      ...
-      -| 555-5553
-      -| 555-3412
-      ...
- 
-    Command-line arguments are made available for explicit examination by
- the `awk' program in the `ARGV' array (*note ARGC and ARGV::).  `awk'
- processes the values of command-line assignments for escape sequences
- (*note Escape Sequences::).  (d.c.)
- 
- 
- File: gawk.info,  Node: Conversion,  Prev: Variables,  Up: Values
- 
- 6.1.4 Conversion of Strings and Numbers
- ---------------------------------------
- 
- Strings are converted to numbers and numbers are converted to strings,
- if the context of the `awk' program demands it.  For example, if the
- value of either `foo' or `bar' in the expression `foo + bar' happens to
- be a string, it is converted to a number before the addition is
- performed.  If numeric values appear in string concatenation, they are
- converted to strings.  Consider the following:
- 
-      two = 2; three = 3
-      print (two three) + 4
- 
- This prints the (numeric) value 27.  The numeric values of the
- variables `two' and `three' are converted to strings and concatenated
- together.  The resulting string is converted back to the number 23, to
- which 4 is then added.
- 
-    If, for some reason, you need to force a number to be converted to a
- string, concatenate that number with the empty string, `""'.  To force
- a string to be converted to a number, add zero to that string.  A
- string is converted to a number by interpreting any numeric prefix of
- the string as numerals: `"2.5"' converts to 2.5, `"1e3"' converts to
- 1000, and `"25fix"' has a numeric value of 25.  Strings that can't be
- interpreted as valid numbers convert to zero.
- 
-    The exact manner in which numbers are converted into strings is
- controlled by the `awk' built-in variable `CONVFMT' (*note Built-in
- Variables::).  Numbers are converted using the `sprintf()' function
- with `CONVFMT' as the format specifier (*note String Functions::).
- 
-    `CONVFMT''s default value is `"%.6g"', which prints a value with at
- most six significant digits.  For some applications, you might want to
- change it to specify more precision.  On most modern machines, 17
- digits is usually enough to capture a floating-point number's value
- exactly.(1)
- 
-    Strange results can occur if you set `CONVFMT' to a string that
- doesn't tell `sprintf()' how to format floating-point numbers in a
- useful way.  For example, if you forget the `%' in the format, `awk'
- converts all numbers to the same constant string.
- 
-    As a special case, if a number is an integer, then the result of
- converting it to a string is _always_ an integer, no matter what the
- value of `CONVFMT' may be.  Given the following code fragment:
- 
-      CONVFMT = "%2.2f"
-      a = 12
-      b = a ""
- 
- `b' has the value `"12"', not `"12.00"'.  (d.c.)
- 
-    Prior to the POSIX standard, `awk' used the value of `OFMT' for
- converting numbers to strings.  `OFMT' specifies the output format to
- use when printing numbers with `print'.  `CONVFMT' was introduced in
- order to separate the semantics of conversion from the semantics of
- printing.  Both `CONVFMT' and `OFMT' have the same default value:
- `"%.6g"'.  In the vast majority of cases, old `awk' programs do not
- change their behavior.  However, these semantics for `OFMT' are
- something to keep in mind if you must port your new-style program to
- older implementations of `awk'.  We recommend that instead of changing
- your programs, just port `gawk' itself.  *Note Print::, for more
- information on the `print' statement.
- 
-    And, once again, where you are can matter when it comes to converting
- between numbers and strings.  In *note Locales::, we mentioned that the
- local character set and language (the locale) can affect how `gawk'
- matches characters.  The locale also affects numeric formats.  In
- particular, for `awk' programs, it affects the decimal point character.
- The `"C"' locale, and most English-language locales, use the period
- character (`.') as the decimal point.  However, many (if not most)
- European and non-English locales use the comma (`,') as the decimal
- point character.
- 
-    The POSIX standard says that `awk' always uses the period as the
- decimal point when reading the `awk' program source code, and for
- command-line variable assignments (*note Other Arguments::).  However,
- when interpreting input data, for `print' and `printf' output, and for
- number to string conversion, the local decimal point character is used.
- Here are some examples indicating the difference in behavior, on a
- GNU/Linux system:
- 
-      $ gawk 'BEGIN { printf "%g\n", 3.1415927 }'
-      -| 3.14159
-      $ LC_ALL=en_DK gawk 'BEGIN { printf "%g\n", 3.1415927 }'
-      -| 3,14159
-      $ echo 4,321 | gawk '{ print $1 + 1 }'
-      -| 5
-      $ echo 4,321 | LC_ALL=en_DK gawk '{ print $1 + 1 }'
-      -| 5,321
- 
- The `en_DK' locale is for English in Denmark, where the comma acts as
- the decimal point separator.  In the normal `"C"' locale, `gawk' treats
- `4,321' as `4', while in the Danish locale, it's treated as the full
- number, 4.321.
- 
-    Some earlier versions of `gawk' fully complied with this aspect of
- the standard.  However, many users in non-English locales complained
- about this behavior, since their data used a period as the decimal
- point, so the default behavior was restored to use a period as the
- decimal point character.  You can use the `--use-lc-numeric' option
- (*note Options::) to force `gawk' to use the locale's decimal point
- character.  (`gawk' also uses the locale's decimal point character when
- in POSIX mode, either via `--posix', or the `POSIXLY_CORRECT'
- environment variable.)
- 
-    *note table-locale-affects:: describes the cases in which the
- locale's decimal point character is used and when a period is used.
- Some of these features have not been described yet.
- 
- Feature     Default        `--posix' or `--use-lc-numeric'
- ------------------------------------------------------------ 
- `%'g'       Use locale     Use locale
- `%g'        Use period     Use locale
- Input       Use period     Use locale
- `strtonum()'Use period     Use locale
- 
- Table 6.1: Locale Decimal Point versus A Period
- 
-    Finally, modern day formal standards and IEEE standard floating point
- representation can have an unusual but important effect on the way
- `gawk' converts some special string values to numbers.  The details are
- presented in *note POSIX Floating Point Problems::.
- 
-    ---------- Footnotes ----------
- 
-    (1) Pathological cases can require up to 752 digits (!), but we
- doubt that you need to worry about this.
- 
- 
- File: gawk.info,  Node: All Operators,  Next: Truth Values and Conditions,  
Prev: Values,  Up: Expressions
- 
- 6.2 Operators: Doing Something With Values
- ==========================================
- 
- This minor node introduces the "operators" which make use of the values
- provided by constants and variables.
- 
- * Menu:
- 
- * Arithmetic Ops::              Arithmetic operations (`+', `-',
-                                 etc.)
- * Concatenation::               Concatenating strings.
- * Assignment Ops::              Changing the value of a variable or a field.
- * Increment Ops::               Incrementing the numeric value of a variable.
- 
- 
- File: gawk.info,  Node: Arithmetic Ops,  Next: Concatenation,  Up: All 
Operators
- 
- 6.2.1 Arithmetic Operators
- --------------------------
- 
- The `awk' language uses the common arithmetic operators when evaluating
- expressions.  All of these arithmetic operators follow normal
- precedence rules and work as you would expect them to.
- 
-    The following example uses a file named `grades', which contains a
- list of student names as well as three test scores per student (it's a
- small class):
- 
-      Pat   100 97 58
-      Sandy  84 72 93
-      Chris  72 92 89
- 
- This program takes the file `grades' and prints the average of the
- scores:
- 
-      $ awk '{ sum = $2 + $3 + $4 ; avg = sum / 3
-      >        print $1, avg }' grades
-      -| Pat 85
-      -| Sandy 83
-      -| Chris 84.3333
- 
-    The following list provides the arithmetic operators in `awk', in
- order from the highest precedence to the lowest:
- 
- `- X'
-      Negation.
- 
- `+ X'
-      Unary plus; the expression is converted to a number.
- 
- `X ^ Y'
- `X ** Y'
-      Exponentiation; X raised to the Y power.  `2 ^ 3' has the value
-      eight; the character sequence `**' is equivalent to `^'. (c.e.)
- 
- `X * Y'
-      Multiplication.
- 
- `X / Y'
-      Division;  because all numbers in `awk' are floating-point
-      numbers, the result is _not_ rounded to an integer--`3 / 4' has
-      the value 0.75.  (It is a common mistake, especially for C
-      programmers, to forget that _all_ numbers in `awk' are
-      floating-point, and that division of integer-looking constants
-      produces a real number, not an integer.)
- 
- `X % Y'
-      Remainder; further discussion is provided in the text, just after
-      this list.
- 
- `X + Y'
-      Addition.
- 
- `X - Y'
-      Subtraction.
- 
-    Unary plus and minus have the same precedence, the multiplication
- operators all have the same precedence, and addition and subtraction
- have the same precedence.
- 
-    When computing the remainder of `X % Y', the quotient is rounded
- toward zero to an integer and multiplied by Y. This result is
- subtracted from X; this operation is sometimes known as "trunc-mod."
- The following relation always holds:
- 
-      b * int(a / b) + (a % b) == a
- 
-    One possibly undesirable effect of this definition of remainder is
- that `X % Y' is negative if X is negative.  Thus:
- 
-      -17 % 8 = -1
- 
-    In other `awk' implementations, the signedness of the remainder may
- be machine-dependent.
- 
-      NOTE: The POSIX standard only specifies the use of `^' for
-      exponentiation.  For maximum portability, do not use the `**'
-      operator.
- 
- 
- File: gawk.info,  Node: Concatenation,  Next: Assignment Ops,  Prev: 
Arithmetic Ops,  Up: All Operators
- 
- 6.2.2 String Concatenation
- --------------------------
- 
-      It seemed like a good idea at the time.
-      Brian Kernighan
- 
-    There is only one string operation: concatenation.  It does not have
- a specific operator to represent it.  Instead, concatenation is
- performed by writing expressions next to one another, with no operator.
- For example:
- 
-      $ awk '{ print "Field number one: " $1 }' BBS-list
-      -| Field number one: aardvark
-      -| Field number one: alpo-net
-      ...
- 
-    Without the space in the string constant after the `:', the line
- runs together.  For example:
- 
-      $ awk '{ print "Field number one:" $1 }' BBS-list
-      -| Field number one:aardvark
-      -| Field number one:alpo-net
-      ...
- 
-    Because string concatenation does not have an explicit operator, it
- is often necessary to insure that it happens at the right time by using
- parentheses to enclose the items to concatenate.  For example, you
- might expect that the following code fragment concatenates `file' and
- `name':
- 
-      file = "file"
-      name = "name"
-      print "something meaningful" > file name
- 
- This produces a syntax error with some versions of Unix `awk'.(1) It is
- necessary to use the following:
- 
-      print "something meaningful" > (file name)
- 
-    Parentheses should be used around concatenation in all but the most
- common contexts, such as on the righthand side of `='.  Be careful
- about the kinds of expressions used in string concatenation.  In
- particular, the order of evaluation of expressions used for
- concatenation is undefined in the `awk' language.  Consider this
- example:
- 
-      BEGIN {
-          a = "don't"
-          print (a " " (a = "panic"))
-      }
- 
- It is not defined whether the assignment to `a' happens before or after
- the value of `a' is retrieved for producing the concatenated value.
- The result could be either `don't panic', or `panic panic'.
- 
-    The precedence of concatenation, when mixed with other operators, is
- often counter-intuitive.  Consider this example:
- 
-      $ awk 'BEGIN { print -12 " " -24 }'
-      -| -12-24
- 
-    This "obviously" is concatenating -12, a space, and -24.  But where
- did the space disappear to?  The answer lies in the combination of
- operator precedences and `awk''s automatic conversion rules.  To get
- the desired result, write the program this way:
- 
-      $ awk 'BEGIN { print -12 " " (-24) }'
-      -| -12 -24
- 
-    This forces `awk' to treat the `-' on the `-24' as unary.
- Otherwise, it's parsed as follows:
- 
-          -12 (`" "' - 24)
-      => -12 (0 - 24)
-      => -12 (-24)
-      => -12-24
- 
-    As mentioned earlier, when doing concatenation, _parenthesize_.
- Otherwise, you're never quite sure what you'll get.
- 
-    ---------- Footnotes ----------
- 
-    (1) It happens that Brian Kernighan's `awk', `gawk' and `mawk' all
- "get it right," but you should not rely on this.
- 
- 
- File: gawk.info,  Node: Assignment Ops,  Next: Increment Ops,  Prev: 
Concatenation,  Up: All Operators
- 
- 6.2.3 Assignment Expressions
- ----------------------------
- 
- An "assignment" is an expression that stores a (usually different)
- value into a variable.  For example, let's assign the value one to the
- variable `z':
- 
-      z = 1
- 
-    After this expression is executed, the variable `z' has the value
- one.  Whatever old value `z' had before the assignment is forgotten.
- 
-    Assignments can also store string values.  For example, the
- following stores the value `"this food is good"' in the variable
- `message':
- 
-      thing = "food"
-      predicate = "good"
-      message = "this " thing " is " predicate
- 
- This also illustrates string concatenation.  The `=' sign is called an
- "assignment operator".  It is the simplest assignment operator because
- the value of the righthand operand is stored unchanged.  Most operators
- (addition, concatenation, and so on) have no effect except to compute a
- value.  If the value isn't used, there's no reason to use the operator.
- An assignment operator is different; it does produce a value, but even
- if you ignore it, the assignment still makes itself felt through the
- alteration of the variable.  We call this a "side effect".
- 
-    The lefthand operand of an assignment need not be a variable (*note
- Variables::); it can also be a field (*note Changing Fields::) or an
- array element (*note Arrays::).  These are all called "lvalues", which
- means they can appear on the lefthand side of an assignment operator.
- The righthand operand may be any expression; it produces the new value
- that the assignment stores in the specified variable, field, or array
- element. (Such values are called "rvalues".)
- 
-    It is important to note that variables do _not_ have permanent types.
- A variable's type is simply the type of whatever value it happens to
- hold at the moment.  In the following program fragment, the variable
- `foo' has a numeric value at first, and a string value later on:
- 
-      foo = 1
-      print foo
-      foo = "bar"
-      print foo
- 
- When the second assignment gives `foo' a string value, the fact that it
- previously had a numeric value is forgotten.
- 
-    String values that do not begin with a digit have a numeric value of
- zero. After executing the following code, the value of `foo' is five:
- 
-      foo = "a string"
-      foo = foo + 5
- 
-      NOTE: Using a variable as a number and then later as a string can
-      be confusing and is poor programming style.  The previous two
-      examples illustrate how `awk' works, _not_ how you should write
-      your programs!
- 
-    An assignment is an expression, so it has a value--the same value
- that is assigned.  Thus, `z = 1' is an expression with the value one.
- One consequence of this is that you can write multiple assignments
- together, such as:
- 
-      x = y = z = 5
- 
- This example stores the value five in all three variables (`x', `y',
- and `z').  It does so because the value of `z = 5', which is five, is
- stored into `y' and then the value of `y = z = 5', which is five, is
- stored into `x'.
- 
-    Assignments may be used anywhere an expression is called for.  For
- example, it is valid to write `x != (y = 1)' to set `y' to one, and
- then test whether `x' equals one.  But this style tends to make
- programs hard to read; such nesting of assignments should be avoided,
- except perhaps in a one-shot program.
- 
-    Aside from `=', there are several other assignment operators that do
- arithmetic with the old value of the variable.  For example, the
- operator `+=' computes a new value by adding the righthand value to the
- old value of the variable.  Thus, the following assignment adds five to
- the value of `foo':
- 
-      foo += 5
- 
- This is equivalent to the following:
- 
-      foo = foo + 5
- 
- Use whichever makes the meaning of your program clearer.
- 
-    There are situations where using `+=' (or any assignment operator)
- is _not_ the same as simply repeating the lefthand operand in the
- righthand expression.  For example:
- 
-      # Thanks to Pat Rankin for this example
-      BEGIN  {
-          foo[rand()] += 5
-          for (x in foo)
-             print x, foo[x]
- 
-          bar[rand()] = bar[rand()] + 5
-          for (x in bar)
-             print x, bar[x]
-      }
- 
- The indices of `bar' are practically guaranteed to be different, because
- `rand()' returns different values each time it is called.  (Arrays and
- the `rand()' function haven't been covered yet.  *Note Arrays::, and
- see *note Numeric Functions::, for more information).  This example
- illustrates an important fact about assignment operators: the lefthand
- expression is only evaluated _once_.  It is up to the implementation as
- to which expression is evaluated first, the lefthand or the righthand.
- Consider this example:
- 
-      i = 1
-      a[i += 2] = i + 1
- 
- The value of `a[3]' could be either two or four.
- 
-    *note table-assign-ops:: lists the arithmetic assignment operators.
- In each case, the righthand operand is an expression whose value is
- converted to a number.
- 
- Operator               Effect
- -------------------------------------------------------------------------- 
- LVALUE `+=' INCREMENT  Adds INCREMENT to the value of LVALUE.
- LVALUE `-=' DECREMENT  Subtracts DECREMENT from the value of LVALUE.
- LVALUE `*='            Multiplies the value of LVALUE by COEFFICIENT.
- COEFFICIENT            
- LVALUE `/=' DIVISOR    Divides the value of LVALUE by DIVISOR.
- LVALUE `%=' MODULUS    Sets LVALUE to its remainder by MODULUS.
- LVALUE `^=' POWER      
- LVALUE `**=' POWER     Raises LVALUE to the power POWER. (c.e.)
- 
- Table 6.2: Arithmetic Assignment Operators
- 
-      NOTE: Only the `^=' operator is specified by POSIX.  For maximum
-      portability, do not use the `**=' operator.
- 
- Advanced Notes: Syntactic Ambiguities Between `/=' and Regular Expressions
- --------------------------------------------------------------------------
- 
- There is a syntactic ambiguity between the `/=' assignment operator and
- regexp constants whose first character is an `='.  (d.c.)  This is most
- notable in commercial `awk' versions.  For example:
- 
-      $ awk /==/ /dev/null
-      error--> awk: syntax error at source line 1
-      error-->  context is
-      error-->         >>> /= <<<
-      error--> awk: bailing out at source line 1
- 
- A workaround is:
- 
-      awk '/[=]=/' /dev/null
- 
-    `gawk' does not have this problem, nor do the other freely available
- versions described in *note Other Versions::.
- 
- 
- File: gawk.info,  Node: Increment Ops,  Prev: Assignment Ops,  Up: All 
Operators
- 
- 6.2.4 Increment and Decrement Operators
- ---------------------------------------
- 
- "Increment" and "decrement operators" increase or decrease the value of
- a variable by one.  An assignment operator can do the same thing, so
- the increment operators add no power to the `awk' language; however,
- they are convenient abbreviations for very common operations.
- 
-    The operator used for adding one is written `++'.  It can be used to
- increment a variable either before or after taking its value.  To
- pre-increment a variable `v', write `++v'.  This adds one to the value
- of `v'--that new value is also the value of the expression. (The
- assignment expression `v += 1' is completely equivalent.)  Writing the
- `++' after the variable specifies post-increment.  This increments the
- variable value just the same; the difference is that the value of the
- increment expression itself is the variable's _old_ value.  Thus, if
- `foo' has the value four, then the expression `foo++' has the value
- four, but it changes the value of `foo' to five.  In other words, the
- operator returns the old value of the variable, but with the side
- effect of incrementing it.
- 
-    The post-increment `foo++' is nearly the same as writing `(foo += 1)
- - 1'.  It is not perfectly equivalent because all numbers in `awk' are
- floating-point--in floating-point, `foo + 1 - 1' does not necessarily
- equal `foo'.  But the difference is minute as long as you stick to
- numbers that are fairly small (less than 10e12).
- 
-    Fields and array elements are incremented just like variables.  (Use
- `$(i++)' when you want to do a field reference and a variable increment
- at the same time.  The parentheses are necessary because of the
- precedence of the field reference operator `$'.)
- 
-    The decrement operator `--' works just like `++', except that it
- subtracts one instead of adding it.  As with `++', it can be used before
- the lvalue to pre-decrement or after it to post-decrement.  Following
- is a summary of increment and decrement expressions:
- 
- `++LVALUE'
-      Increment LVALUE, returning the new value as the value of the
-      expression.
- 
- `LVALUE++'
-      Increment LVALUE, returning the _old_ value of LVALUE as the value
-      of the expression.
- 
- `--LVALUE'
-      Decrement LVALUE, returning the new value as the value of the
-      expression.  (This expression is like `++LVALUE', but instead of
-      adding, it subtracts.)
- 
- `LVALUE--'
-      Decrement LVALUE, returning the _old_ value of LVALUE as the value
-      of the expression.  (This expression is like `LVALUE++', but
-      instead of adding, it subtracts.)
- 
- Advanced Notes: Operator Evaluation Order
- -----------------------------------------
- 
-      Doctor, doctor!  It hurts when I do this!
-      So don't do that!
-      Groucho Marx
- 
- What happens for something like the following?
- 
-      b = 6
-      print b += b++
- 
- Or something even stranger?
- 
-      b = 6
-      b += ++b + b++
-      print b
- 
-    In other words, when do the various side effects prescribed by the
- postfix operators (`b++') take effect?  When side effects happen is
- "implementation defined".  In other words, it is up to the particular
- version of `awk'.  The result for the first example may be 12 or 13,
- and for the second, it may be 22 or 23.
- 
-    In short, doing things like this is not recommended and definitely
- not anything that you can rely upon for portability.  You should avoid
- such things in your own programs.
- 
- 
- File: gawk.info,  Node: Truth Values and Conditions,  Next: Function Calls,  
Prev: All Operators,  Up: Expressions
- 
- 6.3 Truth Values and Conditions
- ===============================
- 
- In certain contexts, expression values also serve as "truth values;"
- i.e., they determine what should happen next as the program runs. This
- minor node describes how `awk' defines "true" and "false" and how
- values are compared.
- 
- * Menu:
- 
- * Truth Values::                What is ``true'' and what is ``false''.
- * Typing and Comparison::       How variables acquire types and how this
-                                 affects comparison of numbers and strings with
-                                 `<', etc.
- * Boolean Ops::                 Combining comparison expressions using boolean
-                                 operators `||' (``or''), `&&'
-                                 (``and'') and `!' (``not'').
- * Conditional Exp::             Conditional expressions select between two
-                                 subexpressions under control of a third
-                                 subexpression.
- 
- 
- File: gawk.info,  Node: Truth Values,  Next: Typing and Comparison,  Up: 
Truth Values and Conditions
- 
- 6.3.1 True and False in `awk'
- -----------------------------
- 
- Many programming languages have a special representation for the
- concepts of "true" and "false."  Such languages usually use the special
- constants `true' and `false', or perhaps their uppercase equivalents.
- However, `awk' is different.  It borrows a very simple concept of true
- and false from C.  In `awk', any nonzero numeric value _or_ any
- nonempty string value is true.  Any other value (zero or the null
- string, `""') is false.  The following program prints `A strange truth
- value' three times:
- 
-      BEGIN {
-         if (3.1415927)
-             print "A strange truth value"
-         if ("Four Score And Seven Years Ago")
-             print "A strange truth value"
-         if (j = 57)
-             print "A strange truth value"
-      }
- 
-    There is a surprising consequence of the "nonzero or non-null" rule:
- the string constant `"0"' is actually true, because it is non-null.
- (d.c.)
- 
- 
- File: gawk.info,  Node: Typing and Comparison,  Next: Boolean Ops,  Prev: 
Truth Values,  Up: Truth Values and Conditions
- 
- 6.3.2 Variable Typing and Comparison Expressions
- ------------------------------------------------
- 
-      The Guide is definitive. Reality is frequently inaccurate.
-      The Hitchhiker's Guide to the Galaxy
- 
-    Unlike other programming languages, `awk' variables do not have a
- fixed type. Instead, they can be either a number or a string, depending
- upon the value that is assigned to them.  We look now at how variables
- are typed, and how `awk' compares variables.
- 
- * Menu:
- 
- * Variable Typing::             String type versus numeric type.
- * Comparison Operators::        The comparison operators.
- * POSIX String Comparison::     String comparison with POSIX rules.
- 
- 
- File: gawk.info,  Node: Variable Typing,  Next: Comparison Operators,  Up: 
Typing and Comparison
- 
- 6.3.2.1 String Type Versus Numeric Type
- .......................................
- 
- The 1992 POSIX standard introduced the concept of a "numeric string",
- which is simply a string that looks like a number--for example,
- `" +2"'.  This concept is used for determining the type of a variable.
- The type of the variable is important because the types of two variables
- determine how they are compared.  The various versions of the POSIX
- standard did not get the rules quite right for several editions.
- Fortunately, as of at least the 2008 standard (and possibly earlier),
- the standard has been fixed, and variable typing follows these rules:(1)
- 
-    * A numeric constant or the result of a numeric operation has the
-      NUMERIC attribute.
- 
-    * A string constant or the result of a string operation has the
-      STRING attribute.
- 
-    * Fields, `getline' input, `FILENAME', `ARGV' elements, `ENVIRON'
-      elements, and the elements of an array created by `patsplit()',
-      `split()' and `match()' that are numeric strings have the STRNUM
-      attribute.  Otherwise, they have the STRING attribute.
-      Uninitialized variables also have the STRNUM attribute.
- 
-    * Attributes propagate across assignments but are not changed by any
-      use.
- 
-    The last rule is particularly important. In the following program,
- `a' has numeric type, even though it is later used in a string
- operation:
- 
-      BEGIN {
-           a = 12.345
-           b = a " is a cute number"
-           print b
-      }
- 
-    When two operands are compared, either string comparison or numeric
- comparison may be used. This depends upon the attributes of the
- operands, according to the following symmetric matrix:
- 
-              +---------------------------------------------
-              |       STRING          NUMERIC         STRNUM
-      -------+---------------------------------------------
-              |
-      STRING  |       string          string          string
-              |
-      NUMERIC |       string          numeric         numeric
-              |
-      STRNUM  |       string          numeric         numeric
-      -------+---------------------------------------------
- 
-    The basic idea is that user input that looks numeric--and _only_
- user input--should be treated as numeric, even though it is actually
- made of characters and is therefore also a string.  Thus, for example,
- the string constant `" +3.14"', when it appears in program source code,
- is a string--even though it looks numeric--and is _never_ treated as
- number for comparison purposes.
- 
-    In short, when one operand is a "pure" string, such as a string
- constant, then a string comparison is performed.  Otherwise, a numeric
- comparison is performed.
- 
-    This point bears additional emphasis: All user input is made of
- characters, and so is first and foremost of STRING type; input strings
- that look numeric are additionally given the STRNUM attribute.  Thus,
- the six-character input string ` +3.14' receives the STRNUM attribute.
- In contrast, the eight-character literal `" +3.14"' appearing in
- program text is a string constant.  The following examples print `1'
- when the comparison between the two different constants is true, `0'
- otherwise:
- 
-      $ echo ' +3.14' | gawk '{ print $0 == " +3.14" }'    True
-      -| 1
-      $ echo ' +3.14' | gawk '{ print $0 == "+3.14" }'     False
-      -| 0
-      $ echo ' +3.14' | gawk '{ print $0 == "3.14" }'      False
-      -| 0
-      $ echo ' +3.14' | gawk '{ print $0 == 3.14 }'        True
-      -| 1
-      $ echo ' +3.14' | gawk '{ print $1 == " +3.14" }'    False
-      -| 0
-      $ echo ' +3.14' | gawk '{ print $1 == "+3.14" }'     True
-      -| 1
-      $ echo ' +3.14' | gawk '{ print $1 == "3.14" }'      False
-      -| 0
-      $ echo ' +3.14' | gawk '{ print $1 == 3.14 }'        True
-      -| 1
- 
-    ---------- Footnotes ----------
- 
-    (1) `gawk' has followed these rules for many years, and it is
- gratifying that the POSIX standard is also now correct.
- 
- 
- File: gawk.info,  Node: Comparison Operators,  Next: POSIX String Comparison, 
 Prev: Variable Typing,  Up: Typing and Comparison
- 
- 6.3.2.2 Comparison Operators
- ............................
- 
- "Comparison expressions" compare strings or numbers for relationships
- such as equality.  They are written using "relational operators", which
- are a superset of those in C.  *note table-relational-ops:: describes
- them.
- 
- Expression         Result
- -------------------------------------------------------------------------- 
- X `<' Y            True if X is less than Y.
- X `<=' Y           True if X is less than or equal to Y.
- X `>' Y            True if X is greater than Y.
- X `>=' Y           True if X is greater than or equal to Y.
- X `==' Y           True if X is equal to Y.
- X `!=' Y           True if X is not equal to Y.
- X `~' Y            True if the string X matches the regexp denoted by Y.
- X `!~' Y           True if the string X does not match the regexp
-                    denoted by Y.
- SUBSCRIPT `in'     True if the array ARRAY has an element with the
- ARRAY              subscript SUBSCRIPT.
- 
- Table 6.3: Relational Operators
- 
-    Comparison expressions have the value one if true and zero if false.
- When comparing operands of mixed types, numeric operands are converted
- to strings using the value of `CONVFMT' (*note Conversion::).
- 
-    Strings are compared by comparing the first character of each, then
- the second character of each, and so on.  Thus, `"10"' is less than
- `"9"'.  If there are two strings where one is a prefix of the other,
- the shorter string is less than the longer one.  Thus, `"abc"' is less
- than `"abcd"'.
- 
-    It is very easy to accidentally mistype the `==' operator and leave
- off one of the `=' characters.  The result is still valid `awk' code,
- but the program does not do what is intended:
- 
-      if (a = b)   # oops! should be a == b
-         ...
-      else
-         ...
- 
- Unless `b' happens to be zero or the null string, the `if' part of the
- test always succeeds.  Because the operators are so similar, this kind
- of error is very difficult to spot when scanning the source code.
- 
-    The following table of expressions illustrates the kind of comparison
- `gawk' performs, as well as what the result of the comparison is:
- 
- `1.5 <= 2.0'
-      numeric comparison (true)
- 
- `"abc" >= "xyz"'
-      string comparison (false)
- 
- `1.5 != " +2"'
-      string comparison (true)
- 
- `"1e2" < "3"'
-      string comparison (true)
- 
- `a = 2; b = "2"'
- `a == b'
-      string comparison (true)
- 
- `a = 2; b = " +2"'
- 
- `a == b'
-      string comparison (false)
- 
-    In this example:
- 
-      $ echo 1e2 3 | awk '{ print ($1 < $2) ? "true" : "false" }'
-      -| false
- 
- the result is `false' because both `$1' and `$2' are user input.  They
- are numeric strings--therefore both have the STRNUM attribute,
- dictating a numeric comparison.  The purpose of the comparison rules
- and the use of numeric strings is to attempt to produce the behavior
- that is "least surprising," while still "doing the right thing."
- 
-    String comparisons and regular expression comparisons are very
- different.  For example:
- 
-      x == "foo"
- 
- has the value one, or is true if the variable `x' is precisely `foo'.
- By contrast:
- 
-      x ~ /foo/
- 
- has the value one if `x' contains `foo', such as `"Oh, what a fool am
- I!"'.
- 
-    The righthand operand of the `~' and `!~' operators may be either a
- regexp constant (`/.../') or an ordinary expression. In the latter
- case, the value of the expression as a string is used as a dynamic
- regexp (*note Regexp Usage::; also *note Computed Regexps::).
- 
-    In modern implementations of `awk', a constant regular expression in
- slashes by itself is also an expression.  The regexp `/REGEXP/' is an
- abbreviation for the following comparison expression:
- 
-      $0 ~ /REGEXP/
- 
-    One special place where `/foo/' is _not_ an abbreviation for `$0 ~
- /foo/' is when it is the righthand operand of `~' or `!~'.  *Note Using
- Constant Regexps::, where this is discussed in more detail.
- 
- 
- File: gawk.info,  Node: POSIX String Comparison,  Prev: Comparison Operators, 
 Up: Typing and Comparison
- 
- 6.3.2.3 String Comparison With POSIX Rules
- ..........................................
- 
- The POSIX standard says that string comparison is performed based on
- the locale's collating order.  This is usually very different from the
- results obtained when doing straight character-by-character
- comparison.(1)
- 
-    Because this behavior differs considerably from existing practice,
- `gawk' only implements it when in POSIX mode (*note Options::).  Here
- is an example to illustrate the difference, in an `en_US.UTF-8' locale:
- 
-      $ gawk 'BEGIN { printf("ABC < abc = %s\n",
-      >                     ("ABC" < "abc" ? "TRUE" : "FALSE")) }'
-      -| ABC < abc = TRUE
-      $ gawk --posix 'BEGIN { printf("ABC < abc = %s\n",
-      >                             ("ABC" < "abc" ? "TRUE" : "FALSE")) }'
-      -| ABC < abc = FALSE
- 
-    ---------- Footnotes ----------
- 
-    (1) Technically, string comparison is supposed to behave the same
- way as if the strings are compared with the C `strcoll()' function.
- 
- 
- File: gawk.info,  Node: Boolean Ops,  Next: Conditional Exp,  Prev: Typing 
and Comparison,  Up: Truth Values and Conditions
- 
- 6.3.3 Boolean Expressions
- -------------------------
- 
- A "Boolean expression" is a combination of comparison expressions or
- matching expressions, using the Boolean operators "or" (`||'), "and"
- (`&&'), and "not" (`!'), along with parentheses to control nesting.
- The truth value of the Boolean expression is computed by combining the
- truth values of the component expressions.  Boolean expressions are
- also referred to as "logical expressions".  The terms are equivalent.
- 
-    Boolean expressions can be used wherever comparison and matching
- expressions can be used.  They can be used in `if', `while', `do', and
- `for' statements (*note Statements::).  They have numeric values (one
- if true, zero if false) that come into play if the result of the
- Boolean expression is stored in a variable or used in arithmetic.
- 
-    In addition, every Boolean expression is also a valid pattern, so
- you can use one as a pattern to control the execution of rules.  The
- Boolean operators are:
- 
- `BOOLEAN1 && BOOLEAN2'
-      True if both BOOLEAN1 and BOOLEAN2 are true.  For example, the
-      following statement prints the current input record if it contains
-      both `2400' and `foo':
- 
-           if ($0 ~ /2400/ && $0 ~ /foo/) print
- 
-      The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is true.
-      This can make a difference when BOOLEAN2 contains expressions that
-      have side effects. In the case of `$0 ~ /foo/ && ($2 == bar++)',
-      the variable `bar' is not incremented if there is no substring
-      `foo' in the record.
- 
- `BOOLEAN1 || BOOLEAN2'
-      True if at least one of BOOLEAN1 or BOOLEAN2 is true.  For
-      example, the following statement prints all records in the input
-      that contain _either_ `2400' or `foo' or both:
- 
-           if ($0 ~ /2400/ || $0 ~ /foo/) print
- 
-      The subexpression BOOLEAN2 is evaluated only if BOOLEAN1 is false.
-      This can make a difference when BOOLEAN2 contains expressions that
-      have side effects.
- 
- `! BOOLEAN'
-      True if BOOLEAN is false.  For example, the following program
-      prints `no home!' in the unusual event that the `HOME' environment
-      variable is not defined:
- 
-           BEGIN { if (! ("HOME" in ENVIRON))
-                          print "no home!" }
- 
-      (The `in' operator is described in *note Reference to Elements::.)
- 
-    The `&&' and `||' operators are called "short-circuit" operators
- because of the way they work.  Evaluation of the full expression is
- "short-circuited" if the result can be determined part way through its
- evaluation.
- 
-    Statements that use `&&' or `||' can be continued simply by putting
- a newline after them.  But you cannot put a newline in front of either
- of these operators without using backslash continuation (*note
- Statements/Lines::).
- 
-    The actual value of an expression using the `!' operator is either
- one or zero, depending upon the truth value of the expression it is
- applied to.  The `!' operator is often useful for changing the sense of
- a flag variable from false to true and back again. For example, the
- following program is one way to print lines in between special
- bracketing lines:
- 
-      $1 == "START"   { interested = ! interested; next }
-      interested == 1 { print }
-      $1 == "END"     { interested = ! interested; next }
- 
- The variable `interested', as with all `awk' variables, starts out
- initialized to zero, which is also false.  When a line is seen whose
- first field is `START', the value of `interested' is toggled to true,
- using `!'. The next rule prints lines as long as `interested' is true.
- When a line is seen whose first field is `END', `interested' is toggled
- back to false.(1)
- 
-      NOTE: The `next' statement is discussed in *note Next Statement::.
-      `next' tells `awk' to skip the rest of the rules, get the next
-      record, and start processing the rules over again at the top.  The
-      reason it's there is to avoid printing the bracketing `START' and
-      `END' lines.
- 
-    ---------- Footnotes ----------
- 
-    (1) This program has a bug; it prints lines starting with `END'. How
- would you fix it?
- 
- 
- File: gawk.info,  Node: Conditional Exp,  Prev: Boolean Ops,  Up: Truth 
Values and Conditions
- 
- 6.3.4 Conditional Expressions
- -----------------------------
- 
- A "conditional expression" is a special kind of expression that has
- three operands.  It allows you to use one expression's value to select
- one of two other expressions.  The conditional expression is the same
- as in the C language, as shown here:
- 
-      SELECTOR ? IF-TRUE-EXP : IF-FALSE-EXP
- 
- There are three subexpressions.  The first, SELECTOR, is always
- computed first.  If it is "true" (not zero or not null), then
- IF-TRUE-EXP is computed next and its value becomes the value of the
- whole expression.  Otherwise, IF-FALSE-EXP is computed next and its
- value becomes the value of the whole expression.  For example, the
- following expression produces the absolute value of `x':
- 
-      x >= 0 ? x : -x
- 
-    Each time the conditional expression is computed, only one of
- IF-TRUE-EXP and IF-FALSE-EXP is used; the other is ignored.  This is
- important when the expressions have side effects.  For example, this
- conditional expression examines element `i' of either array `a' or
- array `b', and increments `i':
- 
-      x == y ? a[i++] : b[i++]
- 
- This is guaranteed to increment `i' exactly once, because each time
- only one of the two increment expressions is executed and the other is
- not.  *Note Arrays::, for more information about arrays.
- 
-    As a minor `gawk' extension, a statement that uses `?:' can be
- continued simply by putting a newline after either character.  However,
- putting a newline in front of either character does not work without
- using backslash continuation (*note Statements/Lines::).  If `--posix'
- is specified (*note Options::), then this extension is disabled.
- 
- 
- File: gawk.info,  Node: Function Calls,  Next: Precedence,  Prev: Truth 
Values and Conditions,  Up: Expressions
- 
- 6.4 Function Calls
- ==================
- 
- A "function" is a name for a particular calculation.  This enables you
- to ask for it by name at any point in the program.  For example, the
- function `sqrt()' computes the square root of a number.
- 
-    A fixed set of functions are "built-in", which means they are
- available in every `awk' program.  The `sqrt()' function is one of
- these.  *Note Built-in::, for a list of built-in functions and their
- descriptions.  In addition, you can define functions for use in your
- program.  *Note User-defined::, for instructions on how to do this.
- 
-    The way to use a function is with a "function call" expression,
- which consists of the function name followed immediately by a list of
- "arguments" in parentheses.  The arguments are expressions that provide
- the raw materials for the function's calculations.  When there is more
- than one argument, they are separated by commas.  If there are no
- arguments, just write `()' after the function name.  The following
- examples show function calls with and without arguments:
- 
-      sqrt(x^2 + y^2)        one argument
-      atan2(y, x)            two arguments
-      rand()                 no arguments
- 
-      CAUTION: Do not put any space between the function name and the
-      open-parenthesis!  A user-defined function name looks just like
-      the name of a variable--a space would make the expression look
-      like concatenation of a variable with an expression inside
-      parentheses.  With built-in functions, space before the
-      parenthesis is harmless, but it is best not to get into the habit
-      of using space to avoid mistakes with user-defined functions.
- 
-    Each function expects a particular number of arguments.  For
- example, the `sqrt()' function must be called with a single argument,
- the number of which to take the square root:
- 
-      sqrt(ARGUMENT)
- 
-    Some of the built-in functions have one or more optional arguments.
- If those arguments are not supplied, the functions use a reasonable
- default value.  *Note Built-in::, for full details.  If arguments are
- omitted in calls to user-defined functions, then those arguments are
- treated as local variables and initialized to the empty string (*note
- User-defined::).
- 
-    As an advanced feature, `gawk' provides indirect function calls,
- which is a way to choose the function to call at runtime, instead of
- when you write the source code to your program. We defer discussion of
- this feature until later; see *note Indirect Calls::.
- 
-    Like every other expression, the function call has a value, which is
- computed by the function based on the arguments you give it.  In this
- example, the value of `sqrt(ARGUMENT)' is the square root of ARGUMENT.
- The following program reads numbers, one number per line, and prints the
- square root of each one:
- 
-      $ awk '{ print "The square root of", $1, "is", sqrt($1) }'
-      1
-      -| The square root of 1 is 1
-      3
-      -| The square root of 3 is 1.73205
-      5
-      -| The square root of 5 is 2.23607
-      Ctrl-d
- 
-    A function can also have side effects, such as assigning values to
- certain variables or doing I/O.  This program shows how the `match()'
- function (*note String Functions::) changes the variables `RSTART' and
- `RLENGTH':
- 
-      {
-          if (match($1, $2))
-              print RSTART, RLENGTH
-          else
-              print "no match"
-      }
- 
- Here is a sample run:
- 
-      $ awk -f matchit.awk
-      aaccdd  c+
-      -| 3 2
-      foo     bar
-      -| no match
-      abcdefg e
-      -| 5 1
- 
- 
- File: gawk.info,  Node: Precedence,  Next: Locales,  Prev: Function Calls,  
Up: Expressions
- 
- 6.5 Operator Precedence (How Operators Nest)
- ============================================
- 
- "Operator precedence" determines how operators are grouped when
- different operators appear close by in one expression.  For example,
- `*' has higher precedence than `+'; thus, `a + b * c' means to multiply
- `b' and `c', and then add `a' to the product (i.e., `a + (b * c)').
- 
-    The normal precedence of the operators can be overruled by using
- parentheses.  Think of the precedence rules as saying where the
- parentheses are assumed to be.  In fact, it is wise to always use
- parentheses whenever there is an unusual combination of operators,
- because other people who read the program may not remember what the
- precedence is in this case.  Even experienced programmers occasionally
- forget the exact rules, which leads to mistakes.  Explicit parentheses
- help prevent any such mistakes.
- 
-    When operators of equal precedence are used together, the leftmost
- operator groups first, except for the assignment, conditional, and
- exponentiation operators, which group in the opposite order.  Thus, `a
- - b + c' groups as `(a - b) + c' and `a = b = c' groups as `a = (b =
- c)'.
- 
-    Normally the precedence of prefix unary operators does not matter,
- because there is only one way to interpret them: innermost first.
- Thus, `$++i' means `$(++i)' and `++$x' means `++($x)'.  However, when
- another operator follows the operand, then the precedence of the unary
- operators can matter.  `$x^2' means `($x)^2', but `-x^2' means
- `-(x^2)', because `-' has lower precedence than `^', whereas `$' has
- higher precedence.  Also, operators cannot be combined in a way that
- violates the precedence rules; for example, `$$0++--' is not a valid
- expression because the first `$' has higher precedence than the `++';
- to avoid the problem the expression can be rewritten as `$($0++)--'.
- 
-    This table presents `awk''s operators, in order of highest to lowest
- precedence:
- 
- `(...)'
-      Grouping.
- 
- `$'
-      Field reference.
- 
- `++ --'
-      Increment, decrement.
- 
- `^ **'
-      Exponentiation.  These operators group right-to-left.
- 
- `+ - !'
-      Unary plus, minus, logical "not."
- 
- `* / %'
-      Multiplication, division, remainder.
- 
- `+ -'
-      Addition, subtraction.
- 
- `String Concatenation'
-      There is no special symbol for concatenation.  The operands are
-      simply written side by side (*note Concatenation::).
- 
- `< <= == != > >= >> | |&'
-      Relational and redirection.  The relational operators and the
-      redirections have the same precedence level.  Characters such as
-      `>' serve both as relationals and as redirections; the context
-      distinguishes between the two meanings.
- 
-      Note that the I/O redirection operators in `print' and `printf'
-      statements belong to the statement level, not to expressions.  The
-      redirection does not produce an expression that could be the
-      operand of another operator.  As a result, it does not make sense
-      to use a redirection operator near another operator of lower
-      precedence without parentheses.  Such combinations (for example,
-      `print foo > a ? b : c'), result in syntax errors.  The correct
-      way to write this statement is `print foo > (a ? b : c)'.
- 
- `~ !~'
-      Matching, nonmatching.
- 
- `in'
-      Array membership.
- 
- `&&'
-      Logical "and".
- 
- `||'
-      Logical "or".
- 
- `?:'
-      Conditional.  This operator groups right-to-left.
- 
- `= += -= *= /= %= ^= **='
-      Assignment.  These operators group right-to-left.
- 
-      NOTE: The `|&', `**', and `**=' operators are not specified by
-      POSIX.  For maximum portability, do not use them.
- 
- 
- File: gawk.info,  Node: Locales,  Prev: Precedence,  Up: Expressions
- 
- 6.6 Where You Are Makes A Difference
- ====================================
- 
- Modern systems support the notion of "locales": a way to tell the
- system about the local character set and language.
- 
-    Once upon a time, the locale setting used to affect regexp matching
- (*note Ranges and Locales::), but this is no longer true.
- 
-    Locales can affect record splitting.  For the normal case of `RS =
- "\n"', the locale is largely irrelevant.  For other single-character
- record separators, setting `LC_ALL=C' in the environment will give you
- much better performance when reading records.  Otherwise, `gawk' has to
- make several function calls, _per input character_, to find the record
- terminator.
- 
-    According to POSIX, string comparison is also affected by locales
- (similar to regular expressions).  The details are presented in *note
- POSIX String Comparison::.
- 
-    Finally, the locale affects the value of the decimal point character
- used when `gawk' parses input data.  This is discussed in detail in
- *note Conversion::.
- 
- 
- File: gawk.info,  Node: Patterns and Actions,  Next: Arrays,  Prev: 
Expressions,  Up: Top
- 
- 7 Patterns, Actions, and Variables
- **********************************
- 
- As you have already seen, each `awk' statement consists of a pattern
- with an associated action.  This major node describes how you build
- patterns and actions, what kinds of things you can do within actions,
- and `awk''s built-in variables.
- 
-    The pattern-action rules and the statements available for use within
- actions form the core of `awk' programming.  In a sense, everything
- covered up to here has been the foundation that programs are built on
- top of.  Now it's time to start building something useful.
- 
- * Menu:
- 
- * Pattern Overview::            What goes into a pattern.
- * Using Shell Variables::       How to use shell variables with `awk'.
- * Action Overview::             What goes into an action.
- * Statements::                  Describes the various control statements in
-                                 detail.
- * Built-in Variables::          Summarizes the built-in variables.
- 
- 
- File: gawk.info,  Node: Pattern Overview,  Next: Using Shell Variables,  Up: 
Patterns and Actions
- 
- 7.1 Pattern Elements
- ====================
- 
- * Menu:
- 
- * Regexp Patterns::             Using regexps as patterns.
- * Expression Patterns::         Any expression can be used as a pattern.
- * Ranges::                      Pairs of patterns specify record ranges.
- * BEGIN/END::                   Specifying initialization and cleanup rules.
- * BEGINFILE/ENDFILE::           Two special patterns for advanced control.
- * Empty::                       The empty pattern, which matches every record.
- 
-    Patterns in `awk' control the execution of rules--a rule is executed
- when its pattern matches the current input record.  The following is a
- summary of the types of `awk' patterns:
- 
- `/REGULAR EXPRESSION/'
-      A regular expression. It matches when the text of the input record
-      fits the regular expression.  (*Note Regexp::.)
- 
- `EXPRESSION'
-      A single expression.  It matches when its value is nonzero (if a
-      number) or non-null (if a string).  (*Note Expression Patterns::.)
- 
- `PAT1, PAT2'
-      A pair of patterns separated by a comma, specifying a range of
-      records.  The range includes both the initial record that matches
-      PAT1 and the final record that matches PAT2.  (*Note Ranges::.)
- 
- `BEGIN'
- `END'
-      Special patterns for you to supply startup or cleanup actions for
-      your `awk' program.  (*Note BEGIN/END::.)
- 
- `BEGINFILE'
- `ENDFILE'
-      Special patterns for you to supply startup or cleanup actions to be
-      done on a per file basis.  (*Note BEGINFILE/ENDFILE::.)
- 
- `EMPTY'
-      The empty pattern matches every input record.  (*Note Empty::.)
- 
- 
- File: gawk.info,  Node: Regexp Patterns,  Next: Expression Patterns,  Up: 
Pattern Overview
- 
- 7.1.1 Regular Expressions as Patterns
- -------------------------------------
- 
- Regular expressions are one of the first kinds of patterns presented in
- this book.  This kind of pattern is simply a regexp constant in the
- pattern part of a rule.  Its  meaning is `$0 ~ /PATTERN/'.  The pattern
- matches when the input record matches the regexp.  For example:
- 
-      /foo|bar|baz/  { buzzwords++ }
-      END            { print buzzwords, "buzzwords seen" }
- 
- 
- File: gawk.info,  Node: Expression Patterns,  Next: Ranges,  Prev: Regexp 
Patterns,  Up: Pattern Overview
- 
- 7.1.2 Expressions as Patterns
- -----------------------------
- 
- Any `awk' expression is valid as an `awk' pattern.  The pattern matches
- if the expression's value is nonzero (if a number) or non-null (if a
- string).  The expression is reevaluated each time the rule is tested
- against a new input record.  If the expression uses fields such as
- `$1', the value depends directly on the new input record's text;
- otherwise, it depends on only what has happened so far in the execution
- of the `awk' program.
- 
-    Comparison expressions, using the comparison operators described in
- *note Typing and Comparison::, are a very common kind of pattern.
- Regexp matching and nonmatching are also very common expressions.  The
- left operand of the `~' and `!~' operators is a string.  The right
- operand is either a constant regular expression enclosed in slashes
- (`/REGEXP/'), or any expression whose string value is used as a dynamic
- regular expression (*note Computed Regexps::).  The following example
- prints the second field of each input record whose first field is
- precisely `foo':
- 
-      $ awk '$1 == "foo" { print $2 }' BBS-list
- 
- (There is no output, because there is no BBS site with the exact name
- `foo'.)  Contrast this with the following regular expression match,
- which accepts any record with a first field that contains `foo':
- 
-      $ awk '$1 ~ /foo/ { print $2 }' BBS-list
-      -| 555-1234
-      -| 555-6699
-      -| 555-6480
-      -| 555-2127
- 
-    A regexp constant as a pattern is also a special case of an
- expression pattern.  The expression `/foo/' has the value one if `foo'
- appears in the current input record. Thus, as a pattern, `/foo/'
- matches any record containing `foo'.
- 
-    Boolean expressions are also commonly used as patterns.  Whether the
- pattern matches an input record depends on whether its subexpressions
- match.  For example, the following command prints all the records in
- `BBS-list' that contain both `2400' and `foo':
- 
-      $ awk '/2400/ && /foo/' BBS-list
-      -| fooey        555-1234     2400/1200/300     B
- 
-    The following command prints all records in `BBS-list' that contain
- _either_ `2400' or `foo' (or both, of course):
- 
-      $ awk '/2400/ || /foo/' BBS-list
-      -| alpo-net     555-3412     2400/1200/300     A
-      -| bites        555-1675     2400/1200/300     A
-      -| fooey        555-1234     2400/1200/300     B
-      -| foot         555-6699     1200/300          B
-      -| macfoo       555-6480     1200/300          A
-      -| sdace        555-3430     2400/1200/300     A
-      -| sabafoo      555-2127     1200/300          C
- 
-    The following command prints all records in `BBS-list' that do _not_
- contain the string `foo':
- 
-      $ awk '! /foo/' BBS-list
-      -| aardvark     555-5553     1200/300          B
-      -| alpo-net     555-3412     2400/1200/300     A
-      -| barfly       555-7685     1200/300          A
-      -| bites        555-1675     2400/1200/300     A
-      -| camelot      555-0542     300               C
-      -| core         555-2912     1200/300          C
-      -| sdace        555-3430     2400/1200/300     A
- 
-    The subexpressions of a Boolean operator in a pattern can be
- constant regular expressions, comparisons, or any other `awk'
- expressions.  Range patterns are not expressions, so they cannot appear
- inside Boolean patterns.  Likewise, the special patterns `BEGIN', `END',
- `BEGINFILE' and `ENDFILE', which never match any input record, are not
- expressions and cannot appear inside Boolean patterns.
- 
-    The precedence of the different operators which can appear in
- patterns is described in *note Precedence::.
- 
- 
- File: gawk.info,  Node: Ranges,  Next: BEGIN/END,  Prev: Expression Patterns, 
 Up: Pattern Overview
- 
- 7.1.3 Specifying Record Ranges with Patterns
- --------------------------------------------
- 
- A "range pattern" is made of two patterns separated by a comma, in the
- form `BEGPAT, ENDPAT'.  It is used to match ranges of consecutive input
- records.  The first pattern, BEGPAT, controls where the range begins,
- while ENDPAT controls where the pattern ends.  For example, the
- following:
- 
-      awk '$1 == "on", $1 == "off"' myfile
- 
- prints every record in `myfile' between `on'/`off' pairs, inclusive.
- 
-    A range pattern starts out by matching BEGPAT against every input
- record.  When a record matches BEGPAT, the range pattern is "turned on"
- and the range pattern matches this record as well.  As long as the
- range pattern stays turned on, it automatically matches every input
- record read.  The range pattern also matches ENDPAT against every input
- record; when this succeeds, the range pattern is turned off again for
- the following record.  Then the range pattern goes back to checking
- BEGPAT against each record.
- 
-    The record that turns on the range pattern and the one that turns it
- off both match the range pattern.  If you don't want to operate on
- these records, you can write `if' statements in the rule's action to
- distinguish them from the records you are interested in.
- 
-    It is possible for a pattern to be turned on and off by the same
- record. If the record satisfies both conditions, then the action is
- executed for just that record.  For example, suppose there is text
- between two identical markers (e.g., the `%' symbol), each on its own
- line, that should be ignored.  A first attempt would be to combine a
- range pattern that describes the delimited text with the `next'
- statement (not discussed yet, *note Next Statement::).  This causes
- `awk' to skip any further processing of the current record and start
- over again with the next input record. Such a program looks like this:
- 
-      /^%$/,/^%$/    { next }
-                     { print }
- 
- This program fails because the range pattern is both turned on and
- turned off by the first line, which just has a `%' on it.  To
- accomplish this task, write the program in the following manner, using
- a flag:
- 
-      /^%$/     { skip = ! skip; next }
-      skip == 1 { next } # skip lines with `skip' set
- 
-    In a range pattern, the comma (`,') has the lowest precedence of all
- the operators (i.e., it is evaluated last).  Thus, the following
- program attempts to combine a range pattern with another, simpler test:
- 
-      echo Yes | awk '/1/,/2/ || /Yes/'
- 
-    The intent of this program is `(/1/,/2/) || /Yes/'.  However, `awk'
- interprets this as `/1/, (/2/ || /Yes/)'.  This cannot be changed or
- worked around; range patterns do not combine with other patterns:
- 
-      $ echo Yes | gawk '(/1/,/2/) || /Yes/'
-      error--> gawk: cmd. line:1: (/1/,/2/) || /Yes/
-      error--> gawk: cmd. line:1:           ^ syntax error
- 
- 
- File: gawk.info,  Node: BEGIN/END,  Next: BEGINFILE/ENDFILE,  Prev: Ranges,  
Up: Pattern Overview
- 
- 7.1.4 The `BEGIN' and `END' Special Patterns
- --------------------------------------------
- 
- All the patterns described so far are for matching input records.  The
- `BEGIN' and `END' special patterns are different.  They supply startup
- and cleanup actions for `awk' programs.  `BEGIN' and `END' rules must
- have actions; there is no default action for these rules because there
- is no current record when they run.  `BEGIN' and `END' rules are often
- referred to as "`BEGIN' and `END' blocks" by long-time `awk'
- programmers.
- 
- * Menu:
- 
- * Using BEGIN/END::             How and why to use BEGIN/END rules.
- * I/O And BEGIN/END::           I/O issues in BEGIN/END rules.
- 
- 
- File: gawk.info,  Node: Using BEGIN/END,  Next: I/O And BEGIN/END,  Up: 
BEGIN/END
- 
- 7.1.4.1 Startup and Cleanup Actions
- ...................................
- 
- A `BEGIN' rule is executed once only, before the first input record is
- read. Likewise, an `END' rule is executed once only, after all the
- input is read.  For example:
- 
-      $ awk '
-      > BEGIN { print "Analysis of \"foo\"" }
-      > /foo/ { ++n }
-      > END   { print "\"foo\" appears", n, "times." }' BBS-list
-      -| Analysis of "foo"
-      -| "foo" appears 4 times.
- 
-    This program finds the number of records in the input file `BBS-list'
- that contain the string `foo'.  The `BEGIN' rule prints a title for the
- report.  There is no need to use the `BEGIN' rule to initialize the
- counter `n' to zero, since `awk' does this automatically (*note
- Variables::).  The second rule increments the variable `n' every time a
- record containing the pattern `foo' is read.  The `END' rule prints the
- value of `n' at the end of the run.
- 
-    The special patterns `BEGIN' and `END' cannot be used in ranges or
- with Boolean operators (indeed, they cannot be used with any operators).
- An `awk' program may have multiple `BEGIN' and/or `END' rules.  They
- are executed in the order in which they appear: all the `BEGIN' rules
- at startup and all the `END' rules at termination.  `BEGIN' and `END'
- rules may be intermixed with other rules.  This feature was added in
- the 1987 version of `awk' and is included in the POSIX standard.  The
- original (1978) version of `awk' required the `BEGIN' rule to be placed
- at the beginning of the program, the `END' rule to be placed at the
- end, and only allowed one of each.  This is no longer required, but it
- is a good idea to follow this template in terms of program organization
- and readability.
- 
-    Multiple `BEGIN' and `END' rules are useful for writing library
- functions, because each library file can have its own `BEGIN' and/or
- `END' rule to do its own initialization and/or cleanup.  The order in
- which library functions are named on the command line controls the
- order in which their `BEGIN' and `END' rules are executed.  Therefore,
- you have to be careful when writing such rules in library files so that
- the order in which they are executed doesn't matter.  *Note Options::,
- for more information on using library functions.  *Note Library
- Functions::, for a number of useful library functions.
- 
-    If an `awk' program has only `BEGIN' rules and no other rules, then
- the program exits after the `BEGIN' rule is run.(1)  However, if an
- `END' rule exists, then the input is read, even if there are no other
- rules in the program.  This is necessary in case the `END' rule checks
- the `FNR' and `NR' variables.
- 
-    ---------- Footnotes ----------
- 
-    (1) The original version of `awk' kept reading and ignoring input
- until the end of the file was seen.
- 
- 
- File: gawk.info,  Node: I/O And BEGIN/END,  Prev: Using BEGIN/END,  Up: 
BEGIN/END
- 
- 7.1.4.2 Input/Output from `BEGIN' and `END' Rules
- .................................................
- 
- There are several (sometimes subtle) points to remember when doing I/O
- from a `BEGIN' or `END' rule.  The first has to do with the value of
- `$0' in a `BEGIN' rule.  Because `BEGIN' rules are executed before any
- input is read, there simply is no input record, and therefore no
- fields, when executing `BEGIN' rules.  References to `$0' and the fields
- yield a null string or zero, depending upon the context.  One way to
- give `$0' a real value is to execute a `getline' command without a
- variable (*note Getline::).  Another way is simply to assign a value to
- `$0'.
- 
-    The second point is similar to the first but from the other
- direction.  Traditionally, due largely to implementation issues, `$0'
- and `NF' were _undefined_ inside an `END' rule.  The POSIX standard
- specifies that `NF' is available in an `END' rule. It contains the
- number of fields from the last input record.  Most probably due to an
- oversight, the standard does not say that `$0' is also preserved,
- although logically one would think that it should be.  In fact, `gawk'
- does preserve the value of `$0' for use in `END' rules.  Be aware,
- however, that Brian Kernighan's `awk', and possibly other
- implementations, do not.
- 
-    The third point follows from the first two.  The meaning of `print'
- inside a `BEGIN' or `END' rule is the same as always: `print $0'.  If
- `$0' is the null string, then this prints an empty record.  Many long
- time `awk' programmers use an unadorned `print' in `BEGIN' and `END'
- rules, to mean `print ""', relying on `$0' being null.  Although one
- might generally get away with this in `BEGIN' rules, it is a very bad
- idea in `END' rules, at least in `gawk'.  It is also poor style, since
- if an empty line is needed in the output, the program should print one
- explicitly.
- 
-    Finally, the `next' and `nextfile' statements are not allowed in a
- `BEGIN' rule, because the implicit
- read-a-record-and-match-against-the-rules loop has not started yet.
- Similarly, those statements are not valid in an `END' rule, since all
- the input has been read.  (*Note Next Statement::, and see *note
- Nextfile Statement::.)
- 
- 
- File: gawk.info,  Node: BEGINFILE/ENDFILE,  Next: Empty,  Prev: BEGIN/END,  
Up: Pattern Overview
- 
- 7.1.5 The `BEGINFILE' and `ENDFILE' Special Patterns
- ----------------------------------------------------
- 
- This minor node describes a `gawk'-specific feature.
- 
-    Two special kinds of rule, `BEGINFILE' and `ENDFILE', give you
- "hooks" into `gawk''s command-line file processing loop.  As with the
- `BEGIN' and `END' rules (*note BEGIN/END::), all `BEGINFILE' rules in a
- program are merged, in the order they are read by `gawk', and all
- `ENDFILE' rules are merged as well.
- 
-    The body of the `BEGINFILE' rules is executed just before `gawk'
- reads the first record from a file.  `FILENAME' is set to the name of
- the current file, and `FNR' is set to zero.
- 
-    The `BEGINFILE' rule provides you the opportunity for two tasks that
- would otherwise be difficult or impossible to perform:
- 
-    * You can test if the file is readable.  Normally, it is a fatal
-      error if a file named on the command line cannot be opened for
-      reading.  However, you can bypass the fatal error and move on to
-      the next file on the command line.
- 
-      You do this by checking if the `ERRNO' variable is not the empty
-      string; if so, then `gawk' was not able to open the file. In this
-      case, your program can execute the `nextfile' statement (*note
-      Nextfile Statement::).  This causes `gawk' to skip the file
-      entirely.  Otherwise, `gawk' exits with the usual fatal error.
- 
-    * If you have written extensions that modify the record handling (by
-      inserting an "open hook"), you can invoke them at this point,
-      before `gawk' has started processing the file.  (This is a _very_
-      advanced feature, currently used only by the XMLgawk project
-      (http://xmlgawk.sourceforge.net).)
- 
-    The `ENDFILE' rule is called when `gawk' has finished processing the
- last record in an input file.  For the last input file, it will be
- called before any `END' rules.  The `ENDFILE' rule is executed even for
- empty input files.
- 
-    Normally, when an error occurs when reading input in the normal input
- processing loop, the error is fatal.  However, if an `ENDFILE' rule is
- present, the error becomes non-fatal, and instead `ERRNO' is set.  This
- makes it possible to catch and process I/O errors at the level of the
- `awk' program.
- 
-    The `next' statement (*note Next Statement::) is not allowed inside
- either a `BEGINFILE' or and `ENDFILE' rule.  The `nextfile' statement
- (*note Nextfile Statement::) is allowed only inside a `BEGINFILE' rule,
- but not inside an `ENDFILE' rule.
- 
-    The `getline' statement (*note Getline::) is restricted inside both
- `BEGINFILE' and `ENDFILE'.  Only the `getline VARIABLE < FILE' form is
- allowed.
- 
-    `BEGINFILE' and `ENDFILE' are `gawk' extensions.  In most other
- `awk' implementations, or if `gawk' is in compatibility mode (*note
- Options::), they are not special.
- 
- 
- File: gawk.info,  Node: Empty,  Prev: BEGINFILE/ENDFILE,  Up: Pattern Overview
- 
- 7.1.6 The Empty Pattern
- -----------------------
- 
- An empty (i.e., nonexistent) pattern is considered to match _every_
- input record.  For example, the program:
- 
-      awk '{ print $1 }' BBS-list
- 
- prints the first field of every record.
- 
- 
- File: gawk.info,  Node: Using Shell Variables,  Next: Action Overview,  Prev: 
Pattern Overview,  Up: Patterns and Actions
- 
- 7.2 Using Shell Variables in Programs
- =====================================
- 
- `awk' programs are often used as components in larger programs written
- in shell.  For example, it is very common to use a shell variable to
- hold a pattern that the `awk' program searches for.  There are two ways
- to get the value of the shell variable into the body of the `awk'
- program.
- 
-    The most common method is to use shell quoting to substitute the
- variable's value into the program inside the script.  For example, in
- the following program:
- 
-      printf "Enter search pattern: "
-      read pattern
-      awk "/$pattern/ "'{ nmatches++ }
-           END { print nmatches, "found" }' /path/to/data
- 
- the `awk' program consists of two pieces of quoted text that are
- concatenated together to form the program.  The first part is
- double-quoted, which allows substitution of the `pattern' shell
- variable inside the quotes.  The second part is single-quoted.
- 
-    Variable substitution via quoting works, but can be potentially
- messy.  It requires a good understanding of the shell's quoting rules
- (*note Quoting::), and it's often difficult to correctly match up the
- quotes when reading the program.
- 
-    A better method is to use `awk''s variable assignment feature (*note
- Assignment Options::) to assign the shell variable's value to an `awk'
- variable's value.  Then use dynamic regexps to match the pattern (*note
- Computed Regexps::).  The following shows how to redo the previous
- example using this technique:
- 
-      printf "Enter search pattern: "
-      read pattern
-      awk -v pat="$pattern" '$0 ~ pat { nmatches++ }
-             END { print nmatches, "found" }' /path/to/data
- 
- Now, the `awk' program is just one single-quoted string.  The
- assignment `-v pat="$pattern"' still requires double quotes, in case
- there is whitespace in the value of `$pattern'.  The `awk' variable
- `pat' could be named `pattern' too, but that would be more confusing.
- Using a variable also provides more flexibility, since the variable can
- be used anywhere inside the program--for printing, as an array
- subscript, or for any other use--without requiring the quoting tricks
- at every point in the program.
- 
- 
- File: gawk.info,  Node: Action Overview,  Next: Statements,  Prev: Using 
Shell Variables,  Up: Patterns and Actions
- 
- 7.3 Actions
- ===========
- 
- An `awk' program or script consists of a series of rules and function
- definitions interspersed.  (Functions are described later.  *Note
- User-defined::.)  A rule contains a pattern and an action, either of
- which (but not both) may be omitted.  The purpose of the "action" is to
- tell `awk' what to do once a match for the pattern is found.  Thus, in
- outline, an `awk' program generally looks like this:
- 
-      [PATTERN]  { ACTION }
-       PATTERN  [{ ACTION }]
-      ...
-      function NAME(ARGS) { ... }
-      ...
- 
-    An action consists of one or more `awk' "statements", enclosed in
- curly braces (`{...}').  Each statement specifies one thing to do.  The
- statements are separated by newlines or semicolons.  The curly braces
- around an action must be used even if the action contains only one
- statement, or if it contains no statements at all.  However, if you
- omit the action entirely, omit the curly braces as well.  An omitted
- action is equivalent to `{ print $0 }':
- 
-      /foo/  { }     match `foo', do nothing -- empty action
-      /foo/          match `foo', print the record -- omitted action
- 
-    The following types of statements are supported in `awk':
- 
- Expressions
-      Call functions or assign values to variables (*note
-      Expressions::).  Executing this kind of statement simply computes
-      the value of the expression.  This is useful when the expression
-      has side effects (*note Assignment Ops::).
- 
- Control statements
-      Specify the control flow of `awk' programs.  The `awk' language
-      gives you C-like constructs (`if', `for', `while', and `do') as
-      well as a few special ones (*note Statements::).
- 
- Compound statements
-      Consist of one or more statements enclosed in curly braces.  A
-      compound statement is used in order to put several statements
-      together in the body of an `if', `while', `do', or `for' statement.
- 
- Input statements
-      Use the `getline' command (*note Getline::).  Also supplied in
-      `awk' are the `next' statement (*note Next Statement::), and the
-      `nextfile' statement (*note Nextfile Statement::).
- 
- Output statements
-      Such as `print' and `printf'.  *Note Printing::.
- 
- Deletion statements
-      For deleting array elements.  *Note Delete::.
- 
- 
- File: gawk.info,  Node: Statements,  Next: Built-in Variables,  Prev: Action 
Overview,  Up: Patterns and Actions
- 
- 7.4 Control Statements in Actions
- =================================
- 
- "Control statements", such as `if', `while', and so on, control the
- flow of execution in `awk' programs.  Most of `awk''s control
- statements are patterned after similar statements in C.
- 
-    All the control statements start with special keywords, such as `if'
- and `while', to distinguish them from simple expressions.  Many control
- statements contain other statements.  For example, the `if' statement
- contains another statement that may or may not be executed.  The
- contained statement is called the "body".  To include more than one
- statement in the body, group them into a single "compound statement"
- with curly braces, separating them with newlines or semicolons.
- 
- * Menu:
- 
- * If Statement::                Conditionally execute some `awk'
-                                 statements.
- * While Statement::             Loop until some condition is satisfied.
- * Do Statement::                Do specified action while looping until some
-                                 condition is satisfied.
- * For Statement::               Another looping statement, that provides
-                                 initialization and increment clauses.
- * Switch Statement::            Switch/case evaluation for conditional
-                                 execution of statements based on a value.
- * Break Statement::             Immediately exit the innermost enclosing loop.
- * Continue Statement::          Skip to the end of the innermost enclosing
-                                 loop.
- * Next Statement::              Stop processing the current input record.
- * Nextfile Statement::          Stop processing the current file.
- * Exit Statement::              Stop execution of `awk'.
- 
- 
- File: gawk.info,  Node: If Statement,  Next: While Statement,  Up: Statements
- 
- 7.4.1 The `if'-`else' Statement
- -------------------------------
- 
- The `if'-`else' statement is `awk''s decision-making statement.  It
- looks like this:
- 
-      if (CONDITION) THEN-BODY [else ELSE-BODY]
- 
- The CONDITION is an expression that controls what the rest of the
- statement does.  If the CONDITION is true, THEN-BODY is executed;
- otherwise, ELSE-BODY is executed.  The `else' part of the statement is
- optional.  The condition is considered false if its value is zero or
- the null string; otherwise, the condition is true.  Refer to the
- following:
- 
-      if (x % 2 == 0)
-          print "x is even"
-      else
-          print "x is odd"
- 
-    In this example, if the expression `x % 2 == 0' is true (that is, if
- the value of `x' is evenly divisible by two), then the first `print'
- statement is executed; otherwise, the second `print' statement is
- executed.  If the `else' keyword appears on the same line as THEN-BODY
- and THEN-BODY is not a compound statement (i.e., not surrounded by
- curly braces), then a semicolon must separate THEN-BODY from the `else'.
- To illustrate this, the previous example can be rewritten as:
- 
-      if (x % 2 == 0) print "x is even"; else
-              print "x is odd"
- 
- If the `;' is left out, `awk' can't interpret the statement and it
- produces a syntax error.  Don't actually write programs this way,
- because a human reader might fail to see the `else' if it is not the
- first thing on its line.
- 
- 
- File: gawk.info,  Node: While Statement,  Next: Do Statement,  Prev: If 
Statement,  Up: Statements
- 
- 7.4.2 The `while' Statement
- ---------------------------
- 
- In programming, a "loop" is a part of a program that can be executed
- two or more times in succession.  The `while' statement is the simplest
- looping statement in `awk'.  It repeatedly executes a statement as long
- as a condition is true.  For example:
- 
-      while (CONDITION)
-        BODY
- 
- BODY is a statement called the "body" of the loop, and CONDITION is an
- expression that controls how long the loop keeps running.  The first
- thing the `while' statement does is test the CONDITION.  If the
- CONDITION is true, it executes the statement BODY.  (The CONDITION is
- true when the value is not zero and not a null string.)  After BODY has
- been executed, CONDITION is tested again, and if it is still true, BODY
- is executed again.  This process repeats until the CONDITION is no
- longer true.  If the CONDITION is initially false, the body of the loop
- is never executed and `awk' continues with the statement following the
- loop.  This example prints the first three fields of each record, one
- per line:
- 
-      awk '{
-             i = 1
-             while (i <= 3) {
-                 print $i
-                 i++
-             }
-      }' inventory-shipped
- 
- The body of this loop is a compound statement enclosed in braces,
- containing two statements.  The loop works in the following manner:
- first, the value of `i' is set to one.  Then, the `while' statement
- tests whether `i' is less than or equal to three.  This is true when
- `i' equals one, so the `i'-th field is printed.  Then the `i++'
- increments the value of `i' and the loop repeats.  The loop terminates
- when `i' reaches four.
- 
-    A newline is not required between the condition and the body;
- however using one makes the program clearer unless the body is a
- compound statement or else is very simple.  The newline after the
- open-brace that begins the compound statement is not required either,
- but the program is harder to read without it.
- 
- 
- File: gawk.info,  Node: Do Statement,  Next: For Statement,  Prev: While 
Statement,  Up: Statements
- 
- 7.4.3 The `do'-`while' Statement
- --------------------------------
- 
- The `do' loop is a variation of the `while' looping statement.  The
- `do' loop executes the BODY once and then repeats the BODY as long as
- the CONDITION is true.  It looks like this:
- 
-      do
-        BODY
-      while (CONDITION)
- 
-    Even if the CONDITION is false at the start, the BODY is executed at
- least once (and only once, unless executing BODY makes CONDITION true).
- Contrast this with the corresponding `while' statement:
- 
-      while (CONDITION)
-        BODY
- 
- This statement does not execute BODY even once if the CONDITION is
- false to begin with.  The following is an example of a `do' statement:
- 
-      {
-             i = 1
-             do {
-                print $0
-                i++
-             } while (i <= 10)
-      }
- 
- This program prints each input record 10 times.  However, it isn't a
- very realistic example, since in this case an ordinary `while' would do
- just as well.  This situation reflects actual experience; only
- occasionally is there a real use for a `do' statement.
- 
- 
- File: gawk.info,  Node: For Statement,  Next: Switch Statement,  Prev: Do 
Statement,  Up: Statements
- 
- 7.4.4 The `for' Statement
- -------------------------
- 
- The `for' statement makes it more convenient to count iterations of a
- loop.  The general form of the `for' statement looks like this:
- 
-      for (INITIALIZATION; CONDITION; INCREMENT)
-        BODY
- 
- The INITIALIZATION, CONDITION, and INCREMENT parts are arbitrary `awk'
- expressions, and BODY stands for any `awk' statement.
- 
-    The `for' statement starts by executing INITIALIZATION.  Then, as
- long as the CONDITION is true, it repeatedly executes BODY and then
- INCREMENT.  Typically, INITIALIZATION sets a variable to either zero or
- one, INCREMENT adds one to it, and CONDITION compares it against the
- desired number of iterations.  For example:
- 
-      awk '{
-             for (i = 1; i <= 3; i++)
-                print $i
-      }' inventory-shipped
- 
- This prints the first three fields of each input record, with one field
- per line.
- 
-    It isn't possible to set more than one variable in the
- INITIALIZATION part without using a multiple assignment statement such
- as `x = y = 0'. This makes sense only if all the initial values are
- equal.  (But it is possible to initialize additional variables by
- writing their assignments as separate statements preceding the `for'
- loop.)
- 
-    The same is true of the INCREMENT part. Incrementing additional
- variables requires separate statements at the end of the loop.  The C
- compound expression, using C's comma operator, is useful in this
- context but it is not supported in `awk'.
- 
-    Most often, INCREMENT is an increment expression, as in the previous
- example.  But this is not required; it can be any expression
- whatsoever.  For example, the following statement prints all the powers
- of two between 1 and 100:
- 
-      for (i = 1; i <= 100; i *= 2)
-        print i
- 
-    If there is nothing to be done, any of the three expressions in the
- parentheses following the `for' keyword may be omitted.  Thus,
- `for (; x > 0;)' is equivalent to `while (x > 0)'.  If the CONDITION is
- omitted, it is treated as true, effectively yielding an "infinite loop"
- (i.e., a loop that never terminates).
- 
-    In most cases, a `for' loop is an abbreviation for a `while' loop,
- as shown here:
- 
-      INITIALIZATION
-      while (CONDITION) {
-        BODY
-        INCREMENT
-      }
- 
- The only exception is when the `continue' statement (*note Continue
- Statement::) is used inside the loop. Changing a `for' statement to a
- `while' statement in this way can change the effect of the `continue'
- statement inside the loop.
- 
-    The `awk' language has a `for' statement in addition to a `while'
- statement because a `for' loop is often both less work to type and more
- natural to think of.  Counting the number of iterations is very common
- in loops.  It can be easier to think of this counting as part of
- looping rather than as something to do inside the loop.
- 
-    There is an alternate version of the `for' loop, for iterating over
- all the indices of an array:
- 
-      for (i in array)
-          DO SOMETHING WITH array[i]
- 
- *Note Scanning an Array::, for more information on this version of the
- `for' loop.
- 
- 
- File: gawk.info,  Node: Switch Statement,  Next: Break Statement,  Prev: For 
Statement,  Up: Statements
- 
- 7.4.5 The `switch' Statement
- ----------------------------
- 
- The `switch' statement allows the evaluation of an expression and the
- execution of statements based on a `case' match. Case statements are
- checked for a match in the order they are defined.  If no suitable
- `case' is found, the `default' section is executed, if supplied.
- 
-    Each `case' contains a single constant, be it numeric, string, or
- regexp.  The `switch' expression is evaluated, and then each `case''s
- constant is compared against the result in turn. The type of constant
- determines the comparison: numeric or string do the usual comparisons.
- A regexp constant does a regular expression match against the string
- value of the original expression.  The general form of the `switch'
- statement looks like this:
- 
-      switch (EXPRESSION) {
-      case VALUE OR REGULAR EXPRESSION:
-          CASE-BODY
-      default:
-          DEFAULT-BODY
-      }
- 
-    Control flow in the `switch' statement works as it does in C. Once a
- match to a given case is made, the case statement bodies execute until
- a `break', `continue', `next', `nextfile'  or `exit' is encountered, or
- the end of the `switch' statement itself. For example:
- 
-      switch (NR * 2 + 1) {
-      case 3:
-      case "11":
-          print NR - 1
-          break
- 
-      case /2[[:digit:]]+/:
-          print NR
- 
-      default:
-          print NR + 1
- 
-      case -1:
-          print NR * -1
-      }
- 
-    Note that if none of the statements specified above halt execution
- of a matched `case' statement, execution falls through to the next
- `case' until execution halts. In the above example, for any case value
- starting with `2' followed by one or more digits, the `print' statement
- is executed and then falls through into the `default' section,
- executing its `print' statement. In turn, the -1 case will also be
- executed since the `default' does not halt execution.
- 
-    This `switch' statement is a `gawk' extension.  If `gawk' is in
- compatibility mode (*note Options::), it is not available.
- 
- 
- File: gawk.info,  Node: Break Statement,  Next: Continue Statement,  Prev: 
Switch Statement,  Up: Statements
- 
- 7.4.6 The `break' Statement
- ---------------------------
- 
- The `break' statement jumps out of the innermost `for', `while', or
- `do' loop that encloses it.  The following example finds the smallest
- divisor of any integer, and also identifies prime numbers:
- 
-      # find smallest divisor of num
-      {
-         num = $1
-         for (div = 2; div * div <= num; div++) {
-           if (num % div == 0)
-             break
-         }
-         if (num % div == 0)
-           printf "Smallest divisor of %d is %d\n", num, div
-         else
-           printf "%d is prime\n", num
-      }
- 
-    When the remainder is zero in the first `if' statement, `awk'
- immediately "breaks out" of the containing `for' loop.  This means that
- `awk' proceeds immediately to the statement following the loop and
- continues processing.  (This is very different from the `exit'
- statement, which stops the entire `awk' program.  *Note Exit
- Statement::.)
- 
-    The following program illustrates how the CONDITION of a `for' or
- `while' statement could be replaced with a `break' inside an `if':
- 
-      # find smallest divisor of num
-      {
-        num = $1
-        for (div = 2; ; div++) {
-          if (num % div == 0) {
-            printf "Smallest divisor of %d is %d\n", num, div
-            break
-          }
-          if (div * div > num) {
-            printf "%d is prime\n", num
-            break
-          }
-        }
-      }
- 
-    The `break' statement is also used to break out of the `switch'
- statement.  This is discussed in *note Switch Statement::.
- 
-    The `break' statement has no meaning when used outside the body of a
- loop or `switch'.  However, although it was never documented,
- historical implementations of `awk' treated the `break' statement
- outside of a loop as if it were a `next' statement (*note Next
- Statement::).  (d.c.)  Recent versions of Brian Kernighan's `awk' no
- longer allow this usage, nor does `gawk'.
- 
- 
- File: gawk.info,  Node: Continue Statement,  Next: Next Statement,  Prev: 
Break Statement,  Up: Statements
- 
- 7.4.7 The `continue' Statement
- ------------------------------
- 
- Similar to `break', the `continue' statement is used only inside `for',
- `while', and `do' loops.  It skips over the rest of the loop body,
- causing the next cycle around the loop to begin immediately.  Contrast
- this with `break', which jumps out of the loop altogether.
- 
-    The `continue' statement in a `for' loop directs `awk' to skip the
- rest of the body of the loop and resume execution with the
- increment-expression of the `for' statement.  The following program
- illustrates this fact:
- 
-      BEGIN {
-           for (x = 0; x <= 20; x++) {
-               if (x == 5)
-                   continue
-               printf "%d ", x
-           }
-           print ""
-      }
- 
- This program prints all the numbers from 0 to 20--except for 5, for
- which the `printf' is skipped.  Because the increment `x++' is not
- skipped, `x' does not remain stuck at 5.  Contrast the `for' loop from
- the previous example with the following `while' loop:
- 
-      BEGIN {
-           x = 0
-           while (x <= 20) {
-               if (x == 5)
-                   continue
-               printf "%d ", x
-               x++
-           }
-           print ""
-      }
- 
- This program loops forever once `x' reaches 5.
- 
-    The `continue' statement has no special meaning with respect to the
- `switch' statement, nor does it have any meaning when used outside the
- body of a loop.  Historical versions of `awk' treated a `continue'
- statement outside a loop the same way they treated a `break' statement
- outside a loop: as if it were a `next' statement (*note Next
- Statement::).  (d.c.)  Recent versions of Brian Kernighan's `awk' no
- longer work this way, nor does `gawk'.
- 
- 
- File: gawk.info,  Node: Next Statement,  Next: Nextfile Statement,  Prev: 
Continue Statement,  Up: Statements
- 
- 7.4.8 The `next' Statement
- --------------------------
- 
- The `next' statement forces `awk' to immediately stop processing the
- current record and go on to the next record.  This means that no
- further rules are executed for the current record, and the rest of the
- current rule's action isn't executed.
- 
-    Contrast this with the effect of the `getline' function (*note
- Getline::).  That also causes `awk' to read the next record
- immediately, but it does not alter the flow of control in any way
- (i.e., the rest of the current action executes with a new input record).
- 
-    At the highest level, `awk' program execution is a loop that reads
- an input record and then tests each rule's pattern against it.  If you
- think of this loop as a `for' statement whose body contains the rules,
- then the `next' statement is analogous to a `continue' statement. It
- skips to the end of the body of this implicit loop and executes the
- increment (which reads another record).
- 
-    For example, suppose an `awk' program works only on records with
- four fields, and it shouldn't fail when given bad input.  To avoid
- complicating the rest of the program, write a "weed out" rule near the
- beginning, in the following manner:
- 
-      NF != 4 {
-        err = sprintf("%s:%d: skipped: NF != 4\n", FILENAME, FNR)
-        print err > "/dev/stderr"
-        next
-      }
- 
- Because of the `next' statement, the program's subsequent rules won't
- see the bad record.  The error message is redirected to the standard
- error output stream, as error messages should be.  For more detail see
- *note Special Files::.
- 
-    If the `next' statement causes the end of the input to be reached,
- then the code in any `END' rules is executed.  *Note BEGIN/END::.
- 
-    The `next' statement is not allowed inside `BEGINFILE' and `ENDFILE'
- rules. *Note BEGINFILE/ENDFILE::.
- 
-    According to the POSIX standard, the behavior is undefined if the
- `next' statement is used in a `BEGIN' or `END' rule.  `gawk' treats it
- as a syntax error.  Although POSIX permits it, some other `awk'
- implementations don't allow the `next' statement inside function bodies
- (*note User-defined::).  Just as with any other `next' statement, a
- `next' statement inside a function body reads the next record and
- starts processing it with the first rule in the program.
- 
- 
- File: gawk.info,  Node: Nextfile Statement,  Next: Exit Statement,  Prev: 
Next Statement,  Up: Statements
- 
- 7.4.9 Using `gawk''s `nextfile' Statement
- -----------------------------------------
- 
- `gawk' provides the `nextfile' statement, which is similar to the
- `next' statement. (c.e.)  However, instead of abandoning processing of
- the current record, the `nextfile' statement instructs `gawk' to stop
- processing the current data file.
- 
-    The `nextfile' statement is a `gawk' extension.  In most other `awk'
- implementations, or if `gawk' is in compatibility mode (*note
- Options::), `nextfile' is not special.
- 
-    Upon execution of the `nextfile' statement, any `ENDFILE' rules are
- executed except in the case as mentioned below, `FILENAME' is updated
- to the name of the next data file listed on the command line, `FNR' is
- reset to one, `ARGIND' is incremented, any `BEGINFILE' rules are
- executed, and processing starts over with the first rule in the program.
- (`ARGIND' hasn't been introduced yet. *Note Built-in Variables::.)  If
- the `nextfile' statement causes the end of the input to be reached,
- then the code in any `END' rules is executed. An exception to this is
- when the `nextfile' is invoked during execution of any statement in an
- `END' rule; In this case, it causes the program to stop immediately.
- *Note BEGIN/END::.
- 
-    The `nextfile' statement is useful when there are many data files to
- process but it isn't necessary to process every record in every file.
- Normally, in order to move on to the next data file, a program has to
- continue scanning the unwanted records.  The `nextfile' statement
- accomplishes this much more efficiently.
- 
-    In addition, `nextfile' is useful inside a `BEGINFILE' rule to skip
- over a file that would otherwise cause `gawk' to exit with a fatal
- error. In this case, `ENDFILE' rules are not executed. *Note
- BEGINFILE/ENDFILE::.
- 
-    While one might think that `close(FILENAME)' would accomplish the
- same as `nextfile', this isn't true.  `close()' is reserved for closing
- files, pipes, and coprocesses that are opened with redirections.  It is
- not related to the main processing that `awk' does with the files
- listed in `ARGV'.
- 
-    The current version of the Brian Kernighan's `awk' (*note Other
- Versions::) also supports `nextfile'.  However, it doesn't allow the
- `nextfile' statement inside function bodies (*note User-defined::).
- `gawk' does; a `nextfile' inside a function body reads the next record
- and starts processing it with the first rule in the program, just as
- any other `nextfile' statement.
- 
- 
- File: gawk.info,  Node: Exit Statement,  Prev: Nextfile Statement,  Up: 
Statements
- 
- 7.4.10 The `exit' Statement
- ---------------------------
- 
- The `exit' statement causes `awk' to immediately stop executing the
- current rule and to stop processing input; any remaining input is
- ignored.  The `exit' statement is written as follows:
- 
-      exit [RETURN CODE]
- 
-    When an `exit' statement is executed from a `BEGIN' rule, the
- program stops processing everything immediately.  No input records are
- read.  However, if an `END' rule is present, as part of executing the
- `exit' statement, the `END' rule is executed (*note BEGIN/END::).  If
- `exit' is used in the body of an `END' rule, it causes the program to
- stop immediately.
- 
-    An `exit' statement that is not part of a `BEGIN' or `END' rule
- stops the execution of any further automatic rules for the current
- record, skips reading any remaining input records, and executes the
- `END' rule if there is one.  Any `ENDFILE' rules are also skipped; they
- are not executed.
- 
-    In such a case, if you don't want the `END' rule to do its job, set
- a variable to nonzero before the `exit' statement and check that
- variable in the `END' rule.  *Note Assert Function::, for an example
- that does this.
- 
-    If an argument is supplied to `exit', its value is used as the exit
- status code for the `awk' process.  If no argument is supplied, `exit'
- causes `awk' to return a "success" status.  In the case where an
- argument is supplied to a first `exit' statement, and then `exit' is
- called a second time from an `END' rule with no argument, `awk' uses
- the previously supplied exit value.  (d.c.)  *Note Exit Status::, for
- more information.
- 
-    For example, suppose an error condition occurs that is difficult or
- impossible to handle.  Conventionally, programs report this by exiting
- with a nonzero status.  An `awk' program can do this using an `exit'
- statement with a nonzero argument, as shown in the following example:
- 
-      BEGIN {
-             if (("date" | getline date_now) <= 0) {
-               print "Can't get system date" > "/dev/stderr"
-               exit 1
-             }
-             print "current date is", date_now
-             close("date")
-      }
- 
-      NOTE: For full portability, exit values should be between zero and
-      126, inclusive.  Negative values, and values of 127 or greater,
-      may not produce consistent results across different operating
-      systems.
- 
- 
- File: gawk.info,  Node: Built-in Variables,  Prev: Statements,  Up: Patterns 
and Actions
- 
- 7.5 Built-in Variables
- ======================
- 
- Most `awk' variables are available to use for your own purposes; they
- never change unless your program assigns values to them, and they never
- affect anything unless your program examines them.  However, a few
- variables in `awk' have special built-in meanings.  `awk' examines some
- of these automatically, so that they enable you to tell `awk' how to do
- certain things.  Others are set automatically by `awk', so that they
- carry information from the internal workings of `awk' to your program.
- 
-    This minor node documents all the built-in variables of `gawk', most
- of which are also documented in the chapters describing their areas of
- activity.
- 
- * Menu:
- 
- * User-modified::               Built-in variables that you change to control
-                                 `awk'.
- * Auto-set::                    Built-in variables where `awk' gives
-                                 you information.
- * ARGC and ARGV::               Ways to use `ARGC' and `ARGV'.
- 
- 
- File: gawk.info,  Node: User-modified,  Next: Auto-set,  Up: Built-in 
Variables
- 
- 7.5.1 Built-in Variables That Control `awk'
- -------------------------------------------
- 
- The following is an alphabetical list of variables that you can change
- to control how `awk' does certain things. The variables that are
- specific to `gawk' are marked with a pound sign (`#').
- 
- `BINMODE #'
-      On non-POSIX systems, this variable specifies use of binary mode
-      for all I/O.  Numeric values of one, two, or three specify that
-      input files, output files, or all files, respectively, should use
-      binary I/O.  A numeric value less than zero is treated as zero,
-      and a numeric value greater than three is treated as three.
-      Alternatively, string values of `"r"' or `"w"' specify that input
-      files and output files, respectively, should use binary I/O.  A
-      string value of `"rw"' or `"wr"' indicates that all files should
-      use binary I/O.  Any other string value is treated the same as
-      `"rw"', but causes `gawk' to generate a warning message.
-      `BINMODE' is described in more detail in *note PC Using::.
- 
-      This variable is a `gawk' extension.  In other `awk'
-      implementations (except `mawk', *note Other Versions::), or if
-      `gawk' is in compatibility mode (*note Options::), it is not
-      special.
- 
- `CONVFMT'
-      This string controls conversion of numbers to strings (*note
-      Conversion::).  It works by being passed, in effect, as the first
-      argument to the `sprintf()' function (*note String Functions::).
-      Its default value is `"%.6g"'.  `CONVFMT' was introduced by the
-      POSIX standard.
- 
- `FIELDWIDTHS #'
-      This is a space-separated list of columns that tells `gawk' how to
-      split input with fixed columnar boundaries.  Assigning a value to
-      `FIELDWIDTHS' overrides the use of `FS' and `FPAT' for field
-      splitting.  *Note Constant Size::, for more information.
- 
-      If `gawk' is in compatibility mode (*note Options::), then
-      `FIELDWIDTHS' has no special meaning, and field-splitting
-      operations occur based exclusively on the value of `FS'.
- 
- `FPAT #'
-      This is a regular expression (as a string) that tells `gawk' to
-      create the fields based on text that matches the regular
-      expression.  Assigning a value to `FPAT' overrides the use of `FS'
-      and `FIELDWIDTHS' for field splitting.  *Note Splitting By
-      Content::, for more information.
- 
-      If `gawk' is in compatibility mode (*note Options::), then `FPAT'
-      has no special meaning, and field-splitting operations occur based
-      exclusively on the value of `FS'.
- 
- `FS'
-      This is the input field separator (*note Field Separators::).  The
-      value is a single-character string or a multi-character regular
-      expression that matches the separations between fields in an input
-      record.  If the value is the null string (`""'), then each
-      character in the record becomes a separate field.  (This behavior
-      is a `gawk' extension. POSIX `awk' does not specify the behavior
-      when `FS' is the null string.  Nonetheless, some other versions of
-      `awk' also treat `""' specially.)
- 
-      The default value is `" "', a string consisting of a single space.
-      As a special exception, this value means that any sequence of
-      spaces, TABs, and/or newlines is a single separator.(1)  It also
-      causes spaces, TABs, and newlines at the beginning and end of a
-      record to be ignored.
- 
-      You can set the value of `FS' on the command line using the `-F'
-      option:
- 
-           awk -F, 'PROGRAM' INPUT-FILES
- 
-      If `gawk' is using `FIELDWIDTHS' or `FPAT' for field splitting,
-      assigning a value to `FS' causes `gawk' to return to the normal,
-      `FS'-based field splitting. An easy way to do this is to simply
-      say `FS = FS', perhaps with an explanatory comment.
- 
- `IGNORECASE #'
-      If `IGNORECASE' is nonzero or non-null, then all string comparisons
-      and all regular expression matching are case independent.  Thus,
-      regexp matching with `~' and `!~', as well as the `gensub()',
-      `gsub()', `index()', `match()', `patsplit()', `split()', and
-      `sub()' functions, record termination with `RS', and field
-      splitting with `FS' and `FPAT', all ignore case when doing their
-      particular regexp operations.  However, the value of `IGNORECASE'
-      does _not_ affect array subscripting and it does not affect field
-      splitting when using a single-character field separator.  *Note
-      Case-sensitivity::.
- 
-      If `gawk' is in compatibility mode (*note Options::), then
-      `IGNORECASE' has no special meaning.  Thus, string and regexp
-      operations are always case-sensitive.
- 
- `LINT #'
-      When this variable is true (nonzero or non-null), `gawk' behaves
-      as if the `--lint' command-line option is in effect.  (*note
-      Options::).  With a value of `"fatal"', lint warnings become fatal
-      errors.  With a value of `"invalid"', only warnings about things
-      that are actually invalid are issued. (This is not fully
-      implemented yet.)  Any other true value prints nonfatal warnings.
-      Assigning a false value to `LINT' turns off the lint warnings.
- 
-      This variable is a `gawk' extension.  It is not special in other
-      `awk' implementations.  Unlike the other special variables,
-      changing `LINT' does affect the production of lint warnings, even
-      if `gawk' is in compatibility mode.  Much as the `--lint' and
-      `--traditional' options independently control different aspects of
-      `gawk''s behavior, the control of lint warnings during program
-      execution is independent of the flavor of `awk' being executed.
- 
- `OFMT'
-      This string controls conversion of numbers to strings (*note
-      Conversion::) for printing with the `print' statement.  It works
-      by being passed as the first argument to the `sprintf()' function
-      (*note String Functions::).  Its default value is `"%.6g"'.
-      Earlier versions of `awk' also used `OFMT' to specify the format
-      for converting numbers to strings in general expressions; this is
-      now done by `CONVFMT'.
- 
- `OFS'
-      This is the output field separator (*note Output Separators::).
-      It is output between the fields printed by a `print' statement.
-      Its default value is `" "', a string consisting of a single space.
- 
- `ORS'
-      This is the output record separator.  It is output at the end of
-      every `print' statement.  Its default value is `"\n"', the newline
-      character.  (*Note Output Separators::.)
- 
- `PREC #'
-      The working precision of arbitrary precision floating-point
-      numbers, 53 by default (*note Setting Precision::).
- 
- `ROUNDMODE #'
-      The rounding mode to use for arbitrary precision arithmetic on
-      numbers, by default `"N"' (`roundTiesToEven' in the IEEE-754
-      standard) (*note Setting Rounding Mode::).
- 
- `RS'
-      This is `awk''s input record separator.  Its default value is a
-      string containing a single newline character, which means that an
-      input record consists of a single line of text.  It can also be
-      the null string, in which case records are separated by runs of
-      blank lines.  If it is a regexp, records are separated by matches
-      of the regexp in the input text.  (*Note Records::.)
- 
-      The ability for `RS' to be a regular expression is a `gawk'
-      extension.  In most other `awk' implementations, or if `gawk' is
-      in compatibility mode (*note Options::), just the first character
-      of `RS''s value is used.
- 
- `SUBSEP'
-      This is the subscript separator.  It has the default value of
-      `"\034"' and is used to separate the parts of the indices of a
-      multidimensional array.  Thus, the expression `foo["A", "B"]'
-      really accesses `foo["A\034B"]' (*note Multi-dimensional::).
- 
- `TEXTDOMAIN #'
-      This variable is used for internationalization of programs at the
-      `awk' level.  It sets the default text domain for specially marked
-      string constants in the source text, as well as for the
-      `dcgettext()', `dcngettext()' and `bindtextdomain()' functions
-      (*note Internationalization::).  The default value of `TEXTDOMAIN'
-      is `"messages"'.
- 
-      This variable is a `gawk' extension.  In other `awk'
-      implementations, or if `gawk' is in compatibility mode (*note
-      Options::), it is not special.
- 
-    ---------- Footnotes ----------
- 
-    (1) In POSIX `awk', newline does not count as whitespace.
- 
- 
- File: gawk.info,  Node: Auto-set,  Next: ARGC and ARGV,  Prev: User-modified, 
 Up: Built-in Variables
- 
- 7.5.2 Built-in Variables That Convey Information
- ------------------------------------------------
- 
- The following is an alphabetical list of variables that `awk' sets
- automatically on certain occasions in order to provide information to
- your program.  The variables that are specific to `gawk' are marked
- with a pound sign (`#').
- 
- `ARGC, ARGV'
-      The command-line arguments available to `awk' programs are stored
-      in an array called `ARGV'.  `ARGC' is the number of command-line
-      arguments present.  *Note Other Arguments::.  Unlike most `awk'
-      arrays, `ARGV' is indexed from 0 to `ARGC' - 1.  In the following
-      example:
- 
-           $ awk 'BEGIN {
-           >         for (i = 0; i < ARGC; i++)
-           >             print ARGV[i]
-           >      }' inventory-shipped BBS-list
-           -| awk
-           -| inventory-shipped
-           -| BBS-list
- 
-      `ARGV[0]' contains `awk', `ARGV[1]' contains `inventory-shipped',
-      and `ARGV[2]' contains `BBS-list'.  The value of `ARGC' is three,
-      one more than the index of the last element in `ARGV', because the
-      elements are numbered from zero.
- 
-      The names `ARGC' and `ARGV', as well as the convention of indexing
-      the array from 0 to `ARGC' - 1, are derived from the C language's
-      method of accessing command-line arguments.
- 
-      The value of `ARGV[0]' can vary from system to system.  Also, you
-      should note that the program text is _not_ included in `ARGV', nor
-      are any of `awk''s command-line options.  *Note ARGC and ARGV::,
-      for information about how `awk' uses these variables.  (d.c.)
- 
- `ARGIND #'
-      The index in `ARGV' of the current file being processed.  Every
-      time `gawk' opens a new data file for processing, it sets `ARGIND'
-      to the index in `ARGV' of the file name.  When `gawk' is
-      processing the input files, `FILENAME == ARGV[ARGIND]' is always
-      true.
- 
-      This variable is useful in file processing; it allows you to tell
-      how far along you are in the list of data files as well as to
-      distinguish between successive instances of the same file name on
-      the command line.
- 
-      While you can change the value of `ARGIND' within your `awk'
-      program, `gawk' automatically sets it to a new value when the next
-      file is opened.
- 
-      This variable is a `gawk' extension.  In other `awk'
-      implementations, or if `gawk' is in compatibility mode (*note
-      Options::), it is not special.
- 
- `ENVIRON'
-      An associative array containing the values of the environment.
-      The array indices are the environment variable names; the elements
-      are the values of the particular environment variables.  For
-      example, `ENVIRON["HOME"]' might be `/home/arnold'.  Changing this
-      array does not affect the environment passed on to any programs
-      that `awk' may spawn via redirection or the `system()' function.
- 
-      Some operating systems may not have environment variables.  On
-      such systems, the `ENVIRON' array is empty (except for
-      `ENVIRON["AWKPATH"]', *note AWKPATH Variable:: and
-      `ENVIRON["AWKLIBPATH"]', *note AWKLIBPATH Variable::).
- 
- `ERRNO #'
-      If a system error occurs during a redirection for `getline',
-      during a read for `getline', or during a `close()' operation, then
-      `ERRNO' contains a string describing the error.
- 
-      In addition, `gawk' clears `ERRNO' before opening each
-      command-line input file. This enables checking if the file is
-      readable inside a `BEGINFILE' pattern (*note BEGINFILE/ENDFILE::).
- 
-      Otherwise, `ERRNO' works similarly to the C variable `errno'.
-      Except for the case just mentioned, `gawk' _never_ clears it (sets
-      it to zero or `""').  Thus, you should only expect its value to be
-      meaningful when an I/O operation returns a failure value, such as
-      `getline' returning -1.  You are, of course, free to clear it
-      yourself before doing an I/O operation.
- 
-      This variable is a `gawk' extension.  In other `awk'
-      implementations, or if `gawk' is in compatibility mode (*note
-      Options::), it is not special.
- 
- `FILENAME'
-      The name of the file that `awk' is currently reading.  When no
-      data files are listed on the command line, `awk' reads from the
-      standard input and `FILENAME' is set to `"-"'.  `FILENAME' is
-      changed each time a new file is read (*note Reading Files::).
-      Inside a `BEGIN' rule, the value of `FILENAME' is `""', since
-      there are no input files being processed yet.(1) (d.c.)  Note,
-      though, that using `getline' (*note Getline::) inside a `BEGIN'
-      rule can give `FILENAME' a value.
- 
- `FNR'
-      The current record number in the current file.  `FNR' is
-      incremented each time a new record is read (*note Records::).  It
-      is reinitialized to zero each time a new input file is started.
- 
- `NF'
-      The number of fields in the current input record.  `NF' is set
-      each time a new record is read, when a new field is created or
-      when `$0' changes (*note Fields::).
- 
-      Unlike most of the variables described in this node, assigning a
-      value to `NF' has the potential to affect `awk''s internal
-      workings.  In particular, assignments to `NF' can be used to
-      create or remove fields from the current record. *Note Changing
-      Fields::.
- 
- `NR'
-      The number of input records `awk' has processed since the
-      beginning of the program's execution (*note Records::).  `NR' is
-      incremented each time a new record is read.
- 
- `PROCINFO #'
-      The elements of this array provide access to information about the
-      running `awk' program.  The following elements (listed
-      alphabetically) are guaranteed to be available:
- 
-     `PROCINFO["egid"]'
-           The value of the `getegid()' system call.
- 
-     `PROCINFO["euid"]'
-           The value of the `geteuid()' system call.
- 
-     `PROCINFO["FS"]'
-           This is `"FS"' if field splitting with `FS' is in effect,
-           `"FIELDWIDTHS"' if field splitting with `FIELDWIDTHS' is in
-           effect, or `"FPAT"' if field matching with `FPAT' is in
-           effect.
- 
-     `PROCINFO["gid"]'
-           The value of the `getgid()' system call.
- 
-     `PROCINFO["pgrpid"]'
-           The process group ID of the current process.
- 
-     `PROCINFO["pid"]'
-           The process ID of the current process.
- 
-     `PROCINFO["ppid"]'
-           The parent process ID of the current process.
- 
-     `PROCINFO["sorted_in"]'
-           If this element exists in `PROCINFO', its value controls the
-           order in which array indices will be processed by `for (index
-           in array) ...' loops.  Since this is an advanced feature, we
-           defer the full description until later; see *note Scanning an
-           Array::.
- 
-     `PROCINFO["strftime"]'
-           The default time format string for `strftime()'.  Assigning a
-           new value to this element changes the default.  *Note Time
-           Functions::.
- 
-     `PROCINFO["uid"]'
-           The value of the `getuid()' system call.
- 
-     `PROCINFO["version"]'
-           The version of `gawk'.
- 
-      The following additional elements in the array are available to
-      provide information about the MPFR and GMP libraries if your
-      version of `gawk' supports arbitrary precision numbers (*note
-      Arbitrary Precision Arithmetic::):
- 
-     `PROCINFO["mpfr_version"]'
-           The version of the GNU MPFR library.
- 
-     `PROCINFO["gmp_version"]'
-           The version of the GNU MP library.
- 
-     `PROCINFO["prec_max"]'
-           The maximum precision supported by MPFR.
- 
-     `PROCINFO["prec_min"]'
-           The minimum precision required by MPFR.
- 
-      On some systems, there may be elements in the array, `"group1"'
-      through `"groupN"' for some N. N is the number of supplementary
-      groups that the process has.  Use the `in' operator to test for
-      these elements (*note Reference to Elements::).
- 
-      The `PROCINFO' array is also used to cause coprocesses to
-      communicate over pseudo-ttys instead of through two-way pipes;
-      this is discussed further in *note Two-way I/O::.
- 
-      This array is a `gawk' extension.  In other `awk' implementations,
-      or if `gawk' is in compatibility mode (*note Options::), it is not
-      special.
- 
- `RLENGTH'
-      The length of the substring matched by the `match()' function
-      (*note String Functions::).  `RLENGTH' is set by invoking the
-      `match()' function.  Its value is the length of the matched
-      string, or -1 if no match is found.
- 
- `RSTART'
-      The start-index in characters of the substring that is matched by
-      the `match()' function (*note String Functions::).  `RSTART' is
-      set by invoking the `match()' function.  Its value is the position
-      of the string where the matched substring starts, or zero if no
-      match was found.
- 
- `RT #'
-      This is set each time a record is read. It contains the input text
-      that matched the text denoted by `RS', the record separator.
- 
-      This variable is a `gawk' extension.  In other `awk'
-      implementations, or if `gawk' is in compatibility mode (*note
-      Options::), it is not special.
- 
- Advanced Notes: Changing `NR' and `FNR'
- ---------------------------------------
- 
- `awk' increments `NR' and `FNR' each time it reads a record, instead of
- setting them to the absolute value of the number of records read.  This
- means that a program can change these variables and their new values
- are incremented for each record.  (d.c.)  The following example shows
- this:
- 
-      $ echo '1
-      > 2
-      > 3
-      > 4' | awk 'NR == 2 { NR = 17 }
-      > { print NR }'
-      -| 1
-      -| 17
-      -| 18
-      -| 19
- 
- Before `FNR' was added to the `awk' language (*note V7/SVR3.1::), many
- `awk' programs used this feature to track the number of records in a
- file by resetting `NR' to zero when `FILENAME' changed.
- 
-    ---------- Footnotes ----------
- 
-    (1) Some early implementations of Unix `awk' initialized `FILENAME'
- to `"-"', even if there were data files to be processed. This behavior
- was incorrect and should not be relied upon in your programs.
- 
- 
- File: gawk.info,  Node: ARGC and ARGV,  Prev: Auto-set,  Up: Built-in 
Variables
- 
- 7.5.3 Using `ARGC' and `ARGV'
- -----------------------------
- 
- *note Auto-set::, presented the following program describing the
- information contained in `ARGC' and `ARGV':
- 
-      $ awk 'BEGIN {
-      >        for (i = 0; i < ARGC; i++)
-      >            print ARGV[i]
-      >      }' inventory-shipped BBS-list
-      -| awk
-      -| inventory-shipped
-      -| BBS-list
- 
- In this example, `ARGV[0]' contains `awk', `ARGV[1]' contains
- `inventory-shipped', and `ARGV[2]' contains `BBS-list'.  Notice that
- the `awk' program is not entered in `ARGV'.  The other command-line
- options, with their arguments, are also not entered.  This includes
- variable assignments done with the `-v' option (*note Options::).
- Normal variable assignments on the command line _are_ treated as
- arguments and do show up in the `ARGV' array.  Given the following
- program in a file named `showargs.awk':
- 
-      BEGIN {
-          printf "A=%d, B=%d\n", A, B
-          for (i = 0; i < ARGC; i++)
-              printf "\tARGV[%d] = %s\n", i, ARGV[i]
-      }
-      END   { printf "A=%d, B=%d\n", A, B }
- 
- Running it produces the following:
- 
-      $ awk -v A=1 -f showargs.awk B=2 /dev/null
-      -| A=1, B=0
-      -|        ARGV[0] = awk
-      -|        ARGV[1] = B=2
-      -|        ARGV[2] = /dev/null
-      -| A=1, B=2
- 
-    A program can alter `ARGC' and the elements of `ARGV'.  Each time
- `awk' reaches the end of an input file, it uses the next element of
- `ARGV' as the name of the next input file.  By storing a different
- string there, a program can change which files are read.  Use `"-"' to
- represent the standard input.  Storing additional elements and
- incrementing `ARGC' causes additional files to be read.
- 
-    If the value of `ARGC' is decreased, that eliminates input files
- from the end of the list.  By recording the old value of `ARGC'
- elsewhere, a program can treat the eliminated arguments as something
- other than file names.
- 
-    To eliminate a file from the middle of the list, store the null
- string (`""') into `ARGV' in place of the file's name.  As a special
- feature, `awk' ignores file names that have been replaced with the null
- string.  Another option is to use the `delete' statement to remove
- elements from `ARGV' (*note Delete::).
- 
-    All of these actions are typically done in the `BEGIN' rule, before
- actual processing of the input begins.  *Note Split Program::, and see
- *note Tee Program::, for examples of each way of removing elements from
- `ARGV'.  The following fragment processes `ARGV' in order to examine,
- and then remove, command-line options:
- 
-      BEGIN {
-          for (i = 1; i < ARGC; i++) {
-              if (ARGV[i] == "-v")
-                  verbose = 1
-              else if (ARGV[i] == "-q")
-                  debug = 1
-              else if (ARGV[i] ~ /^-./) {
-                  e = sprintf("%s: unrecognized option -- %c",
-                          ARGV[0], substr(ARGV[i], 2, 1))
-                  print e > "/dev/stderr"
-              } else
-                  break
-              delete ARGV[i]
-          }
-      }
- 
-    To actually get the options into the `awk' program, end the `awk'
- options with `--' and then supply the `awk' program's options, in the
- following manner:
- 
-      awk -f myprog -- -v -q file1 file2 ...
- 
-    This is not necessary in `gawk'. Unless `--posix' has been
- specified, `gawk' silently puts any unrecognized options into `ARGV'
- for the `awk' program to deal with.  As soon as it sees an unknown
- option, `gawk' stops looking for other options that it might otherwise
- recognize.  The previous example with `gawk' would be:
- 
-      gawk -f myprog -q -v file1 file2 ...
- 
- Because `-q' is not a valid `gawk' option, it and the following `-v'
- are passed on to the `awk' program.  (*Note Getopt Function::, for an
- `awk' library function that parses command-line options.)
- 
- 
- File: gawk.info,  Node: Arrays,  Next: Functions,  Prev: Patterns and 
Actions,  Up: Top
- 
- 8 Arrays in `awk'
- *****************
- 
- An "array" is a table of values called "elements".  The elements of an
- array are distinguished by their "indices".  Indices may be either
- numbers or strings.
- 
-    This major node describes how arrays work in `awk', how to use array
- elements, how to scan through every element in an array, and how to
- remove array elements.  It also describes how `awk' simulates
- multidimensional arrays, as well as some of the less obvious points
- about array usage.  The major node moves on to discuss `gawk''s facility
- for sorting arrays, and ends with a brief description of `gawk''s
- ability to support true multidimensional arrays.
- 
-    `awk' maintains a single set of names that may be used for naming
- variables, arrays, and functions (*note User-defined::).  Thus, you
- cannot have a variable and an array with the same name in the same
- `awk' program.
- 
- * Menu:
- 
- * Array Basics::                The basics of arrays.
- * Delete::                      The `delete' statement removes an element
-                                 from an array.
- * Numeric Array Subscripts::    How to use numbers as subscripts in
-                                 `awk'.
- * Uninitialized Subscripts::    Using Uninitialized variables as subscripts.
- * Multi-dimensional::           Emulating multidimensional arrays in
-                                 `awk'.
- * Arrays of Arrays::            True multidimensional arrays.
- 
- 
- File: gawk.info,  Node: Array Basics,  Next: Delete,  Up: Arrays
- 
- 8.1 The Basics of Arrays
- ========================
- 
- This minor node presents the basics: working with elements in arrays
- one at a time, and traversing all of the elements in an array.
- 
- * Menu:
- 
- * Array Intro::                 Introduction to Arrays
- * Reference to Elements::       How to examine one element of an array.
- * Assigning Elements::          How to change an element of an array.
- * Array Example::               Basic Example of an Array
- * Scanning an Array::           A variation of the `for' statement. It
-                                 loops through the indices of an array's
-                                 existing elements.
- * Controlling Scanning::        Controlling the order in which arrays are
-                                 scanned.
- 
- 
- File: gawk.info,  Node: Array Intro,  Next: Reference to Elements,  Up: Array 
Basics
- 
- 8.1.1 Introduction to Arrays
- ----------------------------
- 
-      Doing linear scans over an associative array is like trying to
-      club someone to death with a loaded Uzi.
-      Larry Wall
- 
-    The `awk' language provides one-dimensional arrays for storing
- groups of related strings or numbers.  Every `awk' array must have a
- name.  Array names have the same syntax as variable names; any valid
- variable name would also be a valid array name.  But one name cannot be
- used in both ways (as an array and as a variable) in the same `awk'
- program.
- 
-    Arrays in `awk' superficially resemble arrays in other programming
- languages, but there are fundamental differences.  In `awk', it isn't
- necessary to specify the size of an array before starting to use it.
- Additionally, any number or string in `awk', not just consecutive
- integers, may be used as an array index.
- 
-    In most other languages, arrays must be "declared" before use,
- including a specification of how many elements or components they
- contain.  In such languages, the declaration causes a contiguous block
- of memory to be allocated for that many elements.  Usually, an index in
- the array must be a positive integer.  For example, the index zero
- specifies the first element in the array, which is actually stored at
- the beginning of the block of memory.  Index one specifies the second
- element, which is stored in memory right after the first element, and
- so on.  It is impossible to add more elements to the array, because it
- has room only for as many elements as given in the declaration.  (Some
- languages allow arbitrary starting and ending indices--e.g., `15 ..
- 27'--but the size of the array is still fixed when the array is
- declared.)
- 
-    A contiguous array of four elements might look like the following
- example, conceptually, if the element values are 8, `"foo"', `""', and
- 30:
- 
-      +---------+---------+--------+---------+
-      |    8    |  "foo"  |   ""   |    30   |    Value
-      +---------+---------+--------+---------+
-           0         1         2         3        Index
- 
- Only the values are stored; the indices are implicit from the order of
- the values. Here, 8 is the value at index zero, because 8 appears in the
- position with zero elements before it.
- 
-    Arrays in `awk' are different--they are "associative".  This means
- that each array is a collection of pairs: an index and its corresponding
- array element value:
- 
-      Index 3     Value 30
-      Index 1     Value "foo"
-      Index 0     Value 8
-      Index 2     Value ""
- 
- The pairs are shown in jumbled order because their order is irrelevant.
- 
-    One advantage of associative arrays is that new pairs can be added
- at any time.  For example, suppose a tenth element is added to the array
- whose value is `"number ten"'.  The result is:
- 
-      Index 10    Value "number ten"
-      Index 3     Value 30
-      Index 1     Value "foo"
-      Index 0     Value 8
-      Index 2     Value ""
- 
- Now the array is "sparse", which just means some indices are missing.
- It has elements 0-3 and 10, but doesn't have elements 4, 5, 6, 7, 8, or
- 9.
- 
-    Another consequence of associative arrays is that the indices don't
- have to be positive integers.  Any number, or even a string, can be an
- index.  For example, the following is an array that translates words
- from English to French:
- 
-      Index "dog" Value "chien"
-      Index "cat" Value "chat"
-      Index "one" Value "un"
-      Index 1     Value "un"
- 
- Here we decided to translate the number one in both spelled-out and
- numeric form--thus illustrating that a single array can have both
- numbers and strings as indices.  In fact, array subscripts are always
- strings; this is discussed in more detail in *note Numeric Array
- Subscripts::.  Here, the number `1' isn't double-quoted, since `awk'
- automatically converts it to a string.
- 
-    The value of `IGNORECASE' has no effect upon array subscripting.
- The identical string value used to store an array element must be used
- to retrieve it.  When `awk' creates an array (e.g., with the `split()'
- built-in function), that array's indices are consecutive integers
- starting at one.  (*Note String Functions::.)
- 
-    `awk''s arrays are efficient--the time to access an element is
- independent of the number of elements in the array.
- 
- 
- File: gawk.info,  Node: Reference to Elements,  Next: Assigning Elements,  
Prev: Array Intro,  Up: Array Basics
- 
- 8.1.2 Referring to an Array Element
- -----------------------------------
- 
- The principal way to use an array is to refer to one of its elements.
- An array reference is an expression as follows:
- 
-      ARRAY[INDEX-EXPRESSION]
- 
- Here, ARRAY is the name of an array.  The expression INDEX-EXPRESSION is
- the index of the desired element of the array.
- 
-    The value of the array reference is the current value of that array
- element.  For example, `foo[4.3]' is an expression for the element of
- array `foo' at index `4.3'.
- 
-    A reference to an array element that has no recorded value yields a
- value of `""', the null string.  This includes elements that have not
- been assigned any value as well as elements that have been deleted
- (*note Delete::).
- 
-      NOTE: A reference to an element that does not exist
-      _automatically_ creates that array element, with the null string
-      as its value.  (In some cases, this is unfortunate, because it
-      might waste memory inside `awk'.)
- 
-      Novice `awk' programmers often make the mistake of checking if an
-      element exists by checking if the value is empty:
- 
-           # Check if "foo" exists in a:         Incorrect!
-           if (a["foo"] != "") ...
- 
-      This is incorrect, since this will _create_ `a["foo"]' if it
-      didn't exist before!
- 
-    To determine whether an element exists in an array at a certain
- index, use the following expression:
- 
-      IND in ARRAY
- 
- This expression tests whether the particular index IND exists, without
- the side effect of creating that element if it is not present.  The
- expression has the value one (true) if `ARRAY[IND]' exists and zero
- (false) if it does not exist.  For example, this statement tests
- whether the array `frequencies' contains the index `2':
- 
-      if (2 in frequencies)
-          print "Subscript 2 is present."
- 
-    Note that this is _not_ a test of whether the array `frequencies'
- contains an element whose _value_ is two.  There is no way to do that
- except to scan all the elements.  Also, this _does not_ create
- `frequencies[2]', while the following (incorrect) alternative does:
- 
-      if (frequencies[2] != "")
-          print "Subscript 2 is present."
- 
- 
- File: gawk.info,  Node: Assigning Elements,  Next: Array Example,  Prev: 
Reference to Elements,  Up: Array Basics
- 
- 8.1.3 Assigning Array Elements
- ------------------------------
- 
- Array elements can be assigned values just like `awk' variables:
- 
-      ARRAY[INDEX-EXPRESSION] = VALUE
- 
- ARRAY is the name of an array.  The expression INDEX-EXPRESSION is the
- index of the element of the array that is assigned a value.  The
- expression VALUE is the value to assign to that element of the array.
- 
- 
- File: gawk.info,  Node: Array Example,  Next: Scanning an Array,  Prev: 
Assigning Elements,  Up: Array Basics
- 
- 8.1.4 Basic Array Example
- -------------------------
- 
- The following program takes a list of lines, each beginning with a line
- number, and prints them out in order of line number.  The line numbers
- are not in order when they are first read--instead they are scrambled.
- This program sorts the lines by making an array using the line numbers
- as subscripts.  The program then prints out the lines in sorted order
- of their numbers.  It is a very simple program and gets confused upon
- encountering repeated numbers, gaps, or lines that don't begin with a
- number:
- 
-      {
-        if ($1 > max)
-          max = $1
-        arr[$1] = $0
-      }
- 
-      END {
-        for (x = 1; x <= max; x++)
-          print arr[x]
-      }
- 
-    The first rule keeps track of the largest line number seen so far;
- it also stores each line into the array `arr', at an index that is the
- line's number.  The second rule runs after all the input has been read,
- to print out all the lines.  When this program is run with the
- following input:
- 
-      5  I am the Five man
-      2  Who are you?  The new number two!
-      4  . . . And four on the floor
-      1  Who is number one?
-      3  I three you.
- 
- Its output is:
- 
-      1  Who is number one?
-      2  Who are you?  The new number two!
-      3  I three you.
-      4  . . . And four on the floor
-      5  I am the Five man
- 
-    If a line number is repeated, the last line with a given number
- overrides the others.  Gaps in the line numbers can be handled with an
- easy improvement to the program's `END' rule, as follows:
- 
-      END {
-        for (x = 1; x <= max; x++)
-          if (x in arr)
-            print arr[x]
-      }
- 
- 
- File: gawk.info,  Node: Scanning an Array,  Next: Controlling Scanning,  
Prev: Array Example,  Up: Array Basics
- 
- 8.1.5 Scanning All Elements of an Array
- ---------------------------------------
- 
- In programs that use arrays, it is often necessary to use a loop that
- executes once for each element of an array.  In other languages, where
- arrays are contiguous and indices are limited to positive integers,
- this is easy: all the valid indices can be found by counting from the
- lowest index up to the highest.  This technique won't do the job in
- `awk', because any number or string can be an array index.  So `awk'
- has a special kind of `for' statement for scanning an array:
- 
-      for (VAR in ARRAY)
-        BODY
- 
- This loop executes BODY once for each index in ARRAY that the program
- has previously used, with the variable VAR set to that index.
- 
-    The following program uses this form of the `for' statement.  The
- first rule scans the input records and notes which words appear (at
- least once) in the input, by storing a one into the array `used' with
- the word as index.  The second rule scans the elements of `used' to
- find all the distinct words that appear in the input.  It prints each
- word that is more than 10 characters long and also prints the number of
- such words.  *Note String Functions::, for more information on the
- built-in function `length()'.
- 
-      # Record a 1 for each word that is used at least once
-      {
-          for (i = 1; i <= NF; i++)
-              used[$i] = 1
-      }
- 
-      # Find number of distinct words more than 10 characters long
-      END {
-          for (x in used) {
-              if (length(x) > 10) {
-                  ++num_long_words
-                  print x
-              }
-          }
-          print num_long_words, "words longer than 10 characters"
-      }
- 
- *Note Word Sorting::, for a more detailed example of this type.
- 
-    The order in which elements of the array are accessed by this
- statement is determined by the internal arrangement of the array
- elements within `awk' and normally cannot be controlled or changed.
- This can lead to problems if new elements are added to ARRAY by
- statements in the loop body; it is not predictable whether the `for'
- loop will reach them.  Similarly, changing VAR inside the loop may
- produce strange results.  It is best to avoid such things.
- 
- 
- File: gawk.info,  Node: Controlling Scanning,  Prev: Scanning an Array,  Up: 
Array Basics
- 
- 8.1.6 Using Predefined Array Scanning Orders
- --------------------------------------------
- 
- By default, when a `for' loop traverses an array, the order is
- undefined, meaning that the `awk' implementation determines the order
- in which the array is traversed.  This order is usually based on the
- internal implementation of arrays and will vary from one version of
- `awk' to the next.
- 
-    Often, though, you may wish to do something simple, such as
- "traverse the array by comparing the indices in ascending order," or
- "traverse the array by on comparing the values in descending order."
- `gawk' provides two mechanisms which give you this control.
- 
-    * Set `PROCINFO["sorted_in"]' to one of a set of predefined values.
-      We describe this now.
- 
-    * Set `PROCINFO["sorted_in"]' to the name of a user-defined function
-      to be used for comparison of array elements. This advanced feature
-      is described later, in *note Array Sorting::.
- 
-    The following special values for `PROCINFO["sorted_in"]' are
- available:
- 
- `"@unsorted"'
-      Array elements are processed in arbitrary order, which is the
-      default `awk' behavior.
- 
- `"@ind_str_asc"'
-      Order by indices compared as strings; this is the most basic sort.
-      (Internally, array indices are always strings, so with `a[2*5] = 1'
-      the index is `"10"' rather than numeric 10.)
- 
- `"@ind_num_asc"'
-      Order by indices but force them to be treated as numbers in the
-      process.  Any index with a non-numeric value will end up
-      positioned as if it were zero.
- 
- `"@val_type_asc"'
-      Order by element values rather than indices.  Ordering is by the
-      type assigned to the element (*note Typing and Comparison::).  All
-      numeric values come before all string values, which in turn come
-      before all subarrays.  (Subarrays have not been described yet;
-      *note Arrays of Arrays::).
- 
- `"@val_str_asc"'
-      Order by element values rather than by indices.  Scalar values are
-      compared as strings.  Subarrays, if present, come out last.
- 
- `"@val_num_asc"'
-      Order by element values rather than by indices.  Scalar values are
-      compared as numbers.  Subarrays, if present, come out last.  When
-      numeric values are equal, the string values are used to provide an
-      ordering: this guarantees consistent results across different
-      versions of the C `qsort()' function,(1) which `gawk' uses
-      internally to perform the sorting.
- 
- `"@ind_str_desc"'
-      Reverse order from the most basic sort.
- 
- `"@ind_num_desc"'
-      Numeric indices ordered from high to low.
- 
- `"@val_type_desc"'
-      Element values, based on type, in descending order.
- 
- `"@val_str_desc"'
-      Element values, treated as strings, ordered from high to low.
-      Subarrays, if present, come out first.
- 
- `"@val_num_desc"'
-      Element values, treated as numbers, ordered from high to low.
-      Subarrays, if present, come out first.
- 
-    The array traversal order is determined before the `for' loop starts
- to run. Changing `PROCINFO["sorted_in"]' in the loop body will not
- affect the loop.
- 
-    For example:
- 
-      $ gawk 'BEGIN {
-      >    a[4] = 4
-      >    a[3] = 3
-      >    for (i in a)
-      >        print i, a[i]
-      > }'
-      -| 4 4
-      -| 3 3
-      $ gawk 'BEGIN {
-      >    PROCINFO["sorted_in"] = "@ind_str_asc"
-      >    a[4] = 4
-      >    a[3] = 3
-      >    for (i in a)
-      >        print i, a[i]
-      > }'
-      -| 3 3
-      -| 4 4
- 
-    When sorting an array by element values, if a value happens to be a
- subarray then it is considered to be greater than any string or numeric
- value, regardless of what the subarray itself contains, and all
- subarrays are treated as being equal to each other.  Their order
- relative to each other is determined by their index strings.
- 
-    Here are some additional things to bear in mind about sorted array
- traversal.
- 
-    * The value of `PROCINFO["sorted_in"]' is global. That is, it affects
-      all array traversal `for' loops.  If you need to change it within
-      your own code, you should see if it's defined and save and restore
-      the value:
- 
-           ...
-           if ("sorted_in" in PROCINFO) {
-               save_sorted = PROCINFO["sorted_in"]
-               PROCINFO["sorted_in"] = "@val_str_desc" # or whatever
-           }
-           ...
-           if (save_sorted)
-               PROCINFO["sorted_in"] = save_sorted
- 
-    * As mentioned, the default array traversal order is represented by
-      `"@unsorted"'.  You can also get the default behavior by assigning
-      the null string to `PROCINFO["sorted_in"]' or by just deleting the
-      `"sorted_in"' element from the `PROCINFO' array with the `delete'
-      statement.  (The `delete' statement hasn't been described yet;
-      *note Delete::.)
- 
-    In addition, `gawk' provides built-in functions for sorting arrays;
- see *note Array Sorting Functions::.
- 
-    ---------- Footnotes ----------
- 
-    (1) When two elements compare as equal, the C `qsort()' function
- does not guarantee that they will maintain their original relative
- order after sorting.  Using the string value to provide a unique
- ordering when the numeric values are equal ensures that `gawk' behaves
- consistently across different environments.
- 
- 
- File: gawk.info,  Node: Delete,  Next: Numeric Array Subscripts,  Prev: Array 
Basics,  Up: Arrays
- 
- 8.2 The `delete' Statement
- ==========================
- 
- To remove an individual element of an array, use the `delete' statement:
- 
-      delete ARRAY[INDEX-EXPRESSION]
- 
-    Once an array element has been deleted, any value the element once
- had is no longer available. It is as if the element had never been
- referred to or been given a value.  The following is an example of
- deleting elements in an array:
- 
-      for (i in frequencies)
-        delete frequencies[i]
- 
- This example removes all the elements from the array `frequencies'.
- Once an element is deleted, a subsequent `for' statement to scan the
- array does not report that element and the `in' operator to check for
- the presence of that element returns zero (i.e., false):
- 
-      delete foo[4]
-      if (4 in foo)
-          print "This will never be printed"
- 
-    It is important to note that deleting an element is _not_ the same
- as assigning it a null value (the empty string, `""').  For example:
- 
-      foo[4] = ""
-      if (4 in foo)
-        print "This is printed, even though foo[4] is empty"
- 
-    It is not an error to delete an element that does not exist.
- However, if `--lint' is provided on the command line (*note Options::),
- `gawk' issues a warning message when an element that is not in the
- array is deleted.
- 
-    All the elements of an array may be deleted with a single statement
- (c.e.)  by leaving off the subscript in the `delete' statement, as
- follows:
- 
-      delete ARRAY
- 
-    This ability is a `gawk' extension; it is not available in
- compatibility mode (*note Options::).
- 
-    Using this version of the `delete' statement is about three times
- more efficient than the equivalent loop that deletes each element one
- at a time.
- 
-    The following statement provides a portable but nonobvious way to
- clear out an array:(1)
- 
-      split("", array)
- 
-    The `split()' function (*note String Functions::) clears out the
- target array first. This call asks it to split apart the null string.
- Because there is no data to split out, the function simply clears the
- array and then returns.
- 
-      CAUTION: Deleting an array does not change its type; you cannot
-      delete an array and then use the array's name as a scalar (i.e., a
-      regular variable). For example, the following does not work:
- 
-           a[1] = 3
-           delete a
-           a = 3
- 
-    ---------- Footnotes ----------
- 
-    (1) Thanks to Michael Brennan for pointing this out.
- 
- 
- File: gawk.info,  Node: Numeric Array Subscripts,  Next: Uninitialized 
Subscripts,  Prev: Delete,  Up: Arrays
- 
- 8.3 Using Numbers to Subscript Arrays
- =====================================
- 
- An important aspect to remember about arrays is that _array subscripts
- are always strings_.  When a numeric value is used as a subscript, it
- is converted to a string value before being used for subscripting
- (*note Conversion::).  This means that the value of the built-in
- variable `CONVFMT' can affect how your program accesses elements of an
- array.  For example:
- 
-      xyz = 12.153
-      data[xyz] = 1
-      CONVFMT = "%2.2f"
-      if (xyz in data)
-          printf "%s is in data\n", xyz
-      else
-          printf "%s is not in data\n", xyz
- 
- This prints `12.15 is not in data'.  The first statement gives `xyz' a
- numeric value.  Assigning to `data[xyz]' subscripts `data' with the
- string value `"12.153"' (using the default conversion value of
- `CONVFMT', `"%.6g"').  Thus, the array element `data["12.153"]' is
- assigned the value one.  The program then changes the value of
- `CONVFMT'.  The test `(xyz in data)' generates a new string value from
- `xyz'--this time `"12.15"'--because the value of `CONVFMT' only allows
- two significant digits.  This test fails, since `"12.15"' is different
- from `"12.153"'.
- 
-    According to the rules for conversions (*note Conversion::), integer
- values are always converted to strings as integers, no matter what the
- value of `CONVFMT' may happen to be.  So the usual case of the
- following works:
- 
-      for (i = 1; i <= maxsub; i++)
-          do something with array[i]
- 
-    The "integer values always convert to strings as integers" rule has
- an additional consequence for array indexing.  Octal and hexadecimal
- constants (*note Nondecimal-numbers::) are converted internally into
- numbers, and their original form is forgotten.  This means, for
- example, that `array[17]', `array[021]', and `array[0x11]' all refer to
- the same element!
- 
-    As with many things in `awk', the majority of the time things work
- as one would expect them to.  But it is useful to have a precise
- knowledge of the actual rules since they can sometimes have a subtle
- effect on your programs.
- 
- 
- File: gawk.info,  Node: Uninitialized Subscripts,  Next: Multi-dimensional,  
Prev: Numeric Array Subscripts,  Up: Arrays
- 
- 8.4 Using Uninitialized Variables as Subscripts
- ===============================================
- 
- Suppose it's necessary to write a program to print the input data in
- reverse order.  A reasonable attempt to do so (with some test data)
- might look like this:
- 
-      $ echo 'line 1
-      > line 2
-      > line 3' | awk '{ l[lines] = $0; ++lines }
-      > END {
-      >     for (i = lines-1; i >= 0; --i)
-      >        print l[i]
-      > }'
-      -| line 3
-      -| line 2
- 
-    Unfortunately, the very first line of input data did not come out in
- the output!
- 
-    Upon first glance, we would think that this program should have
- worked.  The variable `lines' is uninitialized, and uninitialized
- variables have the numeric value zero.  So, `awk' should have printed
- the value of `l[0]'.
- 
-    The issue here is that subscripts for `awk' arrays are _always_
- strings. Uninitialized variables, when used as strings, have the value
- `""', not zero.  Thus, `line 1' ends up stored in `l[""]'.  The
- following version of the program works correctly:
- 
-      { l[lines++] = $0 }
-      END {
-          for (i = lines - 1; i >= 0; --i)
-             print l[i]
-      }
- 
-    Here, the `++' forces `lines' to be numeric, thus making the "old
- value" numeric zero. This is then converted to `"0"' as the array
- subscript.
- 
-    Even though it is somewhat unusual, the null string (`""') is a
- valid array subscript.  (d.c.)  `gawk' warns about the use of the null
- string as a subscript if `--lint' is provided on the command line
- (*note Options::).
- 
- 
- File: gawk.info,  Node: Multi-dimensional,  Next: Arrays of Arrays,  Prev: 
Uninitialized Subscripts,  Up: Arrays
- 
- 8.5 Multidimensional Arrays
- ===========================
- 
- * Menu:
- 
- * Multi-scanning::              Scanning multidimensional arrays.
- 
-    A multidimensional array is an array in which an element is
- identified by a sequence of indices instead of a single index.  For
- example, a two-dimensional array requires two indices.  The usual way
- (in most languages, including `awk') to refer to an element of a
- two-dimensional array named `grid' is with `grid[X,Y]'.
- 
-    Multidimensional arrays are supported in `awk' through concatenation
- of indices into one string.  `awk' converts the indices into strings
- (*note Conversion::) and concatenates them together, with a separator
- between them.  This creates a single string that describes the values
- of the separate indices.  The combined string is used as a single index
- into an ordinary, one-dimensional array.  The separator used is the
- value of the built-in variable `SUBSEP'.
- 
-    For example, suppose we evaluate the expression `foo[5,12] = "value"'
- when the value of `SUBSEP' is `"@"'.  The numbers 5 and 12 are
- converted to strings and concatenated with an `@' between them,
- yielding `"address@hidden"'; thus, the array element `foo["address@hidden"]' 
is set to
- `"value"'.
- 
-    Once the element's value is stored, `awk' has no record of whether
- it was stored with a single index or a sequence of indices.  The two
- expressions `foo[5,12]' and `foo[5 SUBSEP 12]' are always equivalent.
- 
-    The default value of `SUBSEP' is the string `"\034"', which contains
- a nonprinting character that is unlikely to appear in an `awk' program
- or in most input data.  The usefulness of choosing an unlikely
- character comes from the fact that index values that contain a string
- matching `SUBSEP' can lead to combined strings that are ambiguous.
- Suppose that `SUBSEP' is `"@"'; then `foo["address@hidden", "c"]' and
- `foo["a", "address@hidden"]' are indistinguishable because both are actually
- stored as `foo["address@hidden@c"]'.
- 
-    To test whether a particular index sequence exists in a
- multidimensional array, use the same operator (`in') that is used for
- single dimensional arrays.  Write the whole sequence of indices in
- parentheses, separated by commas, as the left operand:
- 
-      (SUBSCRIPT1, SUBSCRIPT2, ...) in ARRAY
- 
-    The following example treats its input as a two-dimensional array of
- fields; it rotates this array 90 degrees clockwise and prints the
- result.  It assumes that all lines have the same number of elements:
- 
-      {
-           if (max_nf < NF)
-                max_nf = NF
-           max_nr = NR
-           for (x = 1; x <= NF; x++)
-                vector[x, NR] = $x
-      }
- 
-      END {
-           for (x = 1; x <= max_nf; x++) {
-                for (y = max_nr; y >= 1; --y)
-                     printf("%s ", vector[x, y])
-                printf("\n")
-           }
-      }
- 
- When given the input:
- 
-      1 2 3 4 5 6
-      2 3 4 5 6 1
-      3 4 5 6 1 2
-      4 5 6 1 2 3
- 
- the program produces the following output:
- 
-      4 3 2 1
-      5 4 3 2
-      6 5 4 3
-      1 6 5 4
-      2 1 6 5
-      3 2 1 6
- 
- 
- File: gawk.info,  Node: Multi-scanning,  Up: Multi-dimensional
- 
- 8.5.1 Scanning Multidimensional Arrays
- --------------------------------------
- 
- There is no special `for' statement for scanning a "multidimensional"
- array. There cannot be one, because, in truth, `awk' does not have
- multidimensional arrays or elements--there is only a multidimensional
- _way of accessing_ an array.
- 
-    However, if your program has an array that is always accessed as
- multidimensional, you can get the effect of scanning it by combining
- the scanning `for' statement (*note Scanning an Array::) with the
- built-in `split()' function (*note String Functions::).  It works in
- the following manner:
- 
-      for (combined in array) {
-          split(combined, separate, SUBSEP)
-          ...
-      }
- 
- This sets the variable `combined' to each concatenated combined index
- in the array, and splits it into the individual indices by breaking it
- apart where the value of `SUBSEP' appears.  The individual indices then
- become the elements of the array `separate'.
- 
-    Thus, if a value is previously stored in `array[1, "foo"]', then an
- element with index `"1\034foo"' exists in `array'.  (Recall that the
- default value of `SUBSEP' is the character with code 034.)  Sooner or
- later, the `for' statement finds that index and does an iteration with
- the variable `combined' set to `"1\034foo"'.  Then the `split()'
- function is called as follows:
- 
-      split("1\034foo", separate, "\034")
- 
- The result is to set `separate[1]' to `"1"' and `separate[2]' to
- `"foo"'.  Presto! The original sequence of separate indices is
- recovered.
- 
- 
- File: gawk.info,  Node: Arrays of Arrays,  Prev: Multi-dimensional,  Up: 
Arrays
- 
- 8.6 Arrays of Arrays
- ====================
- 
- `gawk' goes beyond standard `awk''s multidimensional array access and
- provides true arrays of arrays. Elements of a subarray are referred to
- by their own indices enclosed in square brackets, just like the
- elements of the main array.  For example, the following creates a
- two-element subarray at index `1' of the main array `a':
- 
-      a[1][1] = 1
-      a[1][2] = 2
- 
-    This simulates a true two-dimensional array. Each subarray element
- can contain another subarray as a value, which in turn can hold other
- arrays as well. In this way, you can create arrays of three or more
- dimensions.  The indices can be any `awk' expression, including scalars
- separated by commas (that is, a regular `awk' simulated
- multidimensional subscript). So the following is valid in `gawk':
- 
-      a[1][3][1, "name"] = "barney"
- 
-    Each subarray and the main array can be of different length. In
- fact, the elements of an array or its subarray do not all have to have
- the same type. This means that the main array and any of its subarrays
- can be non-rectangular, or jagged in structure. One can assign a scalar
- value to the index `4' of the main array `a':
- 
-      a[4] = "An element in a jagged array"
- 
-    The terms "dimension", "row" and "column" are meaningless when
- applied to such an array, but we will use "dimension" henceforth to
- imply the maximum number of indices needed to refer to an existing
- element. The type of any element that has already been assigned cannot
- be changed by assigning a value of a different type. You have to first
- delete the current element, which effectively makes `gawk' forget about
- the element at that index:
- 
-      delete a[4]
-      a[4][5][6][7] = "An element in a four-dimensional array"
- 
- This removes the scalar value from index `4' and then inserts a
- subarray of subarray of subarray containing a scalar. You can also
- delete an entire subarray or subarray of subarrays:
- 
-      delete a[4][5]
-      a[4][5] = "An element in subarray a[4]"
- 
-    But recall that you can not delete the main array `a' and then use it
- as a scalar.
- 
-    The built-in functions which take array arguments can also be used
- with subarrays. For example, the following code fragment uses `length()'
- (*note String Functions::) to determine the number of elements in the
- main array `a' and its subarrays:
- 
-      print length(a), length(a[1]), length(a[1][3])
- 
- This results in the following output for our main array `a':
- 
-      2, 3, 1
- 
- The `SUBSCRIPT in ARRAY' expression (*note Reference to Elements::)
- works similarly for both regular `awk'-style arrays and arrays of
- arrays. For example, the tests `1 in a', `3 in a[1]', and `(1, "name")
- in a[1][3]' all evaluate to one (true) for our array `a'.
- 
-    The `for (item in array)' statement (*note Scanning an Array::) can
- be nested to scan all the elements of an array of arrays if it is
- rectangular in structure. In order to print the contents (scalar
- values) of a two-dimensional array of arrays (i.e., in which each
- first-level element is itself an array, not necessarily of the same
- length) you could use the following code:
- 
-      for (i in array)
-          for (j in array[i])
-              print array[i][j]
- 
-    The `isarray()' function (*note Type Functions::) lets you test if
- an array element is itself an array:
- 
-      for (i in array) {
-          if (isarray(array[i]) {
-              for (j in array[i]) {
-                  print array[i][j]
-              }
-          }
-      }
- 
-    If the structure of a jagged array of arrays is known in advance,
- you can often devise workarounds using control statements. For example,
- the following code prints the elements of our main array `a':
- 
-      for (i in a) {
-          for (j in a[i]) {
-              if (j == 3) {
-                  for (k in a[i][j])
-                      print a[i][j][k]
-              } else
-                  print a[i][j]
-          }
-      }
- 
- *Note Walking Arrays::, for a user-defined function that will "walk" an
- arbitrarily-dimensioned array of arrays.
- 
-    Recall that a reference to an uninitialized array element yields a
- value of `""', the null string. This has one important implication when
- you intend to use a subarray as an argument to a function, as
- illustrated by the following example:
- 
-      $ gawk 'BEGIN { split("a b c d", b[1]); print b[1][1] }'
-      error--> gawk: cmd. line:1: fatal: split: second argument is not an array
- 
-    The way to work around this is to first force `b[1]' to be an array
- by creating an arbitrary index:
- 
-      $ gawk 'BEGIN { b[1][1] = ""; split("a b c d", b[1]); print b[1][1] }'
-      -| a
- 
- 
- File: gawk.info,  Node: Functions,  Next: Internationalization,  Prev: 
Arrays,  Up: Top
- 
- 9 Functions
- ***********
- 
- This major node describes `awk''s built-in functions, which fall into
- three categories: numeric, string, and I/O.  `gawk' provides additional
- groups of functions to work with values that represent time, do bit
- manipulation, sort arrays, and internationalize and localize programs.
- 
-    Besides the built-in functions, `awk' has provisions for writing new
- functions that the rest of a program can use.  The second half of this
- major node describes these "user-defined" functions.
- 
- * Menu:
- 
- * Built-in::                    Summarizes the built-in functions.
- * User-defined::                Describes User-defined functions in detail.
- * Indirect Calls::              Choosing the function to call at runtime.
- 
- 
- File: gawk.info,  Node: Built-in,  Next: User-defined,  Up: Functions
- 
- 9.1 Built-in Functions
- ======================
- 
- "Built-in" functions are always available for your `awk' program to
- call.  This minor node defines all the built-in functions in `awk';
- some of these are mentioned in other sections but are summarized here
- for your convenience.
- 
- * Menu:
- 
- * Calling Built-in::            How to call built-in functions.
- * Numeric Functions::           Functions that work with numbers, including
-                                 `int()', `sin()' and `rand()'.
- * String Functions::            Functions for string manipulation, such as
-                                 `split()', `match()' and
-                                 `sprintf()'.
- * I/O Functions::               Functions for files and shell commands.
- * Time Functions::              Functions for dealing with timestamps.
- * Bitwise Functions::           Functions for bitwise operations.
- * Type Functions::              Functions for type information.
- * I18N Functions::              Functions for string translation.
- 
- 
- File: gawk.info,  Node: Calling Built-in,  Next: Numeric Functions,  Up: 
Built-in
- 
- 9.1.1 Calling Built-in Functions
- --------------------------------
- 
- To call one of `awk''s built-in functions, write the name of the
- function followed by arguments in parentheses.  For example, `atan2(y +
- z, 1)' is a call to the function `atan2()' and has two arguments.
- 
-    Whitespace is ignored between the built-in function name and the
- open parenthesis, but nonetheless it is good practice to avoid using
- whitespace there.  User-defined functions do not permit whitespace in
- this way, and it is easier to avoid mistakes by following a simple
- convention that always works--no whitespace after a function name.
- 
-    Each built-in function accepts a certain number of arguments.  In
- some cases, arguments can be omitted. The defaults for omitted
- arguments vary from function to function and are described under the
- individual functions.  In some `awk' implementations, extra arguments
- given to built-in functions are ignored.  However, in `gawk', it is a
- fatal error to give extra arguments to a built-in function.
- 
-    When a function is called, expressions that create the function's
- actual parameters are evaluated completely before the call is performed.
- For example, in the following code fragment:
- 
-      i = 4
-      j = sqrt(i++)
- 
- the variable `i' is incremented to the value five before `sqrt()' is
- called with a value of four for its actual parameter.  The order of
- evaluation of the expressions used for the function's parameters is
- undefined.  Thus, avoid writing programs that assume that parameters
- are evaluated from left to right or from right to left.  For example:
- 
-      i = 5
-      j = atan2(i++, i *= 2)
- 
-    If the order of evaluation is left to right, then `i' first becomes
- 6, and then 12, and `atan2()' is called with the two arguments 6 and
- 12.  But if the order of evaluation is right to left, `i' first becomes
- 10, then 11, and `atan2()' is called with the two arguments 11 and 10.
- 
- 
- File: gawk.info,  Node: Numeric Functions,  Next: String Functions,  Prev: 
Calling Built-in,  Up: Built-in
- 
- 9.1.2 Numeric Functions
- -----------------------
- 
- The following list describes all of the built-in functions that work
- with numbers.  Optional parameters are enclosed in square
- brackets ([ ]):
- 
- `atan2(Y, X)'
-      Return the arctangent of `Y / X' in radians.  You can use `pi =
-      atan2(0, -1)' to retrieve the value of pi.
- 
- `cos(X)'
-      Return the cosine of X, with X in radians.
- 
- `exp(X)'
-      Return the exponential of X (`e ^ X') or report an error if X is
-      out of range.  The range of values X can have depends on your
-      machine's floating-point representation.
- 
- `int(X)'
-      Return the nearest integer to X, located between X and zero and
-      truncated toward zero.
- 
-      For example, `int(3)' is 3, `int(3.9)' is 3, `int(-3.9)' is -3,
-      and `int(-3)' is -3 as well.
- 
- `log(X)'
-      Return the natural logarithm of X, if X is positive; otherwise,
-      report an error.
- 
- `rand()'
-      Return a random number.  The values of `rand()' are uniformly
-      distributed between zero and one.  The value could be zero but is
-      never one.(1)
- 
-      Often random integers are needed instead.  Following is a
-      user-defined function that can be used to obtain a random
-      non-negative integer less than N:
- 
-           function randint(n) {
-                return int(n * rand())
-           }
- 
-      The multiplication produces a random number greater than zero and
-      less than `n'.  Using `int()', this result is made into an integer
-      between zero and `n' - 1, inclusive.
- 
-      The following example uses a similar function to produce random
-      integers between one and N.  This program prints a new random
-      number for each input record:
- 
-           # Function to roll a simulated die.
-           function roll(n) { return 1 + int(rand() * n) }
- 
-           # Roll 3 six-sided dice and
-           # print total number of points.
-           {
-                 printf("%d points\n",
-                        roll(6)+roll(6)+roll(6))
-           }
- 
-           CAUTION: In most `awk' implementations, including `gawk',
-           `rand()' starts generating numbers from the same starting
-           number, or "seed", each time you run `awk'.(2)  Thus, a
-           program generates the same results each time you run it.  The
-           numbers are random within one `awk' run but predictable from
-           run to run.  This is convenient for debugging, but if you want
-           a program to do different things each time it is used, you
-           must change the seed to a value that is different in each
-           run.  To do this, use `srand()'.
- 
- `sin(X)'
-      Return the sine of X, with X in radians.
- 
- `sqrt(X)'
-      Return the positive square root of X.  `gawk' prints a warning
-      message if X is negative.  Thus, `sqrt(4)' is 2.
- 
- `srand([X])'
-      Set the starting point, or seed, for generating random numbers to
-      the value X.
- 
-      Each seed value leads to a particular sequence of random
-      numbers.(3) Thus, if the seed is set to the same value a second
-      time, the same sequence of random numbers is produced again.
- 
-           CAUTION: Different `awk' implementations use different
-           random-number generators internally.  Don't expect the same
-           `awk' program to produce the same series of random numbers
-           when executed by different versions of `awk'.
- 
-      If the argument X is omitted, as in `srand()', then the current
-      date and time of day are used for a seed.  This is the way to get
-      random numbers that are truly unpredictable.
- 
-      The return value of `srand()' is the previous seed.  This makes it
-      easy to keep track of the seeds in case you need to consistently
-      reproduce sequences of random numbers.
- 
-    ---------- Footnotes ----------
- 
-    (1) The C version of `rand()' on many Unix systems is known to
- produce fairly poor sequences of random numbers.  However, nothing
- requires that an `awk' implementation use the C `rand()' to implement
- the `awk' version of `rand()'.  In fact, `gawk' uses the BSD `random()'
- function, which is considerably better than `rand()', to produce random
- numbers.
- 
-    (2) `mawk' uses a different seed each time.
- 
-    (3) Computer-generated random numbers really are not truly random.
- They are technically known as "pseudorandom."  This means that while
- the numbers in a sequence appear to be random, you can in fact generate
- the same sequence of random numbers over and over again.
- 
- 
- File: gawk.info,  Node: String Functions,  Next: I/O Functions,  Prev: 
Numeric Functions,  Up: Built-in
- 
- 9.1.3 String-Manipulation Functions
- -----------------------------------
- 
- The functions in this minor node look at or change the text of one or
- more strings.  `gawk' understands locales (*note Locales::), and does
- all string processing in terms of _characters_, not _bytes_.  This
- distinction is particularly important to understand for locales where
- one character may be represented by multiple bytes.  Thus, for example,
- `length()' returns the number of characters in a string, and not the
- number of bytes used to represent those characters, Similarly,
- `index()' works with character indices, and not byte indices.
- 
-    In the following list, optional parameters are enclosed in square
- brackets ([ ]).  Several functions perform string substitution; the
- full discussion is provided in the description of the `sub()' function,
- which comes towards the end since the list is presented in alphabetic
- order.  Those functions that are specific to `gawk' are marked with a
- pound sign (`#'):
- 
- * Menu:
- 
- * Gory Details::                More than you want to know about `\' and
-                                 `&' with `sub()', `gsub()', and
-                                 `gensub()'.
- 
- `asort(SOURCE [, DEST [, HOW  ] ]) #'
-      Return the number of elements in the array SOURCE.  `gawk' sorts
-      the contents of SOURCE and replaces the indices of the sorted
-      values of SOURCE with sequential integers starting with one.  If
-      the optional array DEST is specified, then SOURCE is duplicated
-      into DEST.  DEST is then sorted, leaving the indices of SOURCE
-      unchanged.  The optional third argument HOW is a string which
-      controls the rule for comparing values, and the sort direction.  A
-      single space is required between the comparison mode, `string' or
-      `number', and the direction specification, `ascending' or
-      `descending'.  You can omit direction and/or mode in which case it
-      will default to `ascending' and `string', respectively.  An empty
-      string "" is the same as the default `"ascending string"' for the
-      value of HOW.  If the `source' array contains subarrays as values,
-      they will come out last(first) in the `dest' array for
-      `ascending'(`descending') order specification.  The value of
-      `IGNORECASE' affects the sorting.  The third argument can also be
-      a user-defined function name in which case the value returned by
-      the function is used to order the array elements before
-      constructing the result array.  *Note Array Sorting Functions::,
-      for more information.
- 
-      For example, if the contents of `a' are as follows:
- 
-           a["last"] = "de"
-           a["first"] = "sac"
-           a["middle"] = "cul"
- 
-      A call to `asort()':
- 
-           asort(a)
- 
-      results in the following contents of `a':
- 
-           a[1] = "cul"
-           a[2] = "de"
-           a[3] = "sac"
- 
-      In order to reverse the direction of the sorted results in the
-      above example, `asort()' can be called with three arguments as
-      follows:
- 
-           asort(a, a, "descending")
- 
-      The `asort()' function is described in more detail in *note Array
-      Sorting Functions::.  `asort()' is a `gawk' extension; it is not
-      available in compatibility mode (*note Options::).
- 
- `asorti(SOURCE [, DEST [, HOW  ] ]) #'
-      Return the number of elements in the array SOURCE.  It works
-      similarly to `asort()', however, the _indices_ are sorted, instead
-      of the values. (Here too, `IGNORECASE' affects the sorting.)
- 
-      The `asorti()' function is described in more detail in *note Array
-      Sorting Functions::.  `asorti()' is a `gawk' extension; it is not
-      available in compatibility mode (*note Options::).
- 
- `gensub(REGEXP, REPLACEMENT, HOW [, TARGET]) #'
-      Search the target string TARGET for matches of the regular
-      expression REGEXP.  If HOW is a string beginning with `g' or `G'
-      (short for "global"), then replace all matches of REGEXP with
-      REPLACEMENT.  Otherwise, HOW is treated as a number indicating
-      which match of REGEXP to replace. If no TARGET is supplied, use
-      `$0'.  It returns the modified string as the result of the
-      function and the original target string is _not_ changed.
- 
-      `gensub()' is a general substitution function.  It's purpose is to
-      provide more features than the standard `sub()' and `gsub()'
-      functions.
- 
-      `gensub()' provides an additional feature that is not available in
-      `sub()' or `gsub()': the ability to specify components of a regexp
-      in the replacement text.  This is done by using parentheses in the
-      regexp to mark the components and then specifying `\N' in the
-      replacement text, where N is a digit from 1 to 9.  For example:
- 
-           $ gawk '
-           > BEGIN {
-           >      a = "abc def"
-           >      b = gensub(/(.+) (.+)/, "\\2 \\1", "g", a)
-           >      print b
-           > }'
-           -| def abc
- 
-      As with `sub()', you must type two backslashes in order to get one
-      into the string.  In the replacement text, the sequence `\0'
-      represents the entire matched text, as does the character `&'.
- 
-      The following example shows how you can use the third argument to
-      control which match of the regexp should be changed:
- 
-           $ echo a b c a b c |
-           > gawk '{ print gensub(/a/, "AA", 2) }'
-           -| a b c AA b c
- 
-      In this case, `$0' is the default target string.  `gensub()'
-      returns the new string as its result, which is passed directly to
-      `print' for printing.
- 
-      If the HOW argument is a string that does not begin with `g' or
-      `G', or if it is a number that is less than or equal to zero, only
-      one substitution is performed.  If HOW is zero, `gawk' issues a
-      warning message.
- 
-      If REGEXP does not match TARGET, `gensub()''s return value is the
-      original unchanged value of TARGET.
- 
-      `gensub()' is a `gawk' extension; it is not available in
-      compatibility mode (*note Options::).
- 
- `gsub(REGEXP, REPLACEMENT [, TARGET])'
-      Search TARGET for _all_ of the longest, leftmost, _nonoverlapping_
-      matching substrings it can find and replace them with REPLACEMENT.
-      The `g' in `gsub()' stands for "global," which means replace
-      everywhere.  For example:
- 
-           { gsub(/Britain/, "United Kingdom"); print }
- 
-      replaces all occurrences of the string `Britain' with `United
-      Kingdom' for all input records.
- 
-      The `gsub()' function returns the number of substitutions made.  If
-      the variable to search and alter (TARGET) is omitted, then the
-      entire input record (`$0') is used.  As in `sub()', the characters
-      `&' and `\' are special, and the third argument must be assignable.
- 
- `index(IN, FIND)'
-      Search the string IN for the first occurrence of the string FIND,
-      and return the position in characters where that occurrence begins
-      in the string IN.  Consider the following example:
- 
-           $ awk 'BEGIN { print index("peanut", "an") }'
-           -| 3
- 
-      If FIND is not found, `index()' returns zero.  (Remember that
-      string indices in `awk' start at one.)
- 
- `length([STRING])'
-      Return the number of characters in STRING.  If STRING is a number,
-      the length of the digit string representing that number is
-      returned.  For example, `length("abcde")' is five.  By contrast,
-      `length(15 * 35)' works out to three. In this example, 15 * 35 =
-      525, and 525 is then converted to the string `"525"', which has
-      three characters.
- 
-      If no argument is supplied, `length()' returns the length of `$0'.
- 
-           NOTE: In older versions of `awk', the `length()' function
-           could be called without any parentheses.  Doing so is
-           considered poor practice, although the 2008 POSIX standard
-           explicitly allows it, to support historical practice.  For
-           programs to be maximally portable, always supply the
-           parentheses.
- 
-      If `length()' is called with a variable that has not been used,
-      `gawk' forces the variable to be a scalar.  Other implementations
-      of `awk' leave the variable without a type.  (d.c.)  Consider:
- 
-           $ gawk 'BEGIN { print length(x) ; x[1] = 1 }'
-           -| 0
-           error--> gawk: fatal: attempt to use scalar `x' as array
- 
-           $ nawk 'BEGIN { print length(x) ; x[1] = 1 }'
-           -| 0
- 
-      If `--lint' has been specified on the command line, `gawk' issues a
-      warning about this.
- 
-      With `gawk' and several other `awk' implementations, when given an
-      array argument, the `length()' function returns the number of
-      elements in the array. (c.e.)  This is less useful than it might
-      seem at first, as the array is not guaranteed to be indexed from
-      one to the number of elements in it.  If `--lint' is provided on
-      the command line (*note Options::), `gawk' warns that passing an
-      array argument is not portable.  If `--posix' is supplied, using
-      an array argument is a fatal error (*note Arrays::).
- 
- `match(STRING, REGEXP [, ARRAY])'
-      Search STRING for the longest, leftmost substring matched by the
-      regular expression, REGEXP and return the character position, or
-      "index", at which that substring begins (one, if it starts at the
-      beginning of STRING).  If no match is found, return zero.
- 
-      The REGEXP argument may be either a regexp constant (`/.../') or a
-      string constant (`"..."').  In the latter case, the string is
-      treated as a regexp to be matched.  *Note Computed Regexps::, for a
-      discussion of the difference between the two forms, and the
-      implications for writing your program correctly.
- 
-      The order of the first two arguments is backwards from most other
-      string functions that work with regular expressions, such as
-      `sub()' and `gsub()'.  It might help to remember that for
-      `match()', the order is the same as for the `~' operator: `STRING
-      ~ REGEXP'.
- 
-      The `match()' function sets the built-in variable `RSTART' to the
-      index.  It also sets the built-in variable `RLENGTH' to the length
-      in characters of the matched substring.  If no match is found,
-      `RSTART' is set to zero, and `RLENGTH' to -1.
- 
-      For example:
- 
-           {
-                  if ($1 == "FIND")
-                    regex = $2
-                  else {
-                    where = match($0, regex)
-                    if (where != 0)
-                      print "Match of", regex, "found at",
-                                where, "in", $0
-                  }
-           }
- 
-      This program looks for lines that match the regular expression
-      stored in the variable `regex'.  This regular expression can be
-      changed.  If the first word on a line is `FIND', `regex' is
-      changed to be the second word on that line.  Therefore, if given:
- 
-           FIND ru+n
-           My program runs
-           but not very quickly
-           FIND Melvin
-           JF+KM
-           This line is property of Reality Engineering Co.
-           Melvin was here.
- 
-      `awk' prints:
- 
-           Match of ru+n found at 12 in My program runs
-           Match of Melvin found at 1 in Melvin was here.
- 
-      If ARRAY is present, it is cleared, and then the zeroth element of
-      ARRAY is set to the entire portion of STRING matched by REGEXP.
-      If REGEXP contains parentheses, the integer-indexed elements of
-      ARRAY are set to contain the portion of STRING matching the
-      corresponding parenthesized subexpression.  For example:
- 
-           $ echo foooobazbarrrrr |
-           > gawk '{ match($0, /(fo+).+(bar*)/, arr)
-           >         print arr[1], arr[2] }'
-           -| foooo barrrrr
- 
-      In addition, multidimensional subscripts are available providing
-      the start index and length of each matched subexpression:
- 
-           $ echo foooobazbarrrrr |
-           > gawk '{ match($0, /(fo+).+(bar*)/, arr)
-           >           print arr[1], arr[2]
-           >           print arr[1, "start"], arr[1, "length"]
-           >           print arr[2, "start"], arr[2, "length"]
-           > }'
-           -| foooo barrrrr
-           -| 1 5
-           -| 9 7
- 
-      There may not be subscripts for the start and index for every
-      parenthesized subexpression, since they may not all have matched
-      text; thus they should be tested for with the `in' operator (*note
-      Reference to Elements::).
- 
-      The ARRAY argument to `match()' is a `gawk' extension.  In
-      compatibility mode (*note Options::), using a third argument is a
-      fatal error.
- 
- `patsplit(STRING, ARRAY [, FIELDPAT [, SEPS ] ]) #'
-      Divide STRING into pieces defined by FIELDPAT and store the pieces
-      in ARRAY and the separator strings in the SEPS array.  The first
-      piece is stored in `ARRAY[1]', the second piece in `ARRAY[2]', and
-      so forth.  The third argument, FIELDPAT, is a regexp describing
-      the fields in STRING (just as `FPAT' is a regexp describing the
-      fields in input records).  It may be either a regexp constant or a
-      string.  If FIELDPAT is omitted, the value of `FPAT' is used.
-      `patsplit()' returns the number of elements created.  `SEPS[I]' is
-      the separator string between `ARRAY[I]' and `ARRAY[I+1]'.  Any
-      leading separator will be in `SEPS[0]'.
- 
-      The `patsplit()' function splits strings into pieces in a manner
-      similar to the way input lines are split into fields using `FPAT'
-      (*note Splitting By Content::.
- 
-      Before splitting the string, `patsplit()' deletes any previously
-      existing elements in the arrays ARRAY and SEPS.
- 
-      The `patsplit()' function is a `gawk' extension.  In compatibility
-      mode (*note Options::), it is not available.
- 
- `split(STRING, ARRAY [, FIELDSEP [, SEPS ] ])'
-      Divide STRING into pieces separated by FIELDSEP and store the
-      pieces in ARRAY and the separator strings in the SEPS array.  The
-      first piece is stored in `ARRAY[1]', the second piece in
-      `ARRAY[2]', and so forth.  The string value of the third argument,
-      FIELDSEP, is a regexp describing where to split STRING (much as
-      `FS' can be a regexp describing where to split input records;
-      *note Regexp Field Splitting::).  If FIELDSEP is omitted, the
-      value of `FS' is used.  `split()' returns the number of elements
-      created.  SEPS is a `gawk' extension with `SEPS[I]' being the
-      separator string between `ARRAY[I]' and `ARRAY[I+1]'.  If FIELDSEP
-      is a single space then any leading whitespace goes into `SEPS[0]'
-      and any trailing whitespace goes into `SEPS[N]' where N is the
-      return value of `split()' (that is, the number of elements in
-      ARRAY).
- 
-      The `split()' function splits strings into pieces in a manner
-      similar to the way input lines are split into fields.  For example:
- 
-           split("cul-de-sac", a, "-", seps)
- 
-      splits the string `cul-de-sac' into three fields using `-' as the
-      separator.  It sets the contents of the array `a' as follows:
- 
-           a[1] = "cul"
-           a[2] = "de"
-           a[3] = "sac"
- 
-      and sets the contents of the array `seps' as follows:
- 
-           seps[1] = "-"
-           seps[2] = "-"
- 
-      The value returned by this call to `split()' is three.
- 
-      As with input field-splitting, when the value of FIELDSEP is
-      `" "', leading and trailing whitespace is ignored in values
-      assigned to the elements of ARRAY but not in SEPS, and the elements
-      are separated by runs of whitespace.  Also as with input
-      field-splitting, if FIELDSEP is the null string, each individual
-      character in the string is split into its own array element.
-      (c.e.)
- 
-      Note, however, that `RS' has no effect on the way `split()' works.
-      Even though `RS = ""' causes newline to also be an input field
-      separator, this does not affect how `split()' splits strings.
- 
-      Modern implementations of `awk', including `gawk', allow the third
-      argument to be a regexp constant (`/abc/') as well as a string.
-      (d.c.)  The POSIX standard allows this as well.  *Note Computed
-      Regexps::, for a discussion of the difference between using a
-      string constant or a regexp constant, and the implications for
-      writing your program correctly.
- 
-      Before splitting the string, `split()' deletes any previously
-      existing elements in the arrays ARRAY and SEPS.
- 
-      If STRING is null, the array has no elements. (So this is a
-      portable way to delete an entire array with one statement.  *Note
-      Delete::.)
- 
-      If STRING does not match FIELDSEP at all (but is not null), ARRAY
-      has one element only. The value of that element is the original
-      STRING.
- 
- `sprintf(FORMAT, EXPRESSION1, ...)'
-      Return (without printing) the string that `printf' would have
-      printed out with the same arguments (*note Printf::).  For example:
- 
-           pival = sprintf("pi = %.2f (approx.)", 22/7)
- 
-      assigns the string `pi = 3.14 (approx.)' to the variable `pival'.
- 
- `strtonum(STR) #'
-      Examine STR and return its numeric value.  If STR begins with a
-      leading `0', `strtonum()' assumes that STR is an octal number.  If
-      STR begins with a leading `0x' or `0X', `strtonum()' assumes that
-      STR is a hexadecimal number.  For example:
- 
-           $ echo 0x11 |
-           > gawk '{ printf "%d\n", strtonum($1) }'
-           -| 17
- 
-      Using the `strtonum()' function is _not_ the same as adding zero
-      to a string value; the automatic coercion of strings to numbers
-      works only for decimal data, not for octal or hexadecimal.(1)
- 
-      Note also that `strtonum()' uses the current locale's decimal point
-      for recognizing numbers (*note Locales::).
- 
-      `strtonum()' is a `gawk' extension; it is not available in
-      compatibility mode (*note Options::).
- 
- `sub(REGEXP, REPLACEMENT [, TARGET])'
-      Search TARGET, which is treated as a string, for the leftmost,
-      longest substring matched by the regular expression REGEXP.
-      Modify the entire string by replacing the matched text with
-      REPLACEMENT.  The modified string becomes the new value of TARGET.
-      Return the number of substitutions made (zero or one).
- 
-      The REGEXP argument may be either a regexp constant (`/.../') or a
-      string constant (`"..."').  In the latter case, the string is
-      treated as a regexp to be matched.  *Note Computed Regexps::, for a
-      discussion of the difference between the two forms, and the
-      implications for writing your program correctly.
- 
-      This function is peculiar because TARGET is not simply used to
-      compute a value, and not just any expression will do--it must be a
-      variable, field, or array element so that `sub()' can store a
-      modified value there.  If this argument is omitted, then the
-      default is to use and alter `$0'.(2) For example:
- 
-           str = "water, water, everywhere"
-           sub(/at/, "ith", str)
- 
-      sets `str' to `wither, water, everywhere', by replacing the
-      leftmost longest occurrence of `at' with `ith'.
- 
-      If the special character `&' appears in REPLACEMENT, it stands for
-      the precise substring that was matched by REGEXP.  (If the regexp
-      can match more than one string, then this precise substring may
-      vary.)  For example:
- 
-           { sub(/candidate/, "& and his wife"); print }
- 
-      changes the first occurrence of `candidate' to `candidate and his
-      wife' on each input line.  Here is another example:
- 
-           $ awk 'BEGIN {
-           >         str = "daabaaa"
-           >         sub(/a+/, "C&C", str)
-           >         print str
-           > }'
-           -| dCaaCbaaa
- 
-      This shows how `&' can represent a nonconstant string and also
-      illustrates the "leftmost, longest" rule in regexp matching (*note
-      Leftmost Longest::).
- 
-      The effect of this special character (`&') can be turned off by
-      putting a backslash before it in the string.  As usual, to insert
-      one backslash in the string, you must write two backslashes.
-      Therefore, write `\\&' in a string constant to include a literal
-      `&' in the replacement.  For example, the following shows how to
-      replace the first `|' on each line with an `&':
- 
-           { sub(/\|/, "\\&"); print }
- 
-      As mentioned, the third argument to `sub()' must be a variable,
-      field or array element.  Some versions of `awk' allow the third
-      argument to be an expression that is not an lvalue.  In such a
-      case, `sub()' still searches for the pattern and returns zero or
-      one, but the result of the substitution (if any) is thrown away
-      because there is no place to put it.  Such versions of `awk'
-      accept expressions like the following:
- 
-           sub(/USA/, "United States", "the USA and Canada")
- 
-      For historical compatibility, `gawk' accepts such erroneous code.
-      However, using any other nonchangeable object as the third
-      parameter causes a fatal error and your program will not run.
- 
-      Finally, if the REGEXP is not a regexp constant, it is converted
-      into a string, and then the value of that string is treated as the
-      regexp to match.
- 
- `substr(STRING, START [, LENGTH])'
-      Return a LENGTH-character-long substring of STRING, starting at
-      character number START.  The first character of a string is
-      character number one.(3) For example, `substr("washington", 5, 3)'
-      returns `"ing"'.
- 
-      If LENGTH is not present, `substr()' returns the whole suffix of
-      STRING that begins at character number START.  For example,
-      `substr("washington", 5)' returns `"ington"'.  The whole suffix is
-      also returned if LENGTH is greater than the number of characters
-      remaining in the string, counting from character START.
- 
-      If START is less than one, `substr()' treats it as if it was one.
-      (POSIX doesn't specify what to do in this case: Brian Kernighan's
-      `awk' acts this way, and therefore `gawk' does too.)  If START is
-      greater than the number of characters in the string, `substr()'
-      returns the null string.  Similarly, if LENGTH is present but less
-      than or equal to zero, the null string is returned.
- 
-      The string returned by `substr()' _cannot_ be assigned.  Thus, it
-      is a mistake to attempt to change a portion of a string, as shown
-      in the following example:
- 
-           string = "abcdef"
-           # try to get "abCDEf", won't work
-           substr(string, 3, 3) = "CDE"
- 
-      It is also a mistake to use `substr()' as the third argument of
-      `sub()' or `gsub()':
- 
-           gsub(/xyz/, "pdq", substr($0, 5, 20))  # WRONG
- 
-      (Some commercial versions of `awk' treat `substr()' as assignable,
-      but doing so is not portable.)
- 
-      If you need to replace bits and pieces of a string, combine
-      `substr()' with string concatenation, in the following manner:
- 
-           string = "abcdef"
-           ...
-           string = substr(string, 1, 2) "CDE" substr(string, 6)
- 
- `tolower(STRING)'
-      Return a copy of STRING, with each uppercase character in the
-      string replaced with its corresponding lowercase character.
-      Nonalphabetic characters are left unchanged.  For example,
-      `tolower("MiXeD cAsE 123")' returns `"mixed case 123"'.
- 
- `toupper(STRING)'
-      Return a copy of STRING, with each lowercase character in the
-      string replaced with its corresponding uppercase character.
-      Nonalphabetic characters are left unchanged.  For example,
-      `toupper("MiXeD cAsE 123")' returns `"MIXED CASE 123"'.
- 
-    ---------- Footnotes ----------
- 
-    (1) Unless you use the `--non-decimal-data' option, which isn't
- recommended.  *Note Nondecimal Data::, for more information.
- 
-    (2) Note that this means that the record will first be regenerated
- using the value of `OFS' if any fields have been changed, and that the
- fields will be updated after the substitution, even if the operation is
- a "no-op" such as `sub(/^/, "")'.
- 
-    (3) This is different from C and C++, in which the first character
- is number zero.
- 
- 
- File: gawk.info,  Node: Gory Details,  Up: String Functions
- 
- 9.1.3.1 More About `\' and `&' with `sub()', `gsub()', and `gensub()'
- .....................................................................
- 
- When using `sub()', `gsub()', or `gensub()', and trying to get literal
- backslashes and ampersands into the replacement text, you need to
- remember that there are several levels of "escape processing" going on.
- 
-    First, there is the "lexical" level, which is when `awk' reads your
- program and builds an internal copy of it that can be executed.  Then
- there is the runtime level, which is when `awk' actually scans the
- replacement string to determine what to generate.
- 
-    At both levels, `awk' looks for a defined set of characters that can
- come after a backslash.  At the lexical level, it looks for the escape
- sequences listed in *note Escape Sequences::.  Thus, for every `\' that
- `awk' processes at the runtime level, you must type two backslashes at
- the lexical level.  When a character that is not valid for an escape
- sequence follows the `\', Brian Kernighan's `awk' and `gawk' both
- simply remove the initial `\' and put the next character into the
- string. Thus, for example, `"a\qb"' is treated as `"aqb"'.
- 
-    At the runtime level, the various functions handle sequences of `\'
- and `&' differently.  The situation is (sadly) somewhat complex.
- Historically, the `sub()' and `gsub()' functions treated the two
- character sequence `\&' specially; this sequence was replaced in the
- generated text with a single `&'.  Any other `\' within the REPLACEMENT
- string that did not precede an `&' was passed through unchanged.  This
- is illustrated in *note table-sub-escapes::.
- 
-       You type         `sub()' sees          `sub()' generates
-       -------         ---------          --------------
-           `\&'              `&'            the matched text
-          `\\&'             `\&'            a literal `&'
-         `\\\&'             `\&'            a literal `&'
-        `\\\\&'            `\\&'            a literal `\&'
-       `\\\\\&'            `\\&'            a literal `\&'
-      `\\\\\\&'           `\\\&'            a literal `\\&'
-          `\\q'             `\q'            a literal `\q'
- 
- Table 9.1: Historical Escape Sequence Processing for `sub()' and
- `gsub()'
- 
- This table shows both the lexical-level processing, where an odd number
- of backslashes becomes an even number at the runtime level, as well as
- the runtime processing done by `sub()'.  (For the sake of simplicity,
- the rest of the following tables only show the case of even numbers of
- backslashes entered at the lexical level.)
- 
-    The problem with the historical approach is that there is no way to
- get a literal `\' followed by the matched text.
- 
-    The 1992 POSIX standard attempted to fix this problem. That standard
- says that `sub()' and `gsub()' look for either a `\' or an `&' after
- the `\'. If either one follows a `\', that character is output
- literally.  The interpretation of `\' and `&' then becomes as shown in
- *note table-sub-posix-92::.
- 
-       You type         `sub()' sees          `sub()' generates
-       -------         ---------          --------------
-            `&'              `&'            the matched text
-          `\\&'             `\&'            a literal `&'
-        `\\\\&'            `\\&'            a literal `\', then the matched 
text
-      `\\\\\\&'           `\\\&'            a literal `\&'
- 
- Table 9.2: 1992 POSIX Rules for sub and gsub Escape Sequence Processing
- 
- This appears to solve the problem.  Unfortunately, the phrasing of the
- standard is unusual. It says, in effect, that `\' turns off the special
- meaning of any following character, but for anything other than `\' and
- `&', such special meaning is undefined.  This wording leads to two
- problems:
- 
-    * Backslashes must now be doubled in the REPLACEMENT string, breaking
-      historical `awk' programs.
- 
-    * To make sure that an `awk' program is portable, _every_ character
-      in the REPLACEMENT string must be preceded with a backslash.(1)
- 
-    Because of the problems just listed, in 1996, the `gawk' maintainer
- submitted proposed text for a revised standard that reverts to rules
- that correspond more closely to the original existing practice. The
- proposed rules have special cases that make it possible to produce a
- `\' preceding the matched text. This is shown in *note
- table-sub-proposed::.
- 
-       You type         `sub()' sees         `sub()' generates
-       -------         ---------         --------------
-      `\\\\\\&'           `\\\&'            a literal `\&'
-        `\\\\&'            `\\&'            a literal `\', followed by the 
matched text
-          `\\&'             `\&'            a literal `&'
-          `\\q'             `\q'            a literal `\q'
-         `\\\\'             `\\'            `\\'
- 
- Table 9.3: Proposed rules for sub and backslash
- 
-    In a nutshell, at the runtime level, there are now three special
- sequences of characters (`\\\&', `\\&' and `\&') whereas historically
- there was only one.  However, as in the historical case, any `\' that
- is not part of one of these three sequences is not special and appears
- in the output literally.
- 
-    `gawk' 3.0 and 3.1 follow these proposed POSIX rules for `sub()' and
- `gsub()'.  The POSIX standard took much longer to be revised than was
- expected in 1996.  The 2001 standard does not follow the above rules.
- Instead, the rules there are somewhat simpler.  The results are similar
- except for one case.
- 
-    The POSIX rules state that `\&' in the replacement string produces a
- literal `&', `\\' produces a literal `\', and `\' followed by anything
- else is not special; the `\' is placed straight into the output.  These
- rules are presented in *note table-posix-sub::.
- 
-       You type         `sub()' sees         `sub()' generates
-       -------         ---------         --------------
-      `\\\\\\&'           `\\\&'            a literal `\&'
-        `\\\\&'            `\\&'            a literal `\', followed by the 
matched text
-          `\\&'             `\&'            a literal `&'
-          `\\q'             `\q'            a literal `\q'
-         `\\\\'             `\\'            `\'
- 
- Table 9.4: POSIX rules for `sub()' and `gsub()'
- 
-    The only case where the difference is noticeable is the last one:
- `\\\\' is seen as `\\' and produces `\' instead of `\\'.
- 
-    Starting with version 3.1.4, `gawk' followed the POSIX rules when
- `--posix' is specified (*note Options::). Otherwise, it continued to
- follow the 1996 proposed rules, since that had been its behavior for
- many years.
- 
-    When version 4.0.0, was released, the `gawk' maintainer made the
- POSIX rules the default, breaking well over a decade's worth of
- backwards compatibility.(2) Needless to say, this was a bad idea, and
- as of version 4.0.1, `gawk' resumed its historical behavior, and only
- follows the POSIX rules when `--posix' is given.
- 
-    The rules for `gensub()' are considerably simpler. At the runtime
- level, whenever `gawk' sees a `\', if the following character is a
- digit, then the text that matched the corresponding parenthesized
- subexpression is placed in the generated output.  Otherwise, no matter
- what character follows the `\', it appears in the generated text and
- the `\' does not, as shown in *note table-gensub-escapes::.
- 
-        You type          `gensub()' sees         `gensub()' generates
-        -------          ------------         -----------------
-            `&'                    `&'            the matched text
-          `\\&'                   `\&'            a literal `&'
-         `\\\\'                   `\\'            a literal `\'
-        `\\\\&'                  `\\&'            a literal `\', then the 
matched text
-      `\\\\\\&'                 `\\\&'            a literal `\&'
-          `\\q'                   `\q'            a literal `q'
- 
- Table 9.5: Escape Sequence Processing for `gensub()'
- 
-    Because of the complexity of the lexical and runtime level processing
- and the special cases for `sub()' and `gsub()', we recommend the use of
- `gawk' and `gensub()' when you have to do substitutions.
- 
- Advanced Notes: Matching the Null String
- ----------------------------------------
- 
- In `awk', the `*' operator can match the null string.  This is
- particularly important for the `sub()', `gsub()', and `gensub()'
- functions.  For example:
- 
-      $ echo abc | awk '{ gsub(/m*/, "X"); print }'
-      -| XaXbXcX
- 
- Although this makes a certain amount of sense, it can be surprising.
- 
-    ---------- Footnotes ----------
- 
-    (1) This consequence was certainly unintended.
- 
-    (2) This was rather naive of him, despite there being a note in this
- section indicating that the next major version would move to the POSIX
- rules.
- 
- 
- File: gawk.info,  Node: I/O Functions,  Next: Time Functions,  Prev: String 
Functions,  Up: Built-in
- 
- 9.1.4 Input/Output Functions
- ----------------------------
- 
- The following functions relate to input/output (I/O).  Optional
- parameters are enclosed in square brackets ([ ]):
- 
- `close(FILENAME [, HOW])'
-      Close the file FILENAME for input or output. Alternatively, the
-      argument may be a shell command that was used for creating a
-      coprocess, or for redirecting to or from a pipe; then the
-      coprocess or pipe is closed.  *Note Close Files And Pipes::, for
-      more information.
- 
-      When closing a coprocess, it is occasionally useful to first close
-      one end of the two-way pipe and then to close the other.  This is
-      done by providing a second argument to `close()'.  This second
-      argument should be one of the two string values `"to"' or `"from"',
-      indicating which end of the pipe to close.  Case in the string does
-      not matter.  *Note Two-way I/O::, which discusses this feature in
-      more detail and gives an example.
- 
- `fflush([FILENAME])'
-      Flush any buffered output associated with FILENAME, which is
-      either a file opened for writing or a shell command for
-      redirecting output to a pipe or coprocess. (c.e.).
- 
-      Many utility programs "buffer" their output; i.e., they save
-      information to write to a disk file or the screen in memory until
-      there is enough for it to be worthwhile to send the data to the
-      output device.  This is often more efficient than writing every
-      little bit of information as soon as it is ready.  However,
-      sometimes it is necessary to force a program to "flush" its
-      buffers; that is, write the information to its destination, even
-      if a buffer is not full.  This is the purpose of the `fflush()'
-      function--`gawk' also buffers its output and the `fflush()'
-      function forces `gawk' to flush its buffers.
- 
-      `fflush()' was added to Brian Kernighan's version of `awk' in
-      1994; it is not part of the POSIX standard and is not available if
-      `--posix' has been specified on the command line (*note Options::).
- 
-      `gawk' extends the `fflush()' function in two ways.  The first is
-      to allow no argument at all. In this case, the buffer for the
-      standard output is flushed.  The second is to allow the null string
-      (`""') as the argument. In this case, the buffers for _all_ open
-      output files and pipes are flushed.  Brian Kernighan's `awk' also
-      supports these extensions.
- 
-      `fflush()' returns zero if the buffer is successfully flushed;
-      otherwise, it returns -1.  In the case where all buffers are
-      flushed, the return value is zero only if all buffers were flushed
-      successfully.  Otherwise, it is -1, and `gawk' warns about the
-      problem FILENAME.
- 
-      `gawk' also issues a warning message if you attempt to flush a
-      file or pipe that was opened for reading (such as with `getline'),
-      or if FILENAME is not an open file, pipe, or coprocess.  In such a
-      case, `fflush()' returns -1, as well.
- 
- `system(COMMAND)'
-      Execute the operating-system command COMMAND and then return to
-      the `awk' program.  Return COMMAND's exit status.
- 
-      For example, if the following fragment of code is put in your `awk'
-      program:
- 
-           END {
-                system("date | mail -s 'awk run done' root")
-           }
- 
-      the system administrator is sent mail when the `awk' program
-      finishes processing input and begins its end-of-input processing.
- 
-      Note that redirecting `print' or `printf' into a pipe is often
-      enough to accomplish your task.  If you need to run many commands,
-      it is more efficient to simply print them down a pipeline to the
-      shell:
- 
-           while (MORE STUFF TO DO)
-               print COMMAND | "/bin/sh"
-           close("/bin/sh")
- 
-      However, if your `awk' program is interactive, `system()' is
-      useful for running large self-contained programs, such as a shell
-      or an editor.  Some operating systems cannot implement the
-      `system()' function.  `system()' causes a fatal error if it is not
-      supported.
- 
-           NOTE: When `--sandbox' is specified, the `system()' function
-           is disabled (*note Options::).
- 
- 
- Advanced Notes: Interactive Versus Noninteractive Buffering
- -----------------------------------------------------------
- 
- As a side point, buffering issues can be even more confusing, depending
- upon whether your program is "interactive", i.e., communicating with a
- user sitting at a keyboard.(1)
- 
-    Interactive programs generally "line buffer" their output; i.e., they
- write out every line.  Noninteractive programs wait until they have a
- full buffer, which may be many lines of output.  Here is an example of
- the difference:
- 
-      $ awk '{ print $1 + $2 }'
-      1 1
-      -| 2
-      2 3
-      -| 5
-      Ctrl-d
- 
- Each line of output is printed immediately. Compare that behavior with
- this example:
- 
-      $ awk '{ print $1 + $2 }' | cat
-      1 1
-      2 3
-      Ctrl-d
-      -| 2
-      -| 5
- 
- Here, no output is printed until after the `Ctrl-d' is typed, because
- it is all buffered and sent down the pipe to `cat' in one shot.
- 
- Advanced Notes: Controlling Output Buffering with `system()'
- ------------------------------------------------------------
- 
- The `fflush()' function provides explicit control over output buffering
- for individual files and pipes.  However, its use is not portable to
- many other `awk' implementations.  An alternative method to flush output
- buffers is to call `system()' with a null string as its argument:
- 
-      system("")   # flush output
- 
- `gawk' treats this use of the `system()' function as a special case and
- is smart enough not to run a shell (or other command interpreter) with
- the empty command.  Therefore, with `gawk', this idiom is not only
- useful, it is also efficient.  While this method should work with other
- `awk' implementations, it does not necessarily avoid starting an
- unnecessary shell.  (Other implementations may only flush the buffer
- associated with the standard output and not necessarily all buffered
- output.)
- 
-    If you think about what a programmer expects, it makes sense that
- `system()' should flush any pending output.  The following program:
- 
-      BEGIN {
-           print "first print"
-           system("echo system echo")
-           print "second print"
-      }
- 
- must print:
- 
-      first print
-      system echo
-      second print
- 
- and not:
- 
-      system echo
-      first print
-      second print
- 
-    If `awk' did not flush its buffers before calling `system()', you
- would see the latter (undesirable) output.
- 
-    ---------- Footnotes ----------
- 
-    (1) A program is interactive if the standard output is connected to
- a terminal device. On modern systems, this means your keyboard and
- screen.
- 
- 
- File: gawk.info,  Node: Time Functions,  Next: Bitwise Functions,  Prev: I/O 
Functions,  Up: Built-in
- 
- 9.1.5 Time Functions
- --------------------
- 
- `awk' programs are commonly used to process log files containing
- timestamp information, indicating when a particular log record was
- written.  Many programs log their timestamp in the form returned by the
- `time()' system call, which is the number of seconds since a particular
- epoch.  On POSIX-compliant systems, it is the number of seconds since
- 1970-01-01 00:00:00 UTC, not counting leap seconds.(1) All known
- POSIX-compliant systems support timestamps from 0 through 2^31 - 1,
- which is sufficient to represent times through 2038-01-19 03:14:07 UTC.
- Many systems support a wider range of timestamps, including negative
- timestamps that represent times before the epoch.
- 
-    In order to make it easier to process such log files and to produce
- useful reports, `gawk' provides the following functions for working
- with timestamps.  They are `gawk' extensions; they are not specified in
- the POSIX standard, nor are they in any other known version of `awk'.(2)
- Optional parameters are enclosed in square brackets ([ ]):
- 
- `mktime(DATESPEC)'
-      Turn DATESPEC into a timestamp in the same form as is returned by
-      `systime()'.  It is similar to the function of the same name in
-      ISO C.  The argument, DATESPEC, is a string of the form
-      `"YYYY MM DD HH MM SS [DST]"'.  The string consists of six or
-      seven numbers representing, respectively, the full year including
-      century, the month from 1 to 12, the day of the month from 1 to
-      31, the hour of the day from 0 to 23, the minute from 0 to 59, the
-      second from 0 to 60,(3) and an optional daylight-savings flag.
- 
-      The values of these numbers need not be within the ranges
-      specified; for example, an hour of -1 means 1 hour before midnight.
-      The origin-zero Gregorian calendar is assumed, with year 0
-      preceding year 1 and year -1 preceding year 0.  The time is
-      assumed to be in the local timezone.  If the daylight-savings flag
-      is positive, the time is assumed to be daylight savings time; if
-      zero, the time is assumed to be standard time; and if negative
-      (the default), `mktime()' attempts to determine whether daylight
-      savings time is in effect for the specified time.
- 
-      If DATESPEC does not contain enough elements or if the resulting
-      time is out of range, `mktime()' returns -1.
- 
- `strftime([FORMAT [, TIMESTAMP [, UTC-FLAG]]])'
-      Format the time specified by TIMESTAMP based on the contents of
-      the FORMAT string and return the result.  It is similar to the
-      function of the same name in ISO C.  If UTC-FLAG is present and is
-      either nonzero or non-null, the value is formatted as UTC
-      (Coordinated Universal Time, formerly GMT or Greenwich Mean Time).
-      Otherwise, the value is formatted for the local time zone.  The
-      TIMESTAMP is in the same format as the value returned by the
-      `systime()' function.  If no TIMESTAMP argument is supplied,
-      `gawk' uses the current time of day as the timestamp.  If no
-      FORMAT argument is supplied, `strftime()' uses the value of
-      `PROCINFO["strftime"]' as the format string (*note Built-in
-      Variables::).  The default string value is
-      `"%a %b %e %H:%M:%S %Z %Y"'.  This format string produces output
-      that is equivalent to that of the `date' utility.  You can assign
-      a new value to `PROCINFO["strftime"]' to change the default format.
- 
- `systime()'
-      Return the current time as the number of seconds since the system
-      epoch.  On POSIX systems, this is the number of seconds since
-      1970-01-01 00:00:00 UTC, not counting leap seconds.  It may be a
-      different number on other systems.
- 
-    The `systime()' function allows you to compare a timestamp from a
- log file with the current time of day.  In particular, it is easy to
- determine how long ago a particular record was logged.  It also allows
- you to produce log records using the "seconds since the epoch" format.
- 
-    The `mktime()' function allows you to convert a textual
- representation of a date and time into a timestamp.   This makes it
- easy to do before/after comparisons of dates and times, particularly
- when dealing with date and time data coming from an external source,
- such as a log file.
- 
-    The `strftime()' function allows you to easily turn a timestamp into
- human-readable information.  It is similar in nature to the `sprintf()'
- function (*note String Functions::), in that it copies nonformat
- specification characters verbatim to the returned string, while
- substituting date and time values for format specifications in the
- FORMAT string.
- 
-    `strftime()' is guaranteed by the 1999 ISO C standard(4) to support
- the following date format specifications:
- 
- `%a'
-      The locale's abbreviated weekday name.
- 
- `%A'
-      The locale's full weekday name.
- 
- `%b'
-      The locale's abbreviated month name.
- 
- `%B'
-      The locale's full month name.
- 
- `%c'
-      The locale's "appropriate" date and time representation.  (This is
-      `%A %B %d %T %Y' in the `"C"' locale.)
- 
- `%C'
-      The century part of the current year.  This is the year divided by
-      100 and truncated to the next lower integer.
- 
- `%d'
-      The day of the month as a decimal number (01-31).
- 
- `%D'
-      Equivalent to specifying `%m/%d/%y'.
- 
- `%e'
-      The day of the month, padded with a space if it is only one digit.
- 
- `%F'
-      Equivalent to specifying `%Y-%m-%d'.  This is the ISO 8601 date
-      format.
- 
- `%g'
-      The year modulo 100 of the ISO 8601 week number, as a decimal
-      number (00-99).  For example, January 1, 1993 is in week 53 of
-      1992. Thus, the year of its ISO 8601 week number is 1992, even
-      though its year is 1993.  Similarly, December 31, 1973 is in week
-      1 of 1974. Thus, the year of its ISO week number is 1974, even
-      though its year is 1973.
- 
- `%G'
-      The full year of the ISO week number, as a decimal number.
- 
- `%h'
-      Equivalent to `%b'.
- 
- `%H'
-      The hour (24-hour clock) as a decimal number (00-23).
- 
- `%I'
-      The hour (12-hour clock) as a decimal number (01-12).
- 
- `%j'
-      The day of the year as a decimal number (001-366).
- 
- `%m'
-      The month as a decimal number (01-12).
- 
- `%M'
-      The minute as a decimal number (00-59).
- 
- `%n'
-      A newline character (ASCII LF).
- 
- `%p'
-      The locale's equivalent of the AM/PM designations associated with
-      a 12-hour clock.
- 
- `%r'
-      The locale's 12-hour clock time.  (This is `%I:%M:%S %p' in the
-      `"C"' locale.)
- 
- `%R'
-      Equivalent to specifying `%H:%M'.
- 
- `%S'
-      The second as a decimal number (00-60).
- 
- `%t'
-      A TAB character.
- 
- `%T'
-      Equivalent to specifying `%H:%M:%S'.
- 
- `%u'
-      The weekday as a decimal number (1-7).  Monday is day one.
- 
- `%U'
-      The week number of the year (the first Sunday as the first day of
-      week one) as a decimal number (00-53).
- 
- `%V'
-      The week number of the year (the first Monday as the first day of
-      week one) as a decimal number (01-53).  The method for determining
-      the week number is as specified by ISO 8601.  (To wit: if the week
-      containing January 1 has four or more days in the new year, then
-      it is week one; otherwise it is week 53 of the previous year and
-      the next week is week one.)
- 
- `%w'
-      The weekday as a decimal number (0-6).  Sunday is day zero.
- 
- `%W'
-      The week number of the year (the first Monday as the first day of
-      week one) as a decimal number (00-53).
- 
- `%x'
-      The locale's "appropriate" date representation.  (This is `%A %B
-      %d %Y' in the `"C"' locale.)
- 
- `%X'
-      The locale's "appropriate" time representation.  (This is `%T' in
-      the `"C"' locale.)
- 
- `%y'
-      The year modulo 100 as a decimal number (00-99).
- 
- `%Y'
-      The full year as a decimal number (e.g., 2011).
- 
- `%z'
-      The timezone offset in a +HHMM format (e.g., the format necessary
-      to produce RFC 822/RFC 1036 date headers).
- 
- `%Z'
-      The time zone name or abbreviation; no characters if no time zone
-      is determinable.
- 
- `%Ec %EC %Ex %EX %Ey %EY %Od %Oe %OH'
- `%OI %Om %OM %OS %Ou %OU %OV %Ow %OW %Oy'
-      "Alternate representations" for the specifications that use only
-      the second letter (`%c', `%C', and so on).(5) (These facilitate
-      compliance with the POSIX `date' utility.)
- 
- `%%'
-      A literal `%'.
- 
-    If a conversion specifier is not one of the above, the behavior is
- undefined.(6)
- 
-    Informally, a "locale" is the geographic place in which a program is
- meant to run.  For example, a common way to abbreviate the date
- September 4, 2012 in the United States is "9/4/12."  In many countries
- in Europe, however, it is abbreviated "4.9.12."  Thus, the `%x'
- specification in a `"US"' locale might produce `9/4/12', while in a
- `"EUROPE"' locale, it might produce `4.9.12'.  The ISO C standard
- defines a default `"C"' locale, which is an environment that is typical
- of what many C programmers are used to.
- 
-    For systems that are not yet fully standards-compliant, `gawk'
- supplies a copy of `strftime()' from the GNU C Library.  It supports
- all of the just-listed format specifications.  If that version is used
- to compile `gawk' (*note Installation::), then the following additional
- format specifications are available:
- 
- `%k'
-      The hour (24-hour clock) as a decimal number (0-23).  Single-digit
-      numbers are padded with a space.
- 
- `%l'
-      The hour (12-hour clock) as a decimal number (1-12).  Single-digit
-      numbers are padded with a space.
- 
- `%s'
-      The time as a decimal timestamp in seconds since the epoch.
- 
- 
-    Additionally, the alternate representations are recognized but their
- normal representations are used.
- 
-    The following example is an `awk' implementation of the POSIX `date'
- utility.  Normally, the `date' utility prints the current date and time
- of day in a well-known format.  However, if you provide an argument to
- it that begins with a `+', `date' copies nonformat specifier characters
- to the standard output and interprets the current time according to the
- format specifiers in the string.  For example:
- 
-      $ date '+Today is %A, %B %d, %Y.'
-      -| Today is Wednesday, March 30, 2011.
- 
-    Here is the `gawk' version of the `date' utility.  It has a shell
- "wrapper" to handle the `-u' option, which requires that `date' run as
- if the time zone is set to UTC:
- 
-      #! /bin/sh
-      #
-      # date --- approximate the POSIX 'date' command
- 
-      case $1 in
-      -u)  TZ=UTC0     # use UTC
-           export TZ
-           shift ;;
-      esac
- 
-      gawk 'BEGIN  {
-          format = "%a %b %e %H:%M:%S %Z %Y"
-          exitval = 0
- 
-          if (ARGC > 2)
-              exitval = 1
-          else if (ARGC == 2) {
-              format = ARGV[1]
-              if (format ~ /^\+/)
-                  format = substr(format, 2)   # remove leading +
-          }
-          print strftime(format)
-          exit exitval
-      }' "$@"
- 
-    ---------- Footnotes ----------
- 
-    (1) *Note Glossary::, especially the entries "Epoch" and "UTC."
- 
-    (2) The GNU `date' utility can also do many of the things described
- here.  Its use may be preferable for simple time-related operations in
- shell scripts.
- 
-    (3) Occasionally there are minutes in a year with a leap second,
- which is why the seconds can go up to 60.
- 
-    (4) Unfortunately, not every system's `strftime()' necessarily
- supports all of the conversions listed here.
- 
-    (5) If you don't understand any of this, don't worry about it; these
- facilities are meant to make it easier to "internationalize" programs.
- Other internationalization features are described in *note
- Internationalization::.
- 
-    (6) This is because ISO C leaves the behavior of the C version of
- `strftime()' undefined and `gawk' uses the system's version of
- `strftime()' if it's there.  Typically, the conversion specifier either
- does not appear in the returned string or appears literally.
- 
- 
- File: gawk.info,  Node: Bitwise Functions,  Next: Type Functions,  Prev: Time 
Functions,  Up: Built-in
- 
- 9.1.6 Bit-Manipulation Functions
- --------------------------------
- 
-      I can explain it for you, but I can't understand it for you.
-      Anonymous
- 
-    Many languages provide the ability to perform "bitwise" operations
- on two integer numbers.  In other words, the operation is performed on
- each successive pair of bits in the operands.  Three common operations
- are bitwise AND, OR, and XOR.  The operations are described in *note
- table-bitwise-ops::.
- 
-                      Bit Operator
-                |  AND  |   OR  |  XOR
-                |--+--+--+--+--+--
-      Operands  | 0 | 1 | 0 | 1 | 0 | 1
-      ---------+--+--+--+--+--+--
-          0     | 0   0 | 0   1 | 0   1
-          1     | 0   1 | 1   1 | 1   0
- 
- Table 9.6: Bitwise Operations
- 
-    As you can see, the result of an AND operation is 1 only when _both_
- bits are 1.  The result of an OR operation is 1 if _either_ bit is 1.
- The result of an XOR operation is 1 if either bit is 1, but not both.
- The next operation is the "complement"; the complement of 1 is 0 and
- the complement of 0 is 1. Thus, this operation "flips" all the bits of
- a given value.
- 
-    Finally, two other common operations are to shift the bits left or
- right.  For example, if you have a bit string `10111001' and you shift
- it right by three bits, you end up with `00010111'.(1) If you start over
- again with `10111001' and shift it left by three bits, you end up with
- `11001000'.  `gawk' provides built-in functions that implement the
- bitwise operations just described. They are:
- 
- `and(V1, V2)'
-      Return the bitwise AND of the values provided by V1 and V2.
- 
- `compl(VAL)'
-      Return the bitwise complement of VAL.
- 
- `lshift(VAL, COUNT)'
-      Return the value of VAL, shifted left by COUNT bits.
- 
- `or(V1, V2)'
-      Return the bitwise OR of the values provided by V1 and V2.
- 
- `rshift(VAL, COUNT)'
-      Return the value of VAL, shifted right by COUNT bits.
- 
- `xor(V1, V2)'
-      Return the bitwise XOR of the values provided by V1 and V2.
- 
-    For all of these functions, first the double precision
- floating-point value is converted to the widest C unsigned integer
- type, then the bitwise operation is performed.  If the result cannot be
- represented exactly as a C `double', leading nonzero bits are removed
- one by one until it can be represented exactly.  The result is then
- converted back into a C `double'.  (If you don't understand this
- paragraph, don't worry about it.)
- 
-    Here is a user-defined function (*note User-defined::) that
- illustrates the use of these functions:
- 
-      # bits2str --- turn a byte into readable 1's and 0's
- 
-      function bits2str(bits,        data, mask)
-      {
-          if (bits == 0)
-              return "0"
- 
-          mask = 1
-          for (; bits != 0; bits = rshift(bits, 1))
-              data = (and(bits, mask) ? "1" : "0") data
- 
-          while ((length(data) % 8) != 0)
-              data = "0" data
- 
-          return data
-      }
- 
-      BEGIN {
-          printf "123 = %s\n", bits2str(123)
-          printf "0123 = %s\n", bits2str(0123)
-          printf "0x99 = %s\n", bits2str(0x99)
-          comp = compl(0x99)
-          printf "compl(0x99) = %#x = %s\n", comp, bits2str(comp)
-          shift = lshift(0x99, 2)
-          printf "lshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)
-          shift = rshift(0x99, 2)
-          printf "rshift(0x99, 2) = %#x = %s\n", shift, bits2str(shift)
-      }
- 
- This program produces the following output when run:
- 
-      $ gawk -f testbits.awk
-      -| 123 = 01111011
-      -| 0123 = 01010011
-      -| 0x99 = 10011001
-      -| compl(0x99) = 0xffffff66 = 11111111111111111111111101100110
-      -| lshift(0x99, 2) = 0x264 = 0000001001100100
-      -| rshift(0x99, 2) = 0x26 = 00100110
- 
-    The `bits2str()' function turns a binary number into a string.  The
- number `1' represents a binary value where the rightmost bit is set to
- 1.  Using this mask, the function repeatedly checks the rightmost bit.
- ANDing the mask with the value indicates whether the rightmost bit is 1
- or not. If so, a `"1"' is concatenated onto the front of the string.
- Otherwise, a `"0"' is added.  The value is then shifted right by one
- bit and the loop continues until there are no more 1 bits.
- 
-    If the initial value is zero it returns a simple `"0"'.  Otherwise,
- at the end, it pads the value with zeros to represent multiples of
- 8-bit quantities. This is typical in modern computers.
- 
-    The main code in the `BEGIN' rule shows the difference between the
- decimal and octal values for the same numbers (*note
- Nondecimal-numbers::), and then demonstrates the results of the
- `compl()', `lshift()', and `rshift()' functions.
- 
-    ---------- Footnotes ----------
- 
-    (1) This example shows that 0's come in on the left side. For
- `gawk', this is always true, but in some languages, it's possible to
- have the left side fill with 1's. Caveat emptor.
- 
- 
- File: gawk.info,  Node: Type Functions,  Next: I18N Functions,  Prev: Bitwise 
Functions,  Up: Built-in
- 
- 9.1.7 Getting Type Information
- ------------------------------
- 
- `gawk' provides a single function that lets you distinguish an array
- from a scalar variable.  This is necessary for writing code that
- traverses every element of a true multidimensional array (*note Arrays
- of Arrays::).
- 
- `isarray(X)'
-      Return a true value if X is an array. Otherwise return false.
- 
- 
- File: gawk.info,  Node: I18N Functions,  Prev: Type Functions,  Up: Built-in
- 
- 9.1.8 String-Translation Functions
- ----------------------------------
- 
- `gawk' provides facilities for internationalizing `awk' programs.
- These include the functions described in the following list.  The
- descriptions here are purposely brief.  *Note Internationalization::,
- for the full story.  Optional parameters are enclosed in square
- brackets ([ ]):
- 
- `bindtextdomain(DIRECTORY [, DOMAIN])'
-      Set the directory in which `gawk' will look for message
-      translation files, in case they will not or cannot be placed in
-      the "standard" locations (e.g., during testing).  It returns the
-      directory in which DOMAIN is "bound."
- 
-      The default DOMAIN is the value of `TEXTDOMAIN'.  If DIRECTORY is
-      the null string (`""'), then `bindtextdomain()' returns the
-      current binding for the given DOMAIN.
- 
- `dcgettext(STRING [, DOMAIN [, CATEGORY]])'
-      Return the translation of STRING in text domain DOMAIN for locale
-      category CATEGORY.  The default value for DOMAIN is the current
-      value of `TEXTDOMAIN'.  The default value for CATEGORY is
-      `"LC_MESSAGES"'.
- 
- `dcngettext(STRING1, STRING2, NUMBER [, DOMAIN [, CATEGORY]])'
-      Return the plural form used for NUMBER of the translation of
-      STRING1 and STRING2 in text domain DOMAIN for locale category
-      CATEGORY. STRING1 is the English singular variant of a message,
-      and STRING2 the English plural variant of the same message.  The
-      default value for DOMAIN is the current value of `TEXTDOMAIN'.
-      The default value for CATEGORY is `"LC_MESSAGES"'.
- 
- 
- File: gawk.info,  Node: User-defined,  Next: Indirect Calls,  Prev: Built-in, 
 Up: Functions
- 
- 9.2 User-Defined Functions
- ==========================
- 
- Complicated `awk' programs can often be simplified by defining your own
- functions.  User-defined functions can be called just like built-in
- ones (*note Function Calls::), but it is up to you to define them,
- i.e., to tell `awk' what they should do.
- 
- * Menu:
- 
- * Definition Syntax::           How to write definitions and what they mean.
- * Function Example::            An example function definition and what it
-                                 does.
- * Function Caveats::            Things to watch out for.
- * Return Statement::            Specifying the value a function returns.
- * Dynamic Typing::              How variable types can change at runtime.
- 
- 
- File: gawk.info,  Node: Definition Syntax,  Next: Function Example,  Up: 
User-defined
- 
- 9.2.1 Function Definition Syntax
- --------------------------------
- 
- Definitions of functions can appear anywhere between the rules of an
- `awk' program.  Thus, the general form of an `awk' program is extended
- to include sequences of rules _and_ user-defined function definitions.
- There is no need to put the definition of a function before all uses of
- the function.  This is because `awk' reads the entire program before
- starting to execute any of it.
- 
-    The definition of a function named NAME looks like this:
- 
-      function NAME([PARAMETER-LIST])
-      {
-           BODY-OF-FUNCTION
-      }
- 
- Here, NAME is the name of the function to define.  A valid function
- name is like a valid variable name: a sequence of letters, digits, and
- underscores that doesn't start with a digit.  Within a single `awk'
- program, any particular name can only be used as a variable, array, or
- function.
- 
-    PARAMETER-LIST is an optional list of the function's arguments and
- local variable names, separated by commas.  When the function is called,
- the argument names are used to hold the argument values given in the
- call.  The local variables are initialized to the empty string.  A
- function cannot have two parameters with the same name, nor may it have
- a parameter with the same name as the function itself.
- 
-    In addition, according to the POSIX standard, function parameters
- cannot have the same name as one of the special built-in variables
- (*note Built-in Variables::.  Not all versions of `awk' enforce this
- restriction.
- 
-    The BODY-OF-FUNCTION consists of `awk' statements.  It is the most
- important part of the definition, because it says what the function
- should actually _do_.  The argument names exist to give the body a way
- to talk about the arguments; local variables exist to give the body
- places to keep temporary values.
- 
-    Argument names are not distinguished syntactically from local
- variable names. Instead, the number of arguments supplied when the
- function is called determines how many argument variables there are.
- Thus, if three argument values are given, the first three names in
- PARAMETER-LIST are arguments and the rest are local variables.
- 
-    It follows that if the number of arguments is not the same in all
- calls to the function, some of the names in PARAMETER-LIST may be
- arguments on some occasions and local variables on others.  Another way
- to think of this is that omitted arguments default to the null string.
- 
-    Usually when you write a function, you know how many names you
- intend to use for arguments and how many you intend to use as local
- variables.  It is conventional to place some extra space between the
- arguments and the local variables, in order to document how your
- function is supposed to be used.
- 
-    During execution of the function body, the arguments and local
- variable values hide, or "shadow", any variables of the same names used
- in the rest of the program.  The shadowed variables are not accessible
- in the function definition, because there is no way to name them while
- their names have been taken away for the local variables.  All other
- variables used in the `awk' program can be referenced or set normally
- in the function's body.
- 
-    The arguments and local variables last only as long as the function
- body is executing.  Once the body finishes, you can once again access
- the variables that were shadowed while the function was running.
- 
-    The function body can contain expressions that call functions.  They
- can even call this function, either directly or by way of another
- function.  When this happens, we say the function is "recursive".  The
- act of a function calling itself is called "recursion".
- 
-    All the built-in functions return a value to their caller.
- User-defined functions can do also, using the `return' statement, which
- is described in detail in *note Return Statement::.  Many of the
- subsequent examples in this minor node use the `return' statement.
- 
-    In many `awk' implementations, including `gawk', the keyword
- `function' may be abbreviated `func'. (c.e.)  However, POSIX only
- specifies the use of the keyword `function'.  This actually has some
- practical implications.  If `gawk' is in POSIX-compatibility mode
- (*note Options::), then the following statement does _not_ define a
- function:
- 
-      func foo() { a = sqrt($1) ; print a }
- 
- Instead it defines a rule that, for each record, concatenates the value
- of the variable `func' with the return value of the function `foo'.  If
- the resulting string is non-null, the action is executed.  This is
- probably not what is desired.  (`awk' accepts this input as
- syntactically valid, because functions may be used before they are
- defined in `awk' programs.(1))
- 
-    To ensure that your `awk' programs are portable, always use the
- keyword `function' when defining a function.
- 
-    ---------- Footnotes ----------
- 
-    (1) This program won't actually run, since `foo()' is undefined.
- 
- 
- File: gawk.info,  Node: Function Example,  Next: Function Caveats,  Prev: 
Definition Syntax,  Up: User-defined
- 
- 9.2.2 Function Definition Examples
- ----------------------------------
- 
- Here is an example of a user-defined function, called `myprint()', that
- takes a number and prints it in a specific format:
- 
-      function myprint(num)
-      {
-           printf "%6.3g\n", num
-      }
- 
- To illustrate, here is an `awk' rule that uses our `myprint' function:
- 
-      $3 > 0     { myprint($3) }
- 
- This program prints, in our special format, all the third fields that
- contain a positive number in our input.  Therefore, when given the
- following input:
- 
-       1.2   3.4    5.6   7.8
-       9.10 11.12 -13.14 15.16
-      17.18 19.20  21.22 23.24
- 
- this program, using our function to format the results, prints:
- 
-         5.6
-        21.2
- 
-    This function deletes all the elements in an array:
- 
-      function delarray(a,    i)
-      {
-          for (i in a)
-             delete a[i]
-      }
- 
-    When working with arrays, it is often necessary to delete all the
- elements in an array and start over with a new list of elements (*note
- Delete::).  Instead of having to repeat this loop everywhere that you
- need to clear out an array, your program can just call `delarray'.
- (This guarantees portability.  The use of `delete ARRAY' to delete the
- contents of an entire array is a nonstandard extension.)
- 
-    The following is an example of a recursive function.  It takes a
- string as an input parameter and returns the string in backwards order.
- Recursive functions must always have a test that stops the recursion.
- In this case, the recursion terminates when the starting position is
- zero, i.e., when there are no more characters left in the string.
- 
-      function rev(str, start)
-      {
-          if (start == 0)
-              return ""
- 
-          return (substr(str, start, 1) rev(str, start - 1))
-      }
- 
-    If this function is in a file named `rev.awk', it can be tested this
- way:
- 
-      $ echo "Don't Panic!" |
-      > gawk --source '{ print rev($0, length($0)) }' -f rev.awk
-      -| !cinaP t'noD
- 
-    The C `ctime()' function takes a timestamp and returns it in a
- string, formatted in a well-known fashion.  The following example uses
- the built-in `strftime()' function (*note Time Functions::) to create
- an `awk' version of `ctime()':
- 
-      # ctime.awk
-      #
-      # awk version of C ctime(3) function
- 
-      function ctime(ts,    format)
-      {
-          format = "%a %b %e %H:%M:%S %Z %Y"
-          if (ts == 0)
-              ts = systime()       # use current time as default
-          return strftime(format, ts)
-      }
- 
- 
- File: gawk.info,  Node: Function Caveats,  Next: Return Statement,  Prev: 
Function Example,  Up: User-defined
- 
- 9.2.3 Calling User-Defined Functions
- ------------------------------------
- 
- This section describes how to call a user-defined function.
- 
- * Menu:
- 
- * Calling A Function::          Don't use spaces.
- * Variable Scope::              Controlling variable scope.
- * Pass By Value/Reference::     Passing parameters.
- 
- 
- File: gawk.info,  Node: Calling A Function,  Next: Variable Scope,  Up: 
Function Caveats
- 
- 9.2.3.1 Writing A Function Call
- ...............................
- 
- "Calling a function" means causing the function to run and do its job.
- A function call is an expression and its value is the value returned by
- the function.
- 
-    A function call consists of the function name followed by the
- arguments in parentheses.  `awk' expressions are what you write in the
- call for the arguments.  Each time the call is executed, these
- expressions are evaluated, and the values become the actual arguments.
- For example, here is a call to `foo()' with three arguments (the first
- being a string concatenation):
- 
-      foo(x y, "lose", 4 * z)
- 
-      CAUTION: Whitespace characters (spaces and TABs) are not allowed
-      between the function name and the open-parenthesis of the argument
-      list.  If you write whitespace by mistake, `awk' might think that
-      you mean to concatenate a variable with an expression in
-      parentheses.  However, it notices that you used a function name
-      and not a variable name, and reports an error.
- 
- 
- File: gawk.info,  Node: Variable Scope,  Next: Pass By Value/Reference,  
Prev: Calling A Function,  Up: Function Caveats
- 
- 9.2.3.2 Controlling Variable Scope
- ..................................
- 
- There is no way to make a variable local to a `{ ... }' block in `awk',
- but you can make a variable local to a function. It is good practice to
- do so whenever a variable is needed only in that function.
- 
-    To make a variable local to a function, simply declare the variable
- as an argument after the actual function arguments (*note Definition
- Syntax::).  Look at the following example where variable `i' is a
- global variable used by both functions `foo()' and `bar()':
- 
-      function bar()
-      {
-          for (i = 0; i < 3; i++)
-              print "bar's i=" i
-      }
- 
-      function foo(j)
-      {
-          i = j + 1
-          print "foo's i=" i
-          bar()
-          print "foo's i=" i
-      }
- 
-      BEGIN {
-            i = 10
-            print "top's i=" i
-            foo(0)
-            print "top's i=" i
-      }
- 
-    Running this script produces the following, because the `i' in
- functions `foo()' and `bar()' and at the top level refer to the same
- variable instance:
- 
-      top's i=10
-      foo's i=1
-      bar's i=0
-      bar's i=1
-      bar's i=2
-      foo's i=3
-      top's i=3
- 
-    If you want `i' to be local to both `foo()' and `bar()' do as
- follows (the extra-space before `i' is a coding convention to indicate
- that `i' is a local variable, not an argument):
- 
-      function bar(    i)
-      {
-          for (i = 0; i < 3; i++)
-              print "bar's i=" i
-      }
- 
-      function foo(j,    i)
-      {
-          i = j + 1
-          print "foo's i=" i
-          bar()
-          print "foo's i=" i
-      }
- 
-      BEGIN {
-            i = 10
-            print "top's i=" i
-            foo(0)
-            print "top's i=" i
-      }
- 
-    Running the corrected script produces the following:
- 
-      top's i=10
-      foo's i=1
-      bar's i=0
-      bar's i=1
-      bar's i=2
-      foo's i=1
-      top's i=10
- 
- 
- File: gawk.info,  Node: Pass By Value/Reference,  Prev: Variable Scope,  Up: 
Function Caveats
- 
- 9.2.3.3 Passing Function Arguments By Value Or By Reference
- ...........................................................
- 
- In `awk', when you declare a function, there is no way to declare
- explicitly whether the arguments are passed "by value" or "by
- reference".
- 
-    Instead the passing convention is determined at runtime when the
- function is called according to the following rule:
- 
-    * If the argument is an array variable, then it is passed by
-      reference,
- 
-    * Otherwise the argument is passed by value.
- 
-    Passing an argument by value means that when a function is called, it
- is given a _copy_ of the value of this argument.  The caller may use a
- variable as the expression for the argument, but the called function
- does not know this--it only knows what value the argument had.  For
- example, if you write the following code:
- 
-      foo = "bar"
-      z = myfunc(foo)
- 
- then you should not think of the argument to `myfunc()' as being "the
- variable `foo'."  Instead, think of the argument as the string value
- `"bar"'.  If the function `myfunc()' alters the values of its local
- variables, this has no effect on any other variables.  Thus, if
- `myfunc()' does this:
- 
-      function myfunc(str)
-      {
-         print str
-         str = "zzz"
-         print str
-      }
- 
- to change its first argument variable `str', it does _not_ change the
- value of `foo' in the caller.  The role of `foo' in calling `myfunc()'
- ended when its value (`"bar"') was computed.  If `str' also exists
- outside of `myfunc()', the function body cannot alter this outer value,
- because it is shadowed during the execution of `myfunc()' and cannot be
- seen or changed from there.
- 
-    However, when arrays are the parameters to functions, they are _not_
- copied.  Instead, the array itself is made available for direct
- manipulation by the function.  This is usually termed "call by
- reference".  Changes made to an array parameter inside the body of a
- function _are_ visible outside that function.
- 
-      NOTE: Changing an array parameter inside a function can be very
-      dangerous if you do not watch what you are doing.  For example:
- 
-           function changeit(array, ind, nvalue)
-           {
-                array[ind] = nvalue
-           }
- 
-           BEGIN {
-               a[1] = 1; a[2] = 2; a[3] = 3
-               changeit(a, 2, "two")
-               printf "a[1] = %s, a[2] = %s, a[3] = %s\n",
-                       a[1], a[2], a[3]
-           }
- 
-      prints `a[1] = 1, a[2] = two, a[3] = 3', because `changeit' stores
-      `"two"' in the second element of `a'.
- 
-    Some `awk' implementations allow you to call a function that has not
- been defined. They only report a problem at runtime when the program
- actually tries to call the function. For example:
- 
-      BEGIN {
-          if (0)
-              foo()
-          else
-              bar()
-      }
-      function bar() { ... }
-      # note that `foo' is not defined
- 
- Because the `if' statement will never be true, it is not really a
- problem that `foo()' has not been defined.  Usually, though, it is a
- problem if a program calls an undefined function.
- 
-    If `--lint' is specified (*note Options::), `gawk' reports calls to
- undefined functions.
- 
-    Some `awk' implementations generate a runtime error if you use the
- `next' statement (*note Next Statement::) inside a user-defined
- function.  `gawk' does not have this limitation.
- 
- 
- File: gawk.info,  Node: Return Statement,  Next: Dynamic Typing,  Prev: 
Function Caveats,  Up: User-defined
- 
- 9.2.4 The `return' Statement
- ----------------------------
- 
- As seen in several earlier examples, the body of a user-defined
- function can contain a `return' statement.  This statement returns
- control to the calling part of the `awk' program.  It can also be used
- to return a value for use in the rest of the `awk' program.  It looks
- like this:
- 
-      return [EXPRESSION]
- 
-    The EXPRESSION part is optional.  Due most likely to an oversight,
- POSIX does not define what the return value is if you omit the
- EXPRESSION.  Technically speaking, this make the returned value
- undefined, and therefore, unpredictable.  In practice, though, all
- versions of `awk' simply return the null string, which acts like zero
- if used in a numeric context.
- 
-    A `return' statement with no value expression is assumed at the end
- of every function definition.  So if control reaches the end of the
- function body, then technically, the function returns an unpredictable
- value.  In practice, it returns the empty string.  `awk' does _not_
- warn you if you use the return value of such a function.
- 
-    Sometimes, you want to write a function for what it does, not for
- what it returns.  Such a function corresponds to a `void' function in
- C, C++ or Java, or to a `procedure' in Ada.  Thus, it may be
- appropriate to not return any value; simply bear in mind that you
- should not be using the return value of such a function.
- 
-    The following is an example of a user-defined function that returns
- a value for the largest number among the elements of an array:
- 
-      function maxelt(vec,   i, ret)
-      {
-           for (i in vec) {
-                if (ret == "" || vec[i] > ret)
-                     ret = vec[i]
-           }
-           return ret
-      }
- 
- You call `maxelt()' with one argument, which is an array name.  The
- local variables `i' and `ret' are not intended to be arguments; while
- there is nothing to stop you from passing more than one argument to
- `maxelt()', the results would be strange.  The extra space before `i'
- in the function parameter list indicates that `i' and `ret' are local
- variables.  You should follow this convention when defining functions.
- 
-    The following program uses the `maxelt()' function.  It loads an
- array, calls `maxelt()', and then reports the maximum number in that
- array:
- 
-      function maxelt(vec,   i, ret)
-      {
-           for (i in vec) {
-                if (ret == "" || vec[i] > ret)
-                     ret = vec[i]
-           }
-           return ret
-      }
- 
-      # Load all fields of each record into nums.
-      {
-           for(i = 1; i <= NF; i++)
-                nums[NR, i] = $i
-      }
- 
-      END {
-           print maxelt(nums)
-      }
- 
-    Given the following input:
- 
-       1 5 23 8 16
-      44 3 5 2 8 26
-      256 291 1396 2962 100
-      -6 467 998 1101
-      99385 11 0 225
- 
- the program reports (predictably) that 99,385 is the largest value in
- the array.
- 
- 
- File: gawk.info,  Node: Dynamic Typing,  Prev: Return Statement,  Up: 
User-defined
- 
- 9.2.5 Functions and Their Effects on Variable Typing
- ----------------------------------------------------
- 
- `awk' is a very fluid language.  It is possible that `awk' can't tell
- if an identifier represents a scalar variable or an array until runtime.
- Here is an annotated sample program:
- 
-      function foo(a)
-      {
-          a[1] = 1   # parameter is an array
-      }
- 
-      BEGIN {
-          b = 1
-          foo(b)  # invalid: fatal type mismatch
- 
-          foo(x)  # x uninitialized, becomes an array dynamically
-          x = 1   # now not allowed, runtime error
-      }
- 
-    Usually, such things aren't a big issue, but it's worth being aware
- of them.
- 
- 
- File: gawk.info,  Node: Indirect Calls,  Prev: User-defined,  Up: Functions
- 
- 9.3 Indirect Function Calls
- ===========================
- 
- This section describes a `gawk'-specific extension.
- 
-    Often, you may wish to defer the choice of function to call until
- runtime.  For example, you may have different kinds of records, each of
- which should be processed differently.
- 
-    Normally, you would have to use a series of `if'-`else' statements
- to decide which function to call.  By using "indirect" function calls,
- you can specify the name of the function to call as a string variable,
- and then call the function.  Let's look at an example.
- 
-    Suppose you have a file with your test scores for the classes you
- are taking.  The first field is the class name. The following fields
- are the functions to call to process the data, up to a "marker" field
- `data:'.  Following the marker, to the end of the record, are the
- various numeric test scores.
- 
-    Here is the initial file; you wish to get the sum and the average of
- your test scores:
- 
-      Biology_101 sum average data: 87.0 92.4 78.5 94.9
-      Chemistry_305 sum average data: 75.2 98.3 94.7 88.2
-      English_401 sum average data: 100.0 95.6 87.1 93.4
- 
-    To process the data, you might write initially:
- 
-      {
-          class = $1
-          for (i = 2; $i != "data:"; i++) {
-              if ($i == "sum")
-                  sum()   # processes the whole record
-              else if ($i == "average")
-                  average()
-              ...           # and so on
-          }
-      }
- 
- This style of programming works, but can be awkward.  With "indirect"
- function calls, you tell `gawk' to use the _value_ of a variable as the
- name of the function to call.
- 
-    The syntax is similar to that of a regular function call: an
- identifier immediately followed by a left parenthesis, any arguments,
- and then a closing right parenthesis, with the addition of a leading `@'
- character:
- 
-      the_func = "sum"
-      result = @the_func()   # calls the `sum' function
- 
-    Here is a full program that processes the previously shown data,
- using indirect function calls.
- 
-      # indirectcall.awk --- Demonstrate indirect function calls
- 
-      # average --- return the average of the values in fields $first - $last
- 
-      function average(first, last,   sum, i)
-      {
-          sum = 0;
-          for (i = first; i <= last; i++)
-              sum += $i
- 
-          return sum / (last - first + 1)
-      }
- 
-      # sum --- return the sum of the values in fields $first - $last
- 
-      function sum(first, last,   ret, i)
-      {
-          ret = 0;
-          for (i = first; i <= last; i++)
-              ret += $i
- 
-          return ret
-      }
- 
-    These two functions expect to work on fields; thus the parameters
- `first' and `last' indicate where in the fields to start and end.
- Otherwise they perform the expected computations and are not unusual.
- 
-      # For each record, print the class name and the requested statistics
- 
-      {
-          class_name = $1
-          gsub(/_/, " ", class_name)  # Replace _ with spaces
- 
-          # find start
-          for (i = 1; i <= NF; i++) {
-              if ($i == "data:") {
-                  start = i + 1
-                  break
-              }
-          }
- 
-          printf("%s:\n", class_name)
-          for (i = 2; $i != "data:"; i++) {
-              the_function = $i
-              printf("\t%s: <%s>\n", $i, @the_function(start, NF) "")
-          }
-          print ""
-      }
- 
-    This is the main processing for each record. It prints the class
- name (with underscores replaced with spaces). It then finds the start
- of the actual data, saving it in `start'.  The last part of the code
- loops through each function name (from `$2' up to the marker, `data:'),
- calling the function named by the field. The indirect function call
- itself occurs as a parameter in the call to `printf'.  (The `printf'
- format string uses `%s' as the format specifier so that we can use
- functions that return strings, as well as numbers. Note that the result
- from the indirect call is concatenated with the empty string, in order
- to force it to be a string value.)
- 
-    Here is the result of running the program:
- 
-      $ gawk -f indirectcall.awk class_data1
-      -| Biology 101:
-      -|     sum: <352.8>
-      -|     average: <88.2>
-      -|
-      -| Chemistry 305:
-      -|     sum: <356.4>
-      -|     average: <89.1>
-      -|
-      -| English 401:
-      -|     sum: <376.1>
-      -|     average: <94.025>
- 
-    The ability to use indirect function calls is more powerful than you
- may think at first.  The C and C++ languages provide "function
- pointers," which are a mechanism for calling a function chosen at
- runtime.  One of the most well-known uses of this ability is the C
- `qsort()' function, which sorts an array using the famous "quick sort"
- algorithm (see the Wikipedia article
- (http://en.wikipedia.org/wiki/Quick_sort) for more information).  To
- use this function, you supply a pointer to a comparison function.  This
- mechanism allows you to sort arbitrary data in an arbitrary fashion.
- 
-    We can do something similar using `gawk', like this:
- 
-      # quicksort.awk --- Quicksort algorithm, with user-supplied
-      #                   comparison function
-      # quicksort --- C.A.R. Hoare's quick sort algorithm. See Wikipedia
-      #               or almost any algorithms or computer science text
- 
-      function quicksort(data, left, right, less_than,    i, last)
-      {
-          if (left >= right)  # do nothing if array contains fewer
-              return          # than two elements
- 
-          quicksort_swap(data, left, int((left + right) / 2))
-          last = left
-          for (i = left + 1; i <= right; i++)
-              if (@less_than(data[i], data[left]))
-                  quicksort_swap(data, ++last, i)
-          quicksort_swap(data, left, last)
-          quicksort(data, left, last - 1, less_than)
-          quicksort(data, last + 1, right, less_than)
-      }
- 
-      # quicksort_swap --- helper function for quicksort, should really be 
inline
- 
-      function quicksort_swap(data, i, j, temp)
-      {
-          temp = data[i]
-          data[i] = data[j]
-          data[j] = temp
-      }
- 
-    The `quicksort()' function receives the `data' array, the starting
- and ending indices to sort (`left' and `right'), and the name of a
- function that performs a "less than" comparison.  It then implements
- the quick sort algorithm.
- 
-    To make use of the sorting function, we return to our previous
- example. The first thing to do is write some comparison functions:
- 
-      # num_lt --- do a numeric less than comparison
- 
-      function num_lt(left, right)
-      {
-          return ((left + 0) < (right + 0))
-      }
- 
-      # num_ge --- do a numeric greater than or equal to comparison
- 
-      function num_ge(left, right)
-      {
-          return ((left + 0) >= (right + 0))
-      }
- 
-    The `num_ge()' function is needed to perform a descending sort; when
- used to perform a "less than" test, it actually does the opposite
- (greater than or equal to), which yields data sorted in descending
- order.
- 
-    Next comes a sorting function.  It is parameterized with the
- starting and ending field numbers and the comparison function. It
- builds an array with the data and calls `quicksort' appropriately, and
- then formats the results as a single string:
- 
-      # do_sort --- sort the data according to `compare'
-      #             and return it as a string
- 
-      function do_sort(first, last, compare,      data, i, retval)
-      {
-          delete data
-          for (i = 1; first <= last; first++) {
-              data[i] = $first
-              i++
-          }
- 
-          quicksort(data, 1, i-1, compare)
- 
-          retval = data[1]
-          for (i = 2; i in data; i++)
-              retval = retval " " data[i]
- 
-          return retval
-      }
- 
-    Finally, the two sorting functions call `do_sort()', passing in the
- names of the two comparison functions:
- 
-      # sort --- sort the data in ascending order and return it as a string
- 
-      function sort(first, last)
-      {
-          return do_sort(first, last, "num_lt")
-      }
- 
-      # rsort --- sort the data in descending order and return it as a string
- 
-      function rsort(first, last)
-      {
-          return do_sort(first, last, "num_ge")
-      }
- 
-    Here is an extended version of the data file:
- 
-      Biology_101 sum average sort rsort data: 87.0 92.4 78.5 94.9
-      Chemistry_305 sum average sort rsort data: 75.2 98.3 94.7 88.2
-      English_401 sum average sort rsort data: 100.0 95.6 87.1 93.4
- 
-    Finally, here are the results when the enhanced program is run:
- 
-      $ gawk -f quicksort.awk -f indirectcall.awk class_data2
-      -| Biology 101:
-      -|     sum: <352.8>
-      -|     average: <88.2>
-      -|     sort: <78.5 87.0 92.4 94.9>
-      -|     rsort: <94.9 92.4 87.0 78.5>
-      -|
-      -| Chemistry 305:
-      -|     sum: <356.4>
-      -|     average: <89.1>
-      -|     sort: <75.2 88.2 94.7 98.3>
-      -|     rsort: <98.3 94.7 88.2 75.2>
-      -|
-      -| English 401:
-      -|     sum: <376.1>
-      -|     average: <94.025>
-      -|     sort: <87.1 93.4 95.6 100.0>
-      -|     rsort: <100.0 95.6 93.4 87.1>
- 
-    Remember that you must supply a leading `@' in front of an indirect
- function call.
- 
-    Unfortunately, indirect function calls cannot be used with the
- built-in functions.  However, you can generally write "wrapper"
- functions which call the built-in ones, and those can be called
- indirectly. (Other than, perhaps, the mathematical functions, there is
- not a lot of reason to try to call the built-in functions indirectly.)
- 
-    `gawk' does its best to make indirect function calls efficient.  For
- example, in the following case:
- 
-      for (i = 1; i <= n; i++)
-          @the_func()
- 
- `gawk' will look up the actual function to call only once.
- 
- 
- File: gawk.info,  Node: Internationalization,  Next: Arbitrary Precision 
Arithmetic,  Prev: Functions,  Up: Top
- 
- 10 Internationalization with `gawk'
- ***********************************
- 
- Once upon a time, computer makers wrote software that worked only in
- English.  Eventually, hardware and software vendors noticed that if
- their systems worked in the native languages of non-English-speaking
- countries, they were able to sell more systems.  As a result,
- internationalization and localization of programs and software systems
- became a common practice.
- 
-    For many years, the ability to provide internationalization was
- largely restricted to programs written in C and C++.  This major node
- describes the underlying library `gawk' uses for internationalization,
- as well as how `gawk' makes internationalization features available at
- the `awk' program level.  Having internationalization available at the
- `awk' level gives software developers additional flexibility--they are
- no longer forced to write in C or C++ when internationalization is a
- requirement.
- 
- * Menu:
- 
- * I18N and L10N::               Internationalization and Localization.
- * Explaining gettext::          How GNU `gettext' works.
- * Programmer i18n::             Features for the programmer.
- * Translator i18n::             Features for the translator.
- * I18N Example::                A simple i18n example.
- * Gawk I18N::                   `gawk' is also internationalized.
- 
- 
- File: gawk.info,  Node: I18N and L10N,  Next: Explaining gettext,  Up: 
Internationalization
- 
- 10.1 Internationalization and Localization
- ==========================================
- 
- "Internationalization" means writing (or modifying) a program once, in
- such a way that it can use multiple languages without requiring further
- source-code changes.  "Localization" means providing the data necessary
- for an internationalized program to work in a particular language.
- Most typically, these terms refer to features such as the language used
- for printing error messages, the language used to read responses, and
- information related to how numerical and monetary values are printed
- and read.
- 
- 
- File: gawk.info,  Node: Explaining gettext,  Next: Programmer i18n,  Prev: 
I18N and L10N,  Up: Internationalization
- 
- 10.2 GNU `gettext'
- ==================
- 
- The facilities in GNU `gettext' focus on messages; strings printed by a
- program, either directly or via formatting with `printf' or
- `sprintf()'.(1)
- 
-    When using GNU `gettext', each application has its own "text
- domain".  This is a unique name, such as `kpilot' or `gawk', that
- identifies the application.  A complete application may have multiple
- components--programs written in C or C++, as well as scripts written in
- `sh' or `awk'.  All of the components use the same text domain.
- 
-    To make the discussion concrete, assume we're writing an application
- named `guide'.  Internationalization consists of the following steps,
- in this order:
- 
-   1. The programmer goes through the source for all of `guide''s
-      components and marks each string that is a candidate for
-      translation.  For example, `"`-F': option required"' is a good
-      candidate for translation.  A table with strings of option names
-      is not (e.g., `gawk''s `--profile' option should remain the same,
-      no matter what the local language).
- 
-   2. The programmer indicates the application's text domain (`"guide"')
-      to the `gettext' library, by calling the `textdomain()' function.
- 
-   3. Messages from the application are extracted from the source code
-      and collected into a portable object template file (`guide.pot'),
-      which lists the strings and their translations.  The translations
-      are initially empty.  The original (usually English) messages
-      serve as the key for lookup of the translations.
- 
-   4. For each language with a translator, `guide.pot' is copied to a
-      portable object file (`.po') and translations are created and
-      shipped with the application.  For example, there might be a
-      `fr.po' for a French translation.
- 
-   5. Each language's `.po' file is converted into a binary message
-      object (`.mo') file.  A message object file contains the original
-      messages and their translations in a binary format that allows
-      fast lookup of translations at runtime.
- 
-   6. When `guide' is built and installed, the binary translation files
-      are installed in a standard place.
- 
-   7. For testing and development, it is possible to tell `gettext' to
-      use `.mo' files in a different directory than the standard one by
-      using the `bindtextdomain()' function.
- 
-   8. At runtime, `guide' looks up each string via a call to
-      `gettext()'.  The returned string is the translated string if
-      available, or the original string if not.
- 
-   9. If necessary, it is possible to access messages from a different
-      text domain than the one belonging to the application, without
-      having to switch the application's default text domain back and
-      forth.
- 
-    In C (or C++), the string marking and dynamic translation lookup are
- accomplished by wrapping each string in a call to `gettext()':
- 
-      printf("%s", gettext("Don't Panic!\n"));
- 
-    The tools that extract messages from source code pull out all
- strings enclosed in calls to `gettext()'.
- 
-    The GNU `gettext' developers, recognizing that typing `gettext(...)'
- over and over again is both painful and ugly to look at, use the macro
- `_' (an underscore) to make things easier:
- 
-      /* In the standard header file: */
-      #define _(str) gettext(str)
- 
-      /* In the program text: */
-      printf("%s", _("Don't Panic!\n"));
- 
- This reduces the typing overhead to just three extra characters per
- string and is considerably easier to read as well.
- 
-    There are locale "categories" for different types of locale-related
- information.  The defined locale categories that `gettext' knows about
- are:
- 
- `LC_MESSAGES'
-      Text messages.  This is the default category for `gettext'
-      operations, but it is possible to supply a different one
-      explicitly, if necessary.  (It is almost never necessary to supply
-      a different category.)
- 
- `LC_COLLATE'
-      Text-collation information; i.e., how different characters and/or
-      groups of characters sort in a given language.
- 
- `LC_CTYPE'
-      Character-type information (alphabetic, digit, upper- or
-      lowercase, and so on).  This information is accessed via the POSIX
-      character classes in regular expressions, such as `/[[:alnum:]]/'
-      (*note Regexp Operators::).
- 
- `LC_MONETARY'
-      Monetary information, such as the currency symbol, and whether the
-      symbol goes before or after a number.
- 
- `LC_NUMERIC'
-      Numeric information, such as which characters to use for the
-      decimal point and the thousands separator.(2)
- 
- `LC_RESPONSE'
-      Response information, such as how "yes" and "no" appear in the
-      local language, and possibly other information as well.
- 
- `LC_TIME'
-      Time- and date-related information, such as 12- or 24-hour clock,
-      month printed before or after the day in a date, local month
-      abbreviations, and so on.
- 
- `LC_ALL'
-      All of the above.  (Not too useful in the context of `gettext'.)
- 
-    ---------- Footnotes ----------
- 
-    (1) For some operating systems, the `gawk' port doesn't support GNU
- `gettext'.  Therefore, these features are not available if you are
- using one of those operating systems. Sorry.
- 
-    (2) Americans use a comma every three decimal places and a period
- for the decimal point, while many Europeans do exactly the opposite:
- 1,234.56 versus 1.234,56.
- 
- 
- File: gawk.info,  Node: Programmer i18n,  Next: Translator i18n,  Prev: 
Explaining gettext,  Up: Internationalization
- 
- 10.3 Internationalizing `awk' Programs
- ======================================
- 
- `gawk' provides the following variables and functions for
- internationalization:
- 
- `TEXTDOMAIN'
-      This variable indicates the application's text domain.  For
-      compatibility with GNU `gettext', the default value is
-      `"messages"'.
- 
- `_"your message here"'
-      String constants marked with a leading underscore are candidates
-      for translation at runtime.  String constants without a leading
-      underscore are not translated.
- 
- `dcgettext(STRING [, DOMAIN [, CATEGORY]])'
-      Return the translation of STRING in text domain DOMAIN for locale
-      category CATEGORY.  The default value for DOMAIN is the current
-      value of `TEXTDOMAIN'.  The default value for CATEGORY is
-      `"LC_MESSAGES"'.
- 
-      If you supply a value for CATEGORY, it must be a string equal to
-      one of the known locale categories described in *note Explaining
-      gettext::.  You must also supply a text domain.  Use `TEXTDOMAIN'
-      if you want to use the current domain.
- 
-           CAUTION: The order of arguments to the `awk' version of the
-           `dcgettext()' function is purposely different from the order
-           for the C version.  The `awk' version's order was chosen to
-           be simple and to allow for reasonable `awk'-style default
-           arguments.
- 
- `dcngettext(STRING1, STRING2, NUMBER [, DOMAIN [, CATEGORY]])'
-      Return the plural form used for NUMBER of the translation of
-      STRING1 and STRING2 in text domain DOMAIN for locale category
-      CATEGORY. STRING1 is the English singular variant of a message,
-      and STRING2 the English plural variant of the same message.  The
-      default value for DOMAIN is the current value of `TEXTDOMAIN'.
-      The default value for CATEGORY is `"LC_MESSAGES"'.
- 
-      The same remarks about argument order as for the `dcgettext()'
-      function apply.
- 
- `bindtextdomain(DIRECTORY [, DOMAIN])'
-      Change the directory in which `gettext' looks for `.mo' files, in
-      case they will not or cannot be placed in the standard locations
-      (e.g., during testing).  Return the directory in which DOMAIN is
-      "bound."
- 
-      The default DOMAIN is the value of `TEXTDOMAIN'.  If DIRECTORY is
-      the null string (`""'), then `bindtextdomain()' returns the
-      current binding for the given DOMAIN.
- 
-    To use these facilities in your `awk' program, follow the steps
- outlined in *note Explaining gettext::, like so:
- 
-   1. Set the variable `TEXTDOMAIN' to the text domain of your program.
-      This is best done in a `BEGIN' rule (*note BEGIN/END::), or it can
-      also be done via the `-v' command-line option (*note Options::):
- 
-           BEGIN {
-               TEXTDOMAIN = "guide"
-               ...
-           }
- 
-   2. Mark all translatable strings with a leading underscore (`_')
-      character.  It _must_ be adjacent to the opening quote of the
-      string.  For example:
- 
-           print _"hello, world"
-           x = _"you goofed"
-           printf(_"Number of users is %d\n", nusers)
- 
-   3. If you are creating strings dynamically, you can still translate
-      them, using the `dcgettext()' built-in function:
- 
-           message = nusers " users logged in"
-           message = dcgettext(message, "adminprog")
-           print message
- 
-      Here, the call to `dcgettext()' supplies a different text domain
-      (`"adminprog"') in which to find the message, but it uses the
-      default `"LC_MESSAGES"' category.
- 
-   4. During development, you might want to put the `.mo' file in a
-      private directory for testing.  This is done with the
-      `bindtextdomain()' built-in function:
- 
-           BEGIN {
-              TEXTDOMAIN = "guide"   # our text domain
-              if (Testing) {
-                  # where to find our files
-                  bindtextdomain("testdir")
-                  # joe is in charge of adminprog
-                  bindtextdomain("../joe/testdir", "adminprog")
-              }
-              ...
-           }
- 
- 
-    *Note I18N Example::, for an example program showing the steps to
- create and use translations from `awk'.
- 
- 
- File: gawk.info,  Node: Translator i18n,  Next: I18N Example,  Prev: 
Programmer i18n,  Up: Internationalization
- 
- 10.4 Translating `awk' Programs
- ===============================
- 
- Once a program's translatable strings have been marked, they must be
- extracted to create the initial `.po' file.  As part of translation, it
- is often helpful to rearrange the order in which arguments to `printf'
- are output.
- 
-    `gawk''s `--gen-pot' command-line option extracts the messages and
- is discussed next.  After that, `printf''s ability to rearrange the
- order for `printf' arguments at runtime is covered.
- 
- * Menu:
- 
- * String Extraction::           Extracting marked strings.
- * Printf Ordering::             Rearranging `printf' arguments.
- * I18N Portability::            `awk'-level portability issues.
- 
- 
- File: gawk.info,  Node: String Extraction,  Next: Printf Ordering,  Up: 
Translator i18n
- 
- 10.4.1 Extracting Marked Strings
- --------------------------------
- 
- Once your `awk' program is working, and all the strings have been
- marked and you've set (and perhaps bound) the text domain, it is time
- to produce translations.  First, use the `--gen-pot' command-line
- option to create the initial `.pot' file:
- 
-      $ gawk --gen-pot -f guide.awk > guide.pot
- 
-    When run with `--gen-pot', `gawk' does not execute your program.
- Instead, it parses it as usual and prints all marked strings to
- standard output in the format of a GNU `gettext' Portable Object file.
- Also included in the output are any constant strings that appear as the
- first argument to `dcgettext()' or as the first and second argument to
- `dcngettext()'.(1) *Note I18N Example::, for the full list of steps to
- go through to create and test translations for `guide'.
- 
-    ---------- Footnotes ----------
- 
-    (1) The `xgettext' utility that comes with GNU `gettext' can handle
- `.awk' files.
- 
- 
- File: gawk.info,  Node: Printf Ordering,  Next: I18N Portability,  Prev: 
String Extraction,  Up: Translator i18n
- 
- 10.4.2 Rearranging `printf' Arguments
- -------------------------------------
- 
- Format strings for `printf' and `sprintf()' (*note Printf::) present a
- special problem for translation.  Consider the following:(1)
- 
-      printf(_"String `%s' has %d characters\n",
-                string, length(string)))
- 
-    A possible German translation for this might be:
- 
-      "%d Zeichen lang ist die Zeichenkette `%s'\n"
- 
-    The problem should be obvious: the order of the format
- specifications is different from the original!  Even though `gettext()'
- can return the translated string at runtime, it cannot change the
- argument order in the call to `printf'.
- 
-    To solve this problem, `printf' format specifiers may have an
- additional optional element, which we call a "positional specifier".
- For example:
- 
-      "%2$d Zeichen lang ist die Zeichenkette `%1$s'\n"
- 
-    Here, the positional specifier consists of an integer count, which
- indicates which argument to use, and a `$'. Counts are one-based, and
- the format string itself is _not_ included.  Thus, in the following
- example, `string' is the first argument and `length(string)' is the
- second:
- 
-      $ gawk 'BEGIN {
-      >     string = "Dont Panic"
-      >     printf _"%2$d characters live in \"%1$s\"\n",
-      >                         string, length(string)
-      > }'
-      -| 10 characters live in "Dont Panic"
- 
-    If present, positional specifiers come first in the format
- specification, before the flags, the field width, and/or the precision.
- 
-    Positional specifiers can be used with the dynamic field width and
- precision capability:
- 
-      $ gawk 'BEGIN {
-      >    printf("%*.*s\n", 10, 20, "hello")
-      >    printf("%3$*2$.*1$s\n", 20, 10, "hello")
-      > }'
-      -|      hello
-      -|      hello
- 
-      NOTE: When using `*' with a positional specifier, the `*' comes
-      first, then the integer position, and then the `$'.  This is
-      somewhat counterintuitive.
- 
-    `gawk' does not allow you to mix regular format specifiers and those
- with positional specifiers in the same string:
- 
-      $ gawk 'BEGIN { printf _"%d %3$s\n", 1, 2, "hi" }'
-      error--> gawk: cmd. line:1: fatal: must use `count$' on all formats or 
none
- 
-      NOTE: There are some pathological cases that `gawk' may fail to
-      diagnose.  In such cases, the output may not be what you expect.
-      It's still a bad idea to try mixing them, even if `gawk' doesn't
-      detect it.
- 
-    Although positional specifiers can be used directly in `awk'
- programs, their primary purpose is to help in producing correct
- translations of format strings into languages different from the one in
- which the program is first written.
- 
-    ---------- Footnotes ----------
- 
-    (1) This example is borrowed from the GNU `gettext' manual.
- 
- 
- File: gawk.info,  Node: I18N Portability,  Prev: Printf Ordering,  Up: 
Translator i18n
- 
- 10.4.3 `awk' Portability Issues
- -------------------------------
- 
- `gawk''s internationalization features were purposely chosen to have as
- little impact as possible on the portability of `awk' programs that use
- them to other versions of `awk'.  Consider this program:
- 
-      BEGIN {
-          TEXTDOMAIN = "guide"
-          if (Test_Guide)   # set with -v
-              bindtextdomain("/test/guide/messages")
-          print _"don't panic!"
-      }
- 
- As written, it won't work on other versions of `awk'.  However, it is
- actually almost portable, requiring very little change:
- 
-    * Assignments to `TEXTDOMAIN' won't have any effect, since
-      `TEXTDOMAIN' is not special in other `awk' implementations.
- 
-    * Non-GNU versions of `awk' treat marked strings as the
-      concatenation of a variable named `_' with the string following
-      it.(1) Typically, the variable `_' has the null string (`""') as
-      its value, leaving the original string constant as the result.
- 
-    * By defining "dummy" functions to replace `dcgettext()',
-      `dcngettext()' and `bindtextdomain()', the `awk' program can be
-      made to run, but all the messages are output in the original
-      language.  For example:
- 
-           function bindtextdomain(dir, domain)
-           {
-               return dir
-           }
- 
-           function dcgettext(string, domain, category)
-           {
-               return string
-           }
- 
-           function dcngettext(string1, string2, number, domain, category)
-           {
-               return (number == 1 ? string1 : string2)
-           }
- 
-    * The use of positional specifications in `printf' or `sprintf()' is
-      _not_ portable.  To support `gettext()' at the C level, many
-      systems' C versions of `sprintf()' do support positional
-      specifiers.  But it works only if enough arguments are supplied in
-      the function call.  Many versions of `awk' pass `printf' formats
-      and arguments unchanged to the underlying C library version of
-      `sprintf()', but only one format and argument at a time.  What
-      happens if a positional specification is used is anybody's guess.
-      However, since the positional specifications are primarily for use
-      in _translated_ format strings, and since non-GNU `awk's never
-      retrieve the translated string, this should not be a problem in
-      practice.
- 
-    ---------- Footnotes ----------
- 
-    (1) This is good fodder for an "Obfuscated `awk'" contest.
- 
- 
- File: gawk.info,  Node: I18N Example,  Next: Gawk I18N,  Prev: Translator 
i18n,  Up: Internationalization
- 
- 10.5 A Simple Internationalization Example
- ==========================================
- 
- Now let's look at a step-by-step example of how to internationalize and
- localize a simple `awk' program, using `guide.awk' as our original
- source:
- 
-      BEGIN {
-          TEXTDOMAIN = "guide"
-          bindtextdomain(".")  # for testing
-          print _"Don't Panic"
-          print _"The Answer Is", 42
-          print "Pardon me, Zaphod who?"
-      }
- 
- Run `gawk --gen-pot' to create the `.pot' file:
- 
-      $ gawk --gen-pot -f guide.awk > guide.pot
- 
- This produces:
- 
-      #: guide.awk:4
-      msgid "Don't Panic"
-      msgstr ""
- 
-      #: guide.awk:5
-      msgid "The Answer Is"
-      msgstr ""
- 
-    This original portable object template file is saved and reused for
- each language into which the application is translated.  The `msgid' is
- the original string and the `msgstr' is the translation.
- 
-      NOTE: Strings not marked with a leading underscore do not appear
-      in the `guide.pot' file.
- 
-    Next, the messages must be translated.  Here is a translation to a
- hypothetical dialect of English, called "Mellow":(1)
- 
-      $ cp guide.pot guide-mellow.po
-      ADD TRANSLATIONS TO guide-mellow.po ...
- 
- Following are the translations:
- 
-      #: guide.awk:4
-      msgid "Don't Panic"
-      msgstr "Hey man, relax!"
- 
-      #: guide.awk:5
-      msgid "The Answer Is"
-      msgstr "Like, the scoop is"
- 
-    The next step is to make the directory to hold the binary message
- object file and then to create the `guide.mo' file.  The directory
- layout shown here is standard for GNU `gettext' on GNU/Linux systems.
- Other versions of `gettext' may use a different layout:
- 
-      $ mkdir en_US en_US/LC_MESSAGES
- 
-    The `msgfmt' utility does the conversion from human-readable `.po'
- file to machine-readable `.mo' file.  By default, `msgfmt' creates a
- file named `messages'.  This file must be renamed and placed in the
- proper directory so that `gawk' can find it:
- 
-      $ msgfmt guide-mellow.po
-      $ mv messages en_US/LC_MESSAGES/guide.mo
- 
-    Finally, we run the program to test it:
- 
-      $ gawk -f guide.awk
-      -| Hey man, relax!
-      -| Like, the scoop is 42
-      -| Pardon me, Zaphod who?
- 
-    If the three replacement functions for `dcgettext()', `dcngettext()'
- and `bindtextdomain()' (*note I18N Portability::) are in a file named
- `libintl.awk', then we can run `guide.awk' unchanged as follows:
- 
-      $ gawk --posix -f guide.awk -f libintl.awk
-      -| Don't Panic
-      -| The Answer Is 42
-      -| Pardon me, Zaphod who?
- 
-    ---------- Footnotes ----------
- 
-    (1) Perhaps it would be better if it were called "Hippy." Ah, well.
- 
- 
- File: gawk.info,  Node: Gawk I18N,  Prev: I18N Example,  Up: 
Internationalization
- 
- 10.6 `gawk' Can Speak Your Language
- ===================================
- 
- `gawk' itself has been internationalized using the GNU `gettext'
- package.  (GNU `gettext' is described in complete detail in *note (GNU
- `gettext' utilities)Top:: gettext, GNU gettext tools.)  As of this
- writing, the latest version of GNU `gettext' is version 0.18.1
- (ftp://ftp.gnu.org/gnu/gettext/gettext-0.18.1.tar.gz).
- 
-    If a translation of `gawk''s messages exists, then `gawk' produces
- usage messages, warnings, and fatal errors in the local language.
- 
- 
- File: gawk.info,  Node: Arbitrary Precision Arithmetic,  Next: Advanced 
Features,  Prev: Internationalization,  Up: Top
- 
- 11 Arithmetic and Arbitrary Precision Arithmetic with `gawk'
- ************************************************************
- 
-      There's a credibility gap: We don't know how much of the
-      computer's answers to believe. Novice computer users solve this
-      problem by implicitly trusting in the computer as an infallible
-      authority; they tend to believe that all digits of a printed
-      answer are significant. Disillusioned computer users have just the
-      opposite approach; they are constantly afraid that their answers
-      are almost meaningless.
-      Donald Knuth(1)
- 
-    This major node discusses issues that you may encounter when
- performing arithmetic.  It begins by discussing some of the general
- atributes of computer arithmetic, along with how this can influence
- what you see when running `awk' programs.  This discussion applies to
- all versions of `awk'.
- 
-    Then the discussion moves on to "arbitrary precsion arithmetic", a
- feature which is specific to `gawk'.
- 
- * Menu:
- 
- * General Arithmetic::          An introduction to computer arithmetic.
- * Floating-point Programming::  Effective floating-point programming.
- * Gawk and MPFR::               How `gawk' provides
-                                 aribitrary-precision arithmetic.
- * Arbitrary Precision Floats::  Arbitrary precision floating-point arithmetic
-                                 with `gawk'.
- * Arbitrary Precision Integers:: Arbitrary precision integer arithmetic with
-                                 `gawk'.
- 
-    ---------- Footnotes ----------
- 
-    (1) Donald E. Knuth.  `The Art of Computer Programming'. Volume 2,
- `Seminumerical Algorithms', third edition, 1998, ISBN 0-201-89683-4, p.
- 229.
- 
- 
- File: gawk.info,  Node: General Arithmetic,  Next: Floating-point 
Programming,  Up: Arbitrary Precision Arithmetic
- 
- 11.1 A General Description of Computer Arithmetic
- =================================================
- 
- Within computers, there are two kinds of numeric values: "integers" and
- "floating-point".  In school, integer values were referred to as
- "whole" numbers--that is, numbers without any fractional part, such as
- 1, 42, or -17.  The advantage to integer numbers is that they represent
- values exactly.  The disadvantage is that their range is limited.  On
- most systems, this range is -2,147,483,648 to 2,147,483,647.  However,
- many systems now support a range from -9,223,372,036,854,775,808 to
- 9,223,372,036,854,775,807.
- 
-    Integer values come in two flavors: "signed" and "unsigned".  Signed
- values may be negative or positive, with the range of values just
- described.  Unsigned values are always positive.  On most systems, the
- range is from 0 to 4,294,967,295.  However, many systems now support a
- range from 0 to 18,446,744,073,709,551,615.
- 
-    Floating-point numbers represent what are called "real" numbers;
- i.e., those that do have a fractional part, such as 3.1415927.  The
- advantage to floating-point numbers is that they can represent a much
- larger range of values.  The disadvantage is that there are numbers
- that they cannot represent exactly.  `awk' uses "double precision"
- floating-point numbers, which can hold more digits than "single
- precision" floating-point numbers.
- 
-    There a several important issues to be aware of, described next.
- 
- * Menu:
- 
- * Floating Point Issues::       Stuff to know about floating-point numbers.
- * Integer Programming::         Effective integer programming.
- 
- 
- File: gawk.info,  Node: Floating Point Issues,  Next: Integer Programming,  
Up: General Arithmetic
- 
- 11.1.1 Floating-Point Number Caveats
- ------------------------------------
- 
- As mentioned earlier, floating-point numbers represent what are called
- "real" numbers, i.e., those that have a fractional part.  `awk' uses
- double precision floating-point numbers to represent all numeric
- values.  This minor node describes some of the issues involved in using
- floating-point numbers.
- 
-    There is a very nice paper on floating-point arithmetic
- (http://www.validlab.com/goldberg/paper.pdf) by David Goldberg, "What
- Every Computer Scientist Should Know About Floating-point Arithmetic,"
- `ACM Computing Surveys' *23*, 1 (1991-03), 5-48.  This is worth reading
- if you are interested in the details, but it does require a background
- in computer science.
- 
- * Menu:
- 
- * String Conversion Precision:: The String Value Can Lie.
- * Unexpected Results::          Floating Point Numbers Are Not Abstract
-                                 Numbers.
- * POSIX Floating Point Problems:: Standards Versus Existing Practice.
- 
- 
- File: gawk.info,  Node: String Conversion Precision,  Next: Unexpected 
Results,  Up: Floating Point Issues
- 
- 11.1.1.1 The String Value Can Lie
- .................................
- 
- Internally, `awk' keeps both the numeric value (double precision
- floating-point) and the string value for a variable.  Separately, `awk'
- keeps track of what type the variable has (*note Typing and
- Comparison::), which plays a role in how variables are used in
- comparisons.
- 
-    It is important to note that the string value for a number may not
- reflect the full value (all the digits) that the numeric value actually
- contains.  The following program (`values.awk') illustrates this:
- 
-      {
-         sum = $1 + $2
-         # see it for what it is
-         printf("sum = %.12g\n", sum)
-         # use CONVFMT
-         a = "<" sum ">"
-         print "a =", a
-         # use OFMT
-         print "sum =", sum
-      }
- 
- This program shows the full value of the sum of `$1' and `$2' using
- `printf', and then prints the string values obtained from both
- automatic conversion (via `CONVFMT') and from printing (via `OFMT').
- 
-    Here is what happens when the program is run:
- 
-      $ echo 3.654321 1.2345678 | awk -f values.awk
-      -| sum = 4.8888888
-      -| a = <4.88889>
-      -| sum = 4.88889
- 
-    This makes it clear that the full numeric value is different from
- what the default string representations show.
- 
-    `CONVFMT''s default value is `"%.6g"', which yields a value with at
- least six significant digits.  For some applications, you might want to
- change it to specify more precision.  On most modern machines, most of
- the time, 17 digits is enough to capture a floating-point number's
- value exactly.(1)
- 
-    ---------- Footnotes ----------
- 
-    (1) Pathological cases can require up to 752 digits (!), but we
- doubt that you need to worry about this.
- 
- 
- File: gawk.info,  Node: Unexpected Results,  Next: POSIX Floating Point 
Problems,  Prev: String Conversion Precision,  Up: Floating Point Issues
- 
- 11.1.1.2 Floating Point Numbers Are Not Abstract Numbers
- ........................................................
- 
- Unlike numbers in the abstract sense (such as what you studied in high
- school or college arithmetic), numbers stored in computers are limited
- in certain ways.  They cannot represent an infinite number of digits,
- nor can they always represent things exactly.  In particular,
- floating-point numbers cannot always represent values exactly.  Here is
- an example:
- 
-      $ awk '{ printf("%010d\n", $1 * 100) }'
-      515.79
-      -| 0000051579
-      515.80
-      -| 0000051579
-      515.81
-      -| 0000051580
-      515.82
-      -| 0000051582
-      Ctrl-d
- 
- This shows that some values can be represented exactly, whereas others
- are only approximated.  This is not a "bug" in `awk', but simply an
- artifact of how computers represent numbers.
- 
-    Another peculiarity of floating-point numbers on modern systems is
- that they often have more than one representation for the number zero!
- In particular, it is possible to represent "minus zero" as well as
- regular, or "positive" zero.
- 
-    This example shows that negative and positive zero are distinct
- values when stored internally, but that they are in fact equal to each
- other, as well as to "regular" zero:
- 
-      $ gawk 'BEGIN { mz = -0 ; pz = 0
-      > printf "-0 = %g, +0 = %g, (-0 == +0) -> %d\n", mz, pz, mz == pz
-      > printf "mz == 0 -> %d, pz == 0 -> %d\n", mz == 0, pz == 0
-      > }'
-      -| -0 = -0, +0 = 0, (-0 == +0) -> 1
-      -| mz == 0 -> 1, pz == 0 -> 1
- 
-    It helps to keep this in mind should you process numeric data that
- contains negative zero values; the fact that the zero is negative is
- noted and can affect comparisons.
- 
- 
- File: gawk.info,  Node: POSIX Floating Point Problems,  Prev: Unexpected 
Results,  Up: Floating Point Issues
- 
- 11.1.1.3 Standards Versus Existing Practice
- ...........................................
- 
- Historically, `awk' has converted any non-numeric looking string to the
- numeric value zero, when required.  Furthermore, the original
- definition of the language and the original POSIX standards specified
- that `awk' only understands decimal numbers (base 10), and not octal
- (base 8) or hexadecimal numbers (base 16).
- 
-    Changes in the language of the 2001 and 2004 POSIX standards can be
- interpreted to imply that `awk' should support additional features.
- These features are:
- 
-    * Interpretation of floating point data values specified in
-      hexadecimal notation (`0xDEADBEEF'). (Note: data values, _not_
-      source code constants.)
- 
-    * Support for the special IEEE 754 floating point values "Not A
-      Number" (NaN), positive Infinity ("inf") and negative Infinity
-      ("-inf").  In particular, the format for these values is as
-      specified by the ISO 1999 C standard, which ignores case and can
-      allow machine-dependent additional characters after the `nan' and
-      allow either `inf' or `infinity'.
- 
-    The first problem is that both of these are clear changes to
- historical practice:
- 
-    * The `gawk' maintainer feels that supporting hexadecimal floating
-      point values, in particular, is ugly, and was never intended by the
-      original designers to be part of the language.
- 
-    * Allowing completely alphabetic strings to have valid numeric
-      values is also a very severe departure from historical practice.
- 
-    The second problem is that the `gawk' maintainer feels that this
- interpretation of the standard, which requires a certain amount of
- "language lawyering" to arrive at in the first place, was not even
- intended by the standard developers.  In other words, "we see how you
- got where you are, but we don't think that that's where you want to be."
- 
-    Recognizing the above issues, but attempting to provide compatibility
- with the earlier versions of the standard, the 2008 POSIX standard
- added explicit wording to allow, but not require, that `awk' support
- hexadecimal floating point values and special values for "Not A Number"
- and infinity.
- 
-    Although the `gawk' maintainer continues to feel that providing
- those features is inadvisable, nevertheless, on systems that support
- IEEE floating point, it seems reasonable to provide _some_ way to
- support NaN and Infinity values.  The solution implemented in `gawk' is
- as follows:
- 
-    * With the `--posix' command-line option, `gawk' becomes "hands
-      off." String values are passed directly to the system library's
-      `strtod()' function, and if it successfully returns a numeric
-      value, that is what's used.(1) By definition, the results are not
-      portable across different systems.  They are also a little
-      surprising:
- 
-           $ echo nanny | gawk --posix '{ print $1 + 0 }'
-           -| nan
-           $ echo 0xDeadBeef | gawk --posix '{ print $1 + 0 }'
-           -| 3735928559
- 
-    * Without `--posix', `gawk' interprets the four strings `+inf',
-      `-inf', `+nan', and `-nan' specially, producing the corresponding
-      special numeric values.  The leading sign acts a signal to `gawk'
-      (and the user) that the value is really numeric.  Hexadecimal
-      floating point is not supported (unless you also use
-      `--non-decimal-data', which is _not_ recommended). For example:
- 
-           $ echo nanny | gawk '{ print $1 + 0 }'
-           -| 0
-           $ echo +nan | gawk '{ print $1 + 0 }'
-           -| nan
-           $ echo 0xDeadBeef | gawk '{ print $1 + 0 }'
-           -| 0
- 
-      `gawk' does ignore case in the four special values.  Thus `+nan'
-      and `+NaN' are the same.
- 
-    ---------- Footnotes ----------
- 
-    (1) You asked for it, you got it.
- 
- 
- File: gawk.info,  Node: Integer Programming,  Prev: Floating Point Issues,  
Up: General Arithmetic
- 
- 11.1.2 Mixing Integers And Floating-point
- -----------------------------------------
- 
- As has been mentioned already, `gawk' ordinarily uses hardware double
- precision with 64-bit IEEE binary floating-point representation for
- numbers on most systems. A large integer like 9007199254740997 has a
- binary representation that, although finite, is more than 53 bits long;
- it must also be rounded to 53 bits.  The biggest integer that can be
- stored in a C `double' is usually the same as the largest possible
- value of a `double'. If your system `double' is an IEEE 64-bit
- `double', this largest possible value is an integer and can be
- represented precisely.  What more should one know about integers?
- 
-    If you want to know what is the largest integer, such that it and
- all smaller integers can be stored in 64-bit doubles without losing
- precision, then the answer is 2^53.  The next representable number is
- the even number 2^53 + 2, meaning it is unlikely that you will be able
- to make `gawk' print 2^53 + 1 in integer format.  The range of integers
- exactly representable by a 64-bit double is [-2^53, 2^53].  If you ever
- see an integer outside this range in `gawk' using 64-bit doubles, you
- have reason to be very suspicious about the accuracy of the output.
- Here is a simple program with erroneous output:
- 
-      $ gawk 'BEGIN { i = 2^53 - 1; for (j = 0; j < 4; j++) print i + j }'
-      -| 9007199254740991
-      -| 9007199254740992
-      -| 9007199254740992
-      -| 9007199254740994
- 
-    The lesson is to not assume that any large integer printed by `gawk'
- represents an exact result from your computation, especially if it wraps
- around on your screen.
- 
- 
- File: gawk.info,  Node: Floating-point Programming,  Next: Gawk and MPFR,  
Prev: General Arithmetic,  Up: Arbitrary Precision Arithmetic
- 
- 11.2 Understanding Floating-point Programming
- =============================================
- 
- Numerical programming is an extensive area; if you need to develop
- sophisticated numerical algorithms then `gawk' may not be the ideal
- tool, and this documentation may not be sufficient.  It might require
- digesting a book or two to really internalize how to compute with ideal
- accuracy and precision and the result often depends on the particular
- application.
- 
-      NOTE: A floating-point calculation's "accuracy" is how close it
-      comes to the real value.  This is as opposed to the "precision",
-      which usually refers to the number of bits used to represent the
-      number (see the Wikipedia article
-      (http://en.wikipedia.org/wiki/Accuracy_and_precision) for more
-      information).
- 
-    There are two options for doing floating-point calculations:
- hardware floating-point (as used by standard `awk' and the default for
- `gawk'), and "arbitrary-precision" floating-point, which is software
- based.  This major node aims to provide enough information to
- understand both, and then will focus on `gawk''s facilities for the
- latter.
- 
-    Binary floating-point representations and arithmetic are inexact.
- Simple values like 0.1 cannot be precisely represented using binary
- floating-point numbers, and the limited precision of floating-point
- numbers means that slight changes in the order of operations or the
- precision of intermediate storage can change the result. To make
- matters worse, with arbitrary precision floating-point, you can set the
- precision before starting a computation, but then you cannot be sure of
- the number of significant decimal places in the final result.
- 
-    Sometimes, before you start to write any code, you should think more
- about what you really want and what's really happening. Consider the
- two numbers in the following example:
- 
-      x = 0.875             # 1/2 + 1/4 + 1/8
-      y = 0.425
- 
-    Unlike the number in `y', the number stored in `x' is exactly
- representable in binary since it can be written as a finite sum of one
- or more fractions whose denominators are all powers of two.  When
- `gawk' reads a floating-point number from program source, it
- automatically rounds that number to whatever precision your machine
- supports. If you try to print the numeric content of a variable using
- an output format string of `"%.17g"', it may not produce the same
- number as you assigned to it:
- 
-      $ gawk 'BEGIN { x = 0.875; y = 0.425
-      >               printf("%0.17g, %0.17g\n", x, y) }'
-      -| 0.875, 0.42499999999999999
- 
-    Often the error is so small you do not even notice it, and if you do,
- you can always specify how much precision you would like in your output.
- Usually this is a format string like `"%.15g"', which when used in the
- previous example, produces an output identical to the input.
- 
-    Because the underlying representation can be little bit off from the
- exact value, comparing floating-point values to see if they are equal
- is generally not a good idea.  Here is an example where it does not
- work like you expect:
- 
-      $ gawk 'BEGIN { print (0.1 + 12.2 == 12.3) }'
-      -| 0
- 
-    The loss of accuracy during a single computation with floating-point
- numbers usually isn't enough to worry about. However, if you compute a
- value which is the result of a sequence of floating point operations,
- the error can accumulate and greatly affect the computation itself.
- Here is an attempt to compute the value of the constant pi using one of
- its many series representations:
- 
-      BEGIN {
-          x = 1.0 / sqrt(3.0)
-          n = 6
-          for (i = 1; i < 30; i++) {
-              n = n * 2.0
-              x = (sqrt(x * x + 1) - 1) / x
-              printf("%.15f\n", n * x)
-          }
-      }
- 
-    When run, the early errors propagating through later computations
- cause the loop to terminate prematurely after an attempt to divide by
- zero.
- 
-      $ gawk -f pi.awk
-      -| 3.215390309173475
-      -| 3.159659942097510
-      -| 3.146086215131467
-      -| 3.142714599645573
-      ...
-      -| 3.224515243534819
-      -| 2.791117213058638
-      -| 0.000000000000000
-      error--> gawk: pi.awk:6: fatal: division by zero attempted
- 
-    Here is one more example where the inaccuracies in internal
- representations yield an unexpected result:
- 
-      $ gawk 'BEGIN {
-      >   for (d = 1.1; d <= 1.5; d += 0.1)
-      >       i++
-      >   print i
-      > }'
-      -| 4
- 
-    Can computation using aribitrary precision help with the previous
- examples?  If you are impatient to know, see *note Exact Arithmetic::.
- 
-    Instead of aribitrary precision floating-point arithmetic, often all
- you need is an adjustment of your logic or a different order for the
- operations in your calculation.  The stability and the accuracy of the
- computation of the constant pi in the previous example can be enhanced
- by using the following simple algebraic transformation:
- 
-      (sqrt(x * x + 1) - 1) / x = x / (sqrt(x * x + 1) + 1)
- 
- After making this, change the program does converge to pi in under 30
- iterations:
- 
-      $ gawk -f /tmp/pi2.awk
-      -| 3.215390309173473
-      -| 3.159659942097501
-      -| 3.146086215131436
-      -| 3.142714599645370
-      -| 3.141873049979825
-      ...
-      -| 3.141592653589797
-      -| 3.141592653589797
- 
-    There is no need to be unduly suspicious about the results from
- floating-point arithmetic. The lesson to remember is that
- floating-point arithmetic is always more complex than the arithmetic
- using pencil and paper. In order to take advantage of the power of
- computer floating-point, you need to know its limitations and work
- within them. For most casual use of floating-point arithmetic, you will
- often get the expected result in the end if you simply round the
- display of your final results to the correct number of significant
- decimal digits. And, avoid presenting numerical data in a manner that
- implies better precision than is actually the case.
- 
- * Menu:
- 
- * Floating-point Representation:: Binary floating-point representation.
- * Floating-point Context::        Floating-point context.
- * Rounding Mode::                 Floating-point rounding mode.
- 
- 
- File: gawk.info,  Node: Floating-point Representation,  Next: Floating-point 
Context,  Up: Floating-point Programming
- 
- 11.2.1 Binary Floating-point Representation
- -------------------------------------------
- 
- Although floating-point representations vary from machine to machine,
- the most commonly encountered representation is that defined by the
- IEEE 754 Standard. An IEEE-754 format value has three components:
- 
-    * A sign bit telling whether the number is positive or negative.
- 
-    * An "exponent" giving its order of magnitude, E.
- 
-    * A "significand", S, specifying the actual digits of the number.
- 
-    The value of the number is then S * 2^E.  The first bit of a
- non-zero binary significand is always one, so the significand in an
- IEEE-754 format only includes the fractional part, leaving the leading
- one implicit.
- 
-    Three of the standard IEEE-754 types are 32-bit single precision,
- 64-bit double precision and 128-bit quadruple precision.  The standard
- also specifies extended precision formats to allow greater precisions
- and larger exponent ranges.
- 
-    The significand is stored in "normalized" format, which means that
- the first bit is always a one.
- 
- 
- File: gawk.info,  Node: Floating-point Context,  Next: Rounding Mode,  Prev: 
Floating-point Representation,  Up: Floating-point Programming
- 
- 11.2.2 Floating-point Context
- -----------------------------
- 
- A floating-point "context" defines the environment for arithmetic
- operations.  It governs precision, sets rules for rounding, and limits
- the range for exponents.  The context has the following primary
- components:
- 
- "Precision"
-      Precision of the floating-point format in bits.
- 
- "emax"
-      Maximum exponent allowed for this format.
- 
- "emin"
-      Minimum exponent allowed for this format.
- 
- "Underflow behavior"
-      The format may or may not support gradual underflow.
- 
- "Rounding"
-      The rounding mode of this context.
- 
-    *note table-ieee-formats:: lists the precision and exponent field
- values for the basic IEEE-754 binary formats:
- 
- Name           Total bits     Precision      emin           emax
- --------------------------------------------------------------------------- 
- Single         32             24             -126           +127
- Double         64             53             -1022          +1023
- Quadruple      128            113            -16382         +16383
- 
- Table 11.1: Basic IEEE Format Context Values
- 
-      NOTE: The precision numbers include the implied leading one that
-      gives them one extra bit of significand.
- 
-    A floating-point context can also determine which signals are treated
- as exceptions, and can set rules for arithmetic with special values.
- Please consult the IEEE-754 standard or other resources for details.
- 
-    `gawk' ordinarily uses the hardware double precision representation
- for numbers.  On most systems, this is IEEE-754 floating-point format,
- corresponding to 64-bit binary with 53 bits of precision.
- 
-      NOTE: In case an underflow occurs, the standard allows, but does
-      not require, the result from an arithmetic operation to be a
-      number smaller than the smallest nonzero normalized number. Such
-      numbers do not have as many significant digits as normal numbers,
-      and are called "denormals" or "subnormals". The alternative,
-      simply returning a zero, is called "flush to zero". The basic
-      IEEE-754 binary formats support subnormal numbers.
- 
- 
- File: gawk.info,  Node: Rounding Mode,  Prev: Floating-point Context,  Up: 
Floating-point Programming
- 
- 11.2.3 Floating-point Rounding Mode
- -----------------------------------
- 
- The "rounding mode" specifies the behavior for the results of numerical
- operations when discarding extra precision. Each rounding mode indicates
- how the least significant returned digit of a rounded result is to be
- calculated.  *note table-rounding-modes:: lists the IEEE-754 defined
- rounding modes:
- 
- Rounding Mode                    IEEE Name
- -------------------------------------------------------------------------- 
- Round to nearest, ties to even   `roundTiesToEven'
- Round toward plus Infinity       `roundTowardPositive'
- Round toward negative Infinity   `roundTowardNegative'
- Round toward zero                `roundTowardZero'
- Round to nearest, ties away      `roundTiesToAway'
- from zero                        
- 
- Table 11.2: IEEE 754 Rounding Modes
- 
-    The default mode `roundTiesToEven' is the most preferred, but the
- least intuitive. This method does the obvious thing for most values, by
- rounding them up or down to the nearest digit.  For example, rounding
- 1.132 to two digits yields 1.13, and rounding 1.157 yields 1.16.
- 
-    However, when it comes to rounding a value that is exactly halfway
- between, things do not work the way you probably learned in school.  In
- this case, the number is rounded to the nearest even digit.  So
- rounding 0.125 to two digits rounds down to 0.12, but rounding 0.6875
- to three digits rounds up to 0.688.  You probably have already
- encountered this rounding mode when using the `printf' routine to
- format floating-point numbers.  For example:
- 
-      BEGIN {
-          x = -4.5
-          for (i = 1; i < 10; i++) {
-              x += 1.0
-              printf("%4.1f => %2.0f\n", x, x)
-          }
-      }
- 
- produces the following output when run:(1)
- 
-      -3.5 => -4
-      -2.5 => -2
-      -1.5 => -2
-      -0.5 => 0
-       0.5 => 0
-       1.5 => 2
-       2.5 => 2
-       3.5 => 4
-       4.5 => 4
- 
-    The theory behind the rounding mode `roundTiesToEven' is that it
- more or less evenly distributes upward and downward rounds of exact
- halves, which might cause the round-off error to cancel itself out.
- This is the default rounding mode used in IEEE-754 computing functions
- and operators.
- 
-    The other rounding modes are rarely used.  Round toward positive
- infinity (`roundTowardPositive') and round toward negative infinity
- (`roundTowardNegative') are often used to implement interval arithmetic,
- where you adjust the rounding mode to calculate upper and lower bounds
- for the range of output. The `roundTowardZero' mode can be used for
- converting floating-point numbers to integers.  The rounding mode
- `roundTiesToAway' rounds the result to the nearest number and selects
- the number with the larger magnitude if a tie occurs.
- 
-    Some numerical analysts will tell you that your choice of rounding
- style has tremendous impact on the final outcome, and advise you to
- wait until final output for any rounding. Instead, you can often avoid
- round-off error problems by setting the precision initially to some
- value sufficiently larger than the final desired precision, so that the
- accumulation of round-off error does not influence the outcome.  If you
- suspect that results from your computation are sensitive to
- accumulation of round-off error, one way to be sure is to look for a
- significant difference in output when you change the rounding mode.
- 
-    ---------- Footnotes ----------
- 
-    (1) It is possible for the output to be completely different if the
- C library in your system does not use the IEEE-754 even-rounding rule
- to round halfway cases for `printf()'.
- 
- 
- File: gawk.info,  Node: Gawk and MPFR,  Next: Arbitrary Precision Floats,  
Prev: Floating-point Programming,  Up: Arbitrary Precision Arithmetic
- 
- 11.3 `gawk' + MPFR = Powerful Arithmetic
- ========================================
- 
- The rest of this major node decsribes how to use the arbitrary precision
- (also known as "multiple precision" or "infinite precision") numeric
- capabilites in `gawk' to produce maximally accurate results when you
- need it.
- 
-    But first you should check if your version of `gawk' supports
- arbitrary precision arithmetic.  The easiest way to find out is to look
- at the output of the following command:
- 
-      $ gawk --version
-      -| GNU Awk 4.1.0 (GNU MPFR 3.1.0, GNU MP 5.0.3)
-      -| Copyright (C) 1989, 1991-2012 Free Software Foundation.
-      ...
- 
-    `gawk' uses the GNU MPFR (http://www.mpfr.org) and GNU MP
- (http://gmplib.org) (GMP) libraries for arbitrary precision arithmetic
- on numbers. So if you do not see the names of these libraries in the
- output, then your version of `gawk' does not support arbitrary
- precision arithmetic.
- 
-    Additionally, there are a few elements available in the `PROCINFO'
- array to provide information about the MPFR and GMP libraries.  *Note
- Auto-set::, for more information.
- 
- 
- File: gawk.info,  Node: Arbitrary Precision Floats,  Next: Arbitrary 
Precision Integers,  Prev: Gawk and MPFR,  Up: Arbitrary Precision Arithmetic
- 
- 11.4 Arbitrary Precision Floating-point Arithmetic with `gawk'
- ==============================================================
- 
- `gawk' uses the GNU MPFR library for arbitrary precision floating-point
- arithmetic.  The MPFR library provides precise control over precisions
- and rounding modes, and gives correctly rounded reproducible
- platform-independent results.  With the command-line option `--bignum'
- or `-M', all floating-point arithmetic operators and numeric functions
- can yield results to any desired precision level supported by MPFR.
- Two built-in variables `PREC' (*note Setting Precision::) and
- `ROUNDMODE' (*note Setting Rounding Mode::) provide control over the
- working precision and the rounding mode.  The precision and the
- rounding mode are set globally for every operation to follow.
- 
-    The default working precision for arbitrary precision floating-point
- values is 53, and the default value for `ROUNDMODE' is `"N"', which
- selects the IEEE-754 `roundTiesToEven' (*note Rounding Mode::) rounding
- mode.(1) `gawk' uses the default exponent range in MPFR (EMAX = 2^30 -
- 1, EMIN = -EMAX) for all floating-point contexts.  There is no explicit
- mechanism to adjust the exponent range.  MPFR does not implement
- subnormal numbers by default, and this behavior cannot be changed in
- `gawk'.
- 
-      NOTE: When emulating an IEEE-754 format (*note Setting
-      Precision::), `gawk' internally adjusts the exponent range to the
-      value defined for the format and also performs computations needed
-      for gradual underflow (subnormal numbers).
- 
-      NOTE: MPFR numbers are variable-size entities, consuming only as
-      much space as needed to store the significant digits. Since the
-      performance using MPFR numbers pales in comparison to doing
-      arithmetic using the underlying machine types, you should consider
-      using only as much precision as needed by your program.
- 
- * Menu:
- 
- * Setting Precision::           Setting the working precision.
- * Setting Rounding Mode::       Setting the rounding mode.
- * Floating-point Constants::    Representing floating-point constants.
- * Changing Precision::          Changing the precision of a number.
- * Exact Arithmetic::            Exact arithmetic with floating-point numbers.
- 
-    ---------- Footnotes ----------
- 
-    (1) The default precision is 53, since according to the MPFR
- documentation, the library should be able to exactly reproduce all
- computations with double-precision machine floating-point numbers
- (`double' type in C), except the default exponent range is much wider
- and subnormal numbers are not implemented.
- 
- 
- File: gawk.info,  Node: Setting Precision,  Next: Setting Rounding Mode,  Up: 
Arbitrary Precision Floats
- 
- 11.4.1 Setting the Working Precision
- ------------------------------------
- 
- `gawk' uses a global working precision; it does not keep track of the
- precision or accuracy of individual numbers. Performing an arithmetic
- operation or calling a built-in function rounds the result to the
- current working precision. The default working precision is 53 which
- can be modified using the built-in variable `PREC'. You can also set the
- value to one of the following pre-defined case-insensitive strings to
- emulate an IEEE-754 binary format:
- 
- `PREC'       IEEE-754 Binary Format
- --------------------------------------------------- 
- `"half"'     16-bit half-precision.
- `"single"'   Basic 32-bit single precision.
- `"double"'   Basic 64-bit double precision.
- `"quad"'     Basic 128-bit quadruple precision.
- `"oct"'      256-bit octuple precision.
- 
-    The following example illustrates the effects of changing precision
- on arithmetic operations:
- 
-      $ gawk -M -vPREC=100 'BEGIN { x = 1.0e-400; print x + 0; \
-      >   PREC = "double"; print x + 0 }'
-      -| 1e-400
-      -| 0
- 
-    Binary and decimal precisions are related approximately according to
- the formula:
- 
-    PREC = 3.322 * DPS
- 
- Here, PREC denotes the binary precision (measured in bits) and DPS
- (short for decimal places) is the decimal digits. We can easily
- calculate how many decimal digits the 53-bit significand of an IEEE
- double is equivalent to: 53 / 3.332 which is equal to about 15.95.  But
- what does 15.95 digits actually mean? It depends whether you are
- concerned about how many digits you can rely on, or how many digits you
- need.
- 
-    It is important to know how many bits it takes to uniquely identify
- a double-precision value (the C type `double').  If you want to convert
- from `double' to decimal and back to `double' (e.g., saving a `double'
- representing an intermediate result to a file, and later reading it
- back to restart the computation), then a few more decimal digits are
- required. 17 digits is generally enough for a `double'.
- 
-    It can also be important to know what decimal numbers can be uniquely
- represented with a `double'. If you want to convert from decimal to
- `double' and back again, 15 digits is the most that you can get. Stated
- differently, you should not present the numbers from your
- floating-point computations with more than 15 significant digits in
- them.
- 
-    Conversely, it takes a precision of 332 bits to hold an approximation
- of the constant pi that is accurate to 100 decimal places.  You should
- always add some extra bits in order to avoid the confusing round-off
- issues that occur because numbers are stored internally in binary.
- 
- 
- File: gawk.info,  Node: Setting Rounding Mode,  Next: Floating-point 
Constants,  Prev: Setting Precision,  Up: Arbitrary Precision Floats
- 
- 11.4.2 Setting the Rounding Mode
- --------------------------------
- 
- The `ROUNDMODE' variable provides program level control over the
- rounding mode.  The correspondance between `ROUNDMODE' and the IEEE
- rounding modes is shown in *note table-gawk-rounding-modes::.
- 
- Rounding Mode                    IEEE Name              `ROUNDMODE'
- --------------------------------------------------------------------------- 
- Round to nearest, ties to even   `roundTiesToEven'      `"N"' or `"n"'
- Round toward plus Infinity       `roundTowardPositive'  `"U"' or `"u"'
- Round toward negative Infinity   `roundTowardNegative'  `"D"' or `"d"'
- Round toward zero                `roundTowardZero'      `"Z"' or `"z"'
- Round to nearest, ties away      `roundTiesToAway'      `"A"' or `"a"'
- from zero                                               
- 
- Table 11.3: `gawk' Rounding Modes
- 
-    `ROUNDMODE' has the default value `"N"', which selects the IEEE-754
- rounding mode `roundTiesToEven'.  Besides the values listed in *note
- Table 11.3: table-gawk-rounding-modes, `gawk' also accepts `"A"' to
- select the IEEE-754 mode `roundTiesToAway' if your version of the MPFR
- library supports it; otherwise setting `ROUNDMODE' to this value has no
- effect. *Note Rounding Mode::, for the meanings of the various rounding
- modes.
- 
-    Here is an example of how to change the default rounding behavior of
- `printf''s output:
- 
-      $ gawk -M -vROUNDMODE="Z" 'BEGIN { printf("%.2f\n", 1.378) }'
-      -| 1.37
- 
- 
- File: gawk.info,  Node: Floating-point Constants,  Next: Changing Precision,  
Prev: Setting Rounding Mode,  Up: Arbitrary Precision Floats
- 
- 11.4.3 Representing Floating-point Constants
- --------------------------------------------
- 
- Be wary of floating-point constants! When reading a floating-point
- constant from program source code, `gawk' uses the default precision,
- unless overridden by an assignment to the special variable `PREC' on
- the command line, to store it internally as a MPFR number.  Changing
- the precision using `PREC' in the program text does not change the
- precision of a constant. If you need to represent a floating-point
- constant at a higher precision than the default and cannot use a
- command line assignment to `PREC', you should either specify the
- constant as a string, or as a rational number whenever possible. The
- following example illustrates the differences among various ways to
- print a floating-point constant:
- 
-      $ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", 0.1) }'
-      -| 0.1000000000000000055511151
-      $ gawk -M -vPREC = 113 'BEGIN { printf("%0.25f\n", 0.1) }'
-      -| 0.1000000000000000000000000
-      $ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", "0.1") }'
-      -| 0.1000000000000000000000000
-      $ gawk -M 'BEGIN { PREC = 113; printf("%0.25f\n", 1/10) }'
-      -| 0.1000000000000000000000000
- 
-    In the first case, the number is stored with the default precision
- of 53.
- 
- 
- File: gawk.info,  Node: Changing Precision,  Next: Exact Arithmetic,  Prev: 
Floating-point Constants,  Up: Arbitrary Precision Floats
- 
- 11.4.4 Changing the Precision of a Number
- -----------------------------------------
- 
-      The point is that in any variable-precision package, a decision is
-      made on how to treat numbers given as data, or arising in
-      intermediate results, which are represented in floating-point
-      format to a precision lower than working precision.  Do we promote
-      them to full membership of the high-precision club, or do we treat
-      them and all their associates as second-class citizens?  Sometimes
-      the first course is proper, sometimes the second, and it takes
-      careful analysis to tell which.
- 
-      Dirk Laurie(1)
- 
-    `gawk' does not implicitly modify the precision of any previously
- computed results when the working precision is changed with an
- assignment to `PREC'.  The precision of a number is always the one that
- was used at the time of its creation, and there is no way for the user
- to explicitly change it afterwards. However, since the result of a
- floating-point arithmetic operation is always an arbitrary precision
- floating-point value--with a precision set by the value of `PREC'--one
- of the following workarounds effectively accomplishes the desired
- behavior:
- 
-      x = x + 0.0
- 
- or:
- 
-      x += 0.0
- 
-    ---------- Footnotes ----------
- 
-    (1) Dirk Laurie.  `Variable-precision Arithmetic Considered Perilous
- -- A Detective Story'.  Electronic Transactions on Numerical Analysis.
- Volume 28, pp. 168-173, 2008.
- 
- 
- File: gawk.info,  Node: Exact Arithmetic,  Prev: Changing Precision,  Up: 
Arbitrary Precision Floats
- 
- 11.4.5 Exact Arithmetic with Floating-point Numbers
- ---------------------------------------------------
- 
-      CAUTION: Never depend on the exactness of floating-point
-      arithmetic, even for apparently simple expressions!
- 
-    Can arbitrary precision arithmetic give exact results? There are no
- easy answers. The standard rules of algebra often do not apply when
- using floating-point arithmetic.  Among other things, the distributive
- and associative laws do not hold completely, and order of operation may
- be important for your computation. Rounding error, cumulative precision
- loss and underflow are often troublesome.
- 
-    When `gawk' tests the expressions `0.1 + 12.2' and `12.3' for
- equality using the machine double precision arithmetic, it decides that
- they are not equal!  (*Note Floating-point Programming::.)  You can get
- the result you want by increasing the precision; 56 in this case will
- get the job done:
- 
-      $ gawk -M -vPREC=56 'BEGIN { print (0.1 + 12.2 == 12.3) }'
-      -| 1
- 
-    If adding more bits is good, perhaps adding even more bits of
- precision is better?  Here is what happens if we use an even larger
- value of `PREC':
- 
-      $ gawk -M -vPREC=201 'BEGIN { print (0.1 + 12.2 == 12.3) }'
-      -| 0
- 
-    This is not a bug in `gawk' or in the MPFR library.  It is easy to
- forget that the finite number of bits used to store the value is often
- just an approximation after proper rounding.  The test for equality
- succeeds if and only if _all_ bits in the two operands are exactly the
- same. Since this is not necessarily true after floating-point
- computations with a particular precision and effective rounding rule, a
- straight test for equality may not work.
- 
-    So, don't assume that floating-point values can be compared for
- equality.  You should also exercise caution when using other forms of
- comparisons.  The standard way to compare between floating-point
- numbers is to determine how much error (or "tolerance") you will allow
- in a comparison and check to see if one value is within this error
- range of the other.
- 
-    In applications where 15 or fewer decimal places suffice, hardware
- double precision arithmetic can be adequate, and is usually much faster.
- But you do need to keep in mind that every floating-point operation can
- suffer a new rounding error with catastrophic consequences as
- illustrated by our attempt to compute the value of the constant pi
- (*note Floating-point Programming::).  Extra precision can greatly
- enhance the stability and the accuracy of your computation in such
- cases.
- 
-    Repeated addition is not necessarily equivalent to multiplication in
- floating-point arithmetic. In the example in *note Floating-point
- Programming:::
- 
-      $ gawk 'BEGIN {
-      >   for (d = 1.1; d <= 1.5; d += 0.1)
-      >       i++
-      >   print i
-      > }'
-      -| 4
- 
- you may or may not succeed in getting the correct result by choosing an
- arbitrarily large value for `PREC'. Reformulation of the problem at
- hand is often the correct approach in such situations.
- 
- 
- File: gawk.info,  Node: Arbitrary Precision Integers,  Prev: Arbitrary 
Precision Floats,  Up: Arbitrary Precision Arithmetic
- 
- 11.5 Arbitrary Precision Integer Arithmetic with `gawk'
- =======================================================
- 
- If the option `--bignum' or `-M' is specified, `gawk' performs all
- integer arithmetic using GMP arbitrary precision integers.  Any number
- that looks like an integer in a program source or data file is stored
- as an arbitrary precision integer.  The size of the integer is limited
- only by your computer's memory.  The current floating-point context has
- no effect on operations involving integers.  For example, the following
- computes 5^4^3^2, the result of which is beyond the limits of ordinary
- `gawk' numbers:
- 
-      $ gawk -M 'BEGIN {
-      >   x = 5^4^3^2
-      >   print "# of digits =", length(x)
-      >   print substr(x, 1, 20), "...", substr(x, length(x) - 19, 20)
-      > }'
-      -| # of digits = 183231
-      -| 62060698786608744707 ... 92256259918212890625
- 
-    If you were to compute the same value using arbitrary precision
- floating-point values instead, the precision needed for correct output
- (using the formula `prec = 3.322 * dps'), would be 3.322 x 183231, or
- 608693.  (Thus, the floating-point representation requires over 30
- times as many decimal digits!)
- 
-    The result from an arithmetic operation with an integer and a
- floating-point value is a floating-point value with a precision equal
- to the working precision.  The following program calculates the eighth
- term in Sylvester's sequence(1) using a recurrence:
- 
-      $ gawk -M 'BEGIN {
-      >   s = 2.0
-      >   for (i = 1; i <= 7; i++)
-      >       s = s * (s - 1) + 1
-      >   print s
-      > }'
-      -| 113423713055421845118910464
- 
-    The output differs from the acutal number,
- 113423713055421844361000443, because the default precision of 53 is not
- enough to represent the floating-point results exactly. You can either
- increase the precision (100 is enough in this case), or replace the
- floating-point constant `2.0' with an integer, to perform all
- computations using integer arithmetic to get the correct output.
- 
-    It will sometimes be necessary for `gawk' to implicitly convert an
- arbitrary precision integer into an arbitrary precision floating-point
- value.  This is primarily because the MPFR library does not always
- provide the relevant interface to process arbitrary precision integers
- or mixed-mode numbers as needed by an operation or function.  In such a
- case, the precision is set to the minimum value necessary for exact
- conversion, and the working precision is not used for this purpose.  If
- this is not what you need or want, you can employ a subterfuge like
- this:
- 
-      gawk -M 'BEGIN { n = 13; print (n + 0.0) % 2.0 }'
- 
-    You can avoid this issue altogether by specifying the number as a
- floating-point value to begin with:
- 
-      gawk -M 'BEGIN { n = 13.0; print n % 2.0 }'
- 
-    Note that for the particular example above, there is likely best to
- just use the following:
- 
-      gawk -M 'BEGIN { n = 13; print n % 2 }'
- 
-    ---------- Footnotes ----------
- 
-    (1) Weisstein, Eric W.  `Sylvester's Sequence'. From MathWorld--A
- Wolfram Web Resource.
- `http://mathworld.wolfram.com/SylvestersSequence.html'
- 
- 
- File: gawk.info,  Node: Advanced Features,  Next: Library Functions,  Prev: 
Arbitrary Precision Arithmetic,  Up: Top
- 
- 12 Advanced Features of `gawk'
- ******************************
- 
-      Write documentation as if whoever reads it is a violent psychopath
-      who knows where you live.
-      Steve English, as quoted by Peter Langston
- 
-    This major node discusses advanced features in `gawk'.  It's a bit
- of a "grab bag" of items that are otherwise unrelated to each other.
- First, a command-line option allows `gawk' to recognize nondecimal
- numbers in input data, not just in `awk' programs.  Then, `gawk''s
- special features for sorting arrays are presented.  Next, two-way I/O,
- discussed briefly in earlier parts of this Info file, is described in
- full detail, along with the basics of TCP/IP networking.  Finally,
- `gawk' can "profile" an `awk' program, making it possible to tune it
- for performance.
- 
-    *note Dynamic Extensions::, discusses the ability to dynamically add
- new built-in functions to `gawk'.  As this feature is still immature
- and likely to change, its description is relegated to an appendix.
- 
- * Menu:
- 
- * Nondecimal Data::             Allowing nondecimal input data.
- * Array Sorting::               Facilities for controlling array traversal and
-                                 sorting arrays.
- * Two-way I/O::                 Two-way communications with another process.
- * TCP/IP Networking::           Using `gawk' for network programming.
- * Profiling::                   Profiling your `awk' programs.
- 
- 
- File: gawk.info,  Node: Nondecimal Data,  Next: Array Sorting,  Up: Advanced 
Features
- 
- 12.1 Allowing Nondecimal Input Data
- ===================================
- 
- If you run `gawk' with the `--non-decimal-data' option, you can have
- nondecimal constants in your input data:
- 
-      $ echo 0123 123 0x123 |
-      > gawk --non-decimal-data '{ printf "%d, %d, %d\n",
-      >                                         $1, $2, $3 }'
-      -| 83, 123, 291
- 
-    For this feature to work, write your program so that `gawk' treats
- your data as numeric:
- 
-      $ echo 0123 123 0x123 | gawk '{ print $1, $2, $3 }'
-      -| 0123 123 0x123
- 
- The `print' statement treats its expressions as strings.  Although the
- fields can act as numbers when necessary, they are still strings, so
- `print' does not try to treat them numerically.  You may need to add
- zero to a field to force it to be treated as a number.  For example:
- 
-      $ echo 0123 123 0x123 | gawk --non-decimal-data '
-      > { print $1, $2, $3
-      >   print $1 + 0, $2 + 0, $3 + 0 }'
-      -| 0123 123 0x123
-      -| 83 123 291
- 
-    Because it is common to have decimal data with leading zeros, and
- because using this facility could lead to surprising results, the
- default is to leave it disabled.  If you want it, you must explicitly
- request it.
- 
-      CAUTION: _Use of this option is not recommended._ It can break old
-      programs very badly.  Instead, use the `strtonum()' function to
-      convert your data (*note Nondecimal-numbers::).  This makes your
-      programs easier to write and easier to read, and leads to less
-      surprising results.
- 
- 
- File: gawk.info,  Node: Array Sorting,  Next: Two-way I/O,  Prev: Nondecimal 
Data,  Up: Advanced Features
- 
- 12.2 Controlling Array Traversal and Array Sorting
- ==================================================
- 
- `gawk' lets you control the order in which a `for (i in array)' loop
- traverses an array.
- 
-    In addition, two built-in functions, `asort()' and `asorti()', let
- you sort arrays based on the array values and indices, respectively.
- These two functions also provide control over the sorting criteria used
- to order the elements during sorting.
- 
- * Menu:
- 
- * Controlling Array Traversal:: How to use PROCINFO["sorted_in"].
- * Array Sorting Functions::     How to use `asort()' and `asorti()'.
- 
- 
- File: gawk.info,  Node: Controlling Array Traversal,  Next: Array Sorting 
Functions,  Up: Array Sorting
- 
- 12.2.1 Controlling Array Traversal
- ----------------------------------
- 
- By default, the order in which a `for (i in array)' loop scans an array
- is not defined; it is generally based upon the internal implementation
- of arrays inside `awk'.
- 
-    Often, though, it is desirable to be able to loop over the elements
- in a particular order that you, the programmer, choose.  `gawk' lets
- you do this.
- 
-    *note Controlling Scanning::, describes how you can assign special,
- pre-defined values to `PROCINFO["sorted_in"]' in order to control the
- order in which `gawk' will traverse an array during a `for' loop.
- 
-    In addition, the value of `PROCINFO["sorted_in"]' can be a function
- name.  This lets you traverse an array based on any custom criterion.
- The array elements are ordered according to the return value of this
- function.  The comparison function should be defined with at least four
- arguments:
- 
-      function comp_func(i1, v1, i2, v2)
-      {
-          COMPARE ELEMENTS 1 AND 2 IN SOME FASHION
-          RETURN < 0; 0; OR > 0
-      }
- 
-    Here, I1 and I2 are the indices, and V1 and V2 are the corresponding
- values of the two elements being compared.  Either V1 or V2, or both,
- can be arrays if the array being traversed contains subarrays as values.
- (*Note Arrays of Arrays::, for more information about subarrays.)  The
- three possible return values are interpreted as follows:
- 
- `comp_func(i1, v1, i2, v2) < 0'
-      Index I1 comes before index I2 during loop traversal.
- 
- `comp_func(i1, v1, i2, v2) == 0'
-      Indices I1 and I2 come together but the relative order with
-      respect to each other is undefined.
- 
- `comp_func(i1, v1, i2, v2) > 0'
-      Index I1 comes after index I2 during loop traversal.
- 
-    Our first comparison function can be used to scan an array in
- numerical order of the indices:
- 
-      function cmp_num_idx(i1, v1, i2, v2)
-      {
-           # numerical index comparison, ascending order
-           return (i1 - i2)
-      }
- 
-    Our second function traverses an array based on the string order of
- the element values rather than by indices:
- 
-      function cmp_str_val(i1, v1, i2, v2)
-      {
-          # string value comparison, ascending order
-          v1 = v1 ""
-          v2 = v2 ""
-          if (v1 < v2)
-              return -1
-          return (v1 != v2)
-      }
- 
-    The third comparison function makes all numbers, and numeric strings
- without any leading or trailing spaces, come out first during loop
- traversal:
- 
-      function cmp_num_str_val(i1, v1, i2, v2,   n1, n2)
-      {
-           # numbers before string value comparison, ascending order
-           n1 = v1 + 0
-           n2 = v2 + 0
-           if (n1 == v1)
-               return (n2 == v2) ? (n1 - n2) : -1
-           else if (n2 == v2)
-               return 1
-           return (v1 < v2) ? -1 : (v1 != v2)
-      }
- 
-    Here is a main program to demonstrate how `gawk' behaves using each
- of the previous functions:
- 
-      BEGIN {
-          data["one"] = 10
-          data["two"] = 20
-          data[10] = "one"
-          data[100] = 100
-          data[20] = "two"
- 
-          f[1] = "cmp_num_idx"
-          f[2] = "cmp_str_val"
-          f[3] = "cmp_num_str_val"
-          for (i = 1; i <= 3; i++) {
-              printf("Sort function: %s\n", f[i])
-              PROCINFO["sorted_in"] = f[i]
-              for (j in data)
-                  printf("\tdata[%s] = %s\n", j, data[j])
-              print ""
-          }
-      }
- 
-    Here are the results when the program is run:
- 
-      $ gawk -f compdemo.awk
-      -| Sort function: cmp_num_idx      Sort by numeric index
-      -|     data[two] = 20
-      -|     data[one] = 10              Both strings are numerically zero
-      -|     data[10] = one
-      -|     data[20] = two
-      -|     data[100] = 100
-      -|
-      -| Sort function: cmp_str_val      Sort by element values as strings
-      -|     data[one] = 10
-      -|     data[100] = 100             String 100 is less than string 20
-      -|     data[two] = 20
-      -|     data[10] = one
-      -|     data[20] = two
-      -|
-      -| Sort function: cmp_num_str_val  Sort all numeric values before all 
strings
-      -|     data[one] = 10
-      -|     data[two] = 20
-      -|     data[100] = 100
-      -|     data[10] = one
-      -|     data[20] = two
- 
-    Consider sorting the entries of a GNU/Linux system password file
- according to login name.  The following program sorts records by a
- specific field position and can be used for this purpose:
- 
-      # sort.awk --- simple program to sort by field position
-      # field position is specified by the global variable POS
- 
-      function cmp_field(i1, v1, i2, v2)
-      {
-          # comparison by value, as string, and ascending order
-          return v1[POS] < v2[POS] ? -1 : (v1[POS] != v2[POS])
-      }
- 
-      {
-          for (i = 1; i <= NF; i++)
-              a[NR][i] = $i
-      }
- 
-      END {
-          PROCINFO["sorted_in"] = "cmp_field"
-          if (POS < 1 || POS > NF)
-              POS = 1
-          for (i in a) {
-              for (j = 1; j <= NF; j++)
-                  printf("%s%c", a[i][j], j < NF ? ":" : "")
-              print ""
-          }
-      }
- 
-    The first field in each entry of the password file is the user's
- login name, and the fields are separated by colons.  Each record
- defines a subarray, with each field as an element in the subarray.
- Running the program produces the following output:
- 
-      $ gawk -vPOS=1 -F: -f sort.awk /etc/passwd
-      -| adm:x:3:4:adm:/var/adm:/sbin/nologin
-      -| apache:x:48:48:Apache:/var/www:/sbin/nologin
-      -| avahi:x:70:70:Avahi daemon:/:/sbin/nologin
-      ...
- 
-    The comparison should normally always return the same value when
- given a specific pair of array elements as its arguments.  If
- inconsistent results are returned then the order is undefined.  This
- behavior can be exploited to introduce random order into otherwise
- seemingly ordered data:
- 
-      function cmp_randomize(i1, v1, i2, v2)
-      {
-          # random order
-          return (2 - 4 * rand())
-      }
- 
-    As mentioned above, the order of the indices is arbitrary if two
- elements compare equal.  This is usually not a problem, but letting the
- tied elements come out in arbitrary order can be an issue, especially
- when comparing item values.  The partial ordering of the equal elements
- may change during the next loop traversal, if other elements are added
- or removed from the array.  One way to resolve ties when comparing
- elements with otherwise equal values is to include the indices in the
- comparison rules.  Note that doing this may make the loop traversal
- less efficient, so consider it only if necessary.  The following
- comparison functions force a deterministic order, and are based on the
- fact that the indices of two elements are never equal:
- 
-      function cmp_numeric(i1, v1, i2, v2)
-      {
-          # numerical value (and index) comparison, descending order
-          return (v1 != v2) ? (v2 - v1) : (i2 - i1)
-      }
- 
-      function cmp_string(i1, v1, i2, v2)
-      {
-          # string value (and index) comparison, descending order
-          v1 = v1 i1
-          v2 = v2 i2
-          return (v1 > v2) ? -1 : (v1 != v2)
-      }
- 
-    A custom comparison function can often simplify ordered loop
- traversal, and the sky is really the limit when it comes to designing
- such a function.
- 
-    When string comparisons are made during a sort, either for element
- values where one or both aren't numbers, or for element indices handled
- as strings, the value of `IGNORECASE' (*note Built-in Variables::)
- controls whether the comparisons treat corresponding uppercase and
- lowercase letters as equivalent or distinct.
- 
-    Another point to keep in mind is that in the case of subarrays the
- element values can themselves be arrays; a production comparison
- function should use the `isarray()' function (*note Type Functions::),
- to check for this, and choose a defined sorting order for subarrays.
- 
-    All sorting based on `PROCINFO["sorted_in"]' is disabled in POSIX
- mode, since the `PROCINFO' array is not special in that case.
- 
-    As a side note, sorting the array indices before traversing the
- array has been reported to add 15% to 20% overhead to the execution
- time of `awk' programs. For this reason, sorted array traversal is not
- the default.
- 
- 
- File: gawk.info,  Node: Array Sorting Functions,  Prev: Controlling Array 
Traversal,  Up: Array Sorting
- 
- 12.2.2 Sorting Array Values and Indices with `gawk'
- ---------------------------------------------------
- 
- In most `awk' implementations, sorting an array requires writing a
- `sort()' function.  While this can be educational for exploring
- different sorting algorithms, usually that's not the point of the
- program.  `gawk' provides the built-in `asort()' and `asorti()'
- functions (*note String Functions::) for sorting arrays.  For example:
- 
-      POPULATE THE ARRAY data
-      n = asort(data)
-      for (i = 1; i <= n; i++)
-          DO SOMETHING WITH data[i]
- 
-    After the call to `asort()', the array `data' is indexed from 1 to
- some number N, the total number of elements in `data'.  (This count is
- `asort()''s return value.)  `data[1]' <= `data[2]' <= `data[3]', and so
- on.  The comparison is based on the type of the elements (*note Typing
- and Comparison::).  All numeric values come before all string values,
- which in turn come before all subarrays.
- 
-    An important side effect of calling `asort()' is that _the array's
- original indices are irrevocably lost_.  As this isn't always
- desirable, `asort()' accepts a second argument:
- 
-      POPULATE THE ARRAY source
-      n = asort(source, dest)
-      for (i = 1; i <= n; i++)
-          DO SOMETHING WITH dest[i]
- 
-    In this case, `gawk' copies the `source' array into the `dest' array
- and then sorts `dest', destroying its indices.  However, the `source'
- array is not affected.
- 
-    `asort()' accepts a third string argument to control comparison of
- array elements.  As with `PROCINFO["sorted_in"]', this argument may be
- one of the predefined names that `gawk' provides (*note Controlling
- Scanning::), or the name of a user-defined function (*note Controlling
- Array Traversal::).
- 
-      NOTE: In all cases, the sorted element values consist of the
-      original array's element values.  The ability to control
-      comparison merely affects the way in which they are sorted.
- 
-    Often, what's needed is to sort on the values of the _indices_
- instead of the values of the elements.  To do that, use the `asorti()'
- function.  The interface is identical to that of `asort()', except that
- the index values are used for sorting, and become the values of the
- result array:
- 
-      { source[$0] = some_func($0) }
- 
-      END {
-          n = asorti(source, dest)
-          for (i = 1; i <= n; i++) {
-              Work with sorted indices directly:
-              DO SOMETHING WITH dest[i]
-              ...
-              Access original array via sorted indices:
-              DO SOMETHING WITH source[dest[i]]
-          }
-      }
- 
-    Similar to `asort()', in all cases, the sorted element values
- consist of the original array's indices.  The ability to control
- comparison merely affects the way in which they are sorted.
- 
-    Sorting the array by replacing the indices provides maximal
- flexibility.  To traverse the elements in decreasing order, use a loop
- that goes from N down to 1, either over the elements or over the
- indices.(1)
- 
-    Copying array indices and elements isn't expensive in terms of
- memory.  Internally, `gawk' maintains "reference counts" to data.  For
- example, when `asort()' copies the first array to the second one, there
- is only one copy of the original array elements' data, even though both
- arrays use the values.
- 
-    Because `IGNORECASE' affects string comparisons, the value of
- `IGNORECASE' also affects sorting for both `asort()' and `asorti()'.
- Note also that the locale's sorting order does _not_ come into play;
- comparisons are based on character values only.(2) Caveat Emptor.
- 
-    ---------- Footnotes ----------
- 
-    (1) You may also use one of the predefined sorting names that sorts
- in decreasing order.
- 
-    (2) This is true because locale-based comparison occurs only when in
- POSIX compatibility mode, and since `asort()' and `asorti()' are `gawk'
- extensions, they are not available in that case.
- 
- 
- File: gawk.info,  Node: Two-way I/O,  Next: TCP/IP Networking,  Prev: Array 
Sorting,  Up: Advanced Features
- 
- 12.3 Two-Way Communications with Another Process
- ================================================
- 
-      From: address@hidden (Mike Brennan)
-      Newsgroups: comp.lang.awk
-      Subject: Re: Learn the SECRET to Attract Women Easily
-      Date: 4 Aug 1997 17:34:46 GMT
-      Message-ID: <address@hidden>
- 
-      On 3 Aug 1997 13:17:43 GMT, Want More Dates???
-      <address@hidden> wrote:
-      >Learn the SECRET to Attract Women Easily
-      >
-      >The SCENT(tm)  Pheromone Sex Attractant For Men to Attract Women
- 
-      The scent of awk programmers is a lot more attractive to women than
-      the scent of perl programmers.
-      --
-      Mike Brennan
- 
-    It is often useful to be able to send data to a separate program for
- processing and then read the result.  This can always be done with
- temporary files:
- 
-      # Write the data for processing
-      tempfile = ("mydata." PROCINFO["pid"])
-      while (NOT DONE WITH DATA)
-          print DATA | ("subprogram > " tempfile)
-      close("subprogram > " tempfile)
- 
-      # Read the results, remove tempfile when done
-      while ((getline newdata < tempfile) > 0)
-          PROCESS newdata APPROPRIATELY
-      close(tempfile)
-      system("rm " tempfile)
- 
- This works, but not elegantly.  Among other things, it requires that
- the program be run in a directory that cannot be shared among users;
- for example, `/tmp' will not do, as another user might happen to be
- using a temporary file with the same name.
- 
-    However, with `gawk', it is possible to open a _two-way_ pipe to
- another process.  The second process is termed a "coprocess", since it
- runs in parallel with `gawk'.  The two-way connection is created using
- the `|&' operator (borrowed from the Korn shell, `ksh'):(1)
- 
-      do {
-          print DATA |& "subprogram"
-          "subprogram" |& getline results
-      } while (DATA LEFT TO PROCESS)
-      close("subprogram")
- 
-    The first time an I/O operation is executed using the `|&' operator,
- `gawk' creates a two-way pipeline to a child process that runs the
- other program.  Output created with `print' or `printf' is written to
- the program's standard input, and output from the program's standard
- output can be read by the `gawk' program using `getline'.  As is the
- case with processes started by `|', the subprogram can be any program,
- or pipeline of programs, that can be started by the shell.
- 
-    There are some cautionary items to be aware of:
- 
-    * As the code inside `gawk' currently stands, the coprocess's
-      standard error goes to the same place that the parent `gawk''s
-      standard error goes. It is not possible to read the child's
-      standard error separately.
- 
-    * I/O buffering may be a problem.  `gawk' automatically flushes all
-      output down the pipe to the coprocess.  However, if the coprocess
-      does not flush its output, `gawk' may hang when doing a `getline'
-      in order to read the coprocess's results.  This could lead to a
-      situation known as "deadlock", where each process is waiting for
-      the other one to do something.
- 
-    It is possible to close just one end of the two-way pipe to a
- coprocess, by supplying a second argument to the `close()' function of
- either `"to"' or `"from"' (*note Close Files And Pipes::).  These
- strings tell `gawk' to close the end of the pipe that sends data to the
- coprocess or the end that reads from it, respectively.
- 
-    This is particularly necessary in order to use the system `sort'
- utility as part of a coprocess; `sort' must read _all_ of its input
- data before it can produce any output.  The `sort' program does not
- receive an end-of-file indication until `gawk' closes the write end of
- the pipe.
- 
-    When you have finished writing data to the `sort' utility, you can
- close the `"to"' end of the pipe, and then start reading sorted data
- via `getline'.  For example:
- 
-      BEGIN {
-          command = "LC_ALL=C sort"
-          n = split("abcdefghijklmnopqrstuvwxyz", a, "")
- 
-          for (i = n; i > 0; i--)
-              print a[i] |& command
-          close(command, "to")
- 
-          while ((command |& getline line) > 0)
-              print "got", line
-          close(command)
-      }
- 
-    This program writes the letters of the alphabet in reverse order, one
- per line, down the two-way pipe to `sort'.  It then closes the write
- end of the pipe, so that `sort' receives an end-of-file indication.
- This causes `sort' to sort the data and write the sorted data back to
- the `gawk' program.  Once all of the data has been read, `gawk'
- terminates the coprocess and exits.
- 
-    As a side note, the assignment `LC_ALL=C' in the `sort' command
- ensures traditional Unix (ASCII) sorting from `sort'.
- 
-    You may also use pseudo-ttys (ptys) for two-way communication
- instead of pipes, if your system supports them.  This is done on a
- per-command basis, by setting a special element in the `PROCINFO' array
- (*note Auto-set::), like so:
- 
-      command = "sort -nr"           # command, save in convenience variable
-      PROCINFO[command, "pty"] = 1   # update PROCINFO
-      print ... |& command       # start two-way pipe
-      ...
- 
- Using ptys avoids the buffer deadlock issues described earlier, at some
- loss in performance.  If your system does not have ptys, or if all the
- system's ptys are in use, `gawk' automatically falls back to using
- regular pipes.
- 
-    ---------- Footnotes ----------
- 
-    (1) This is very different from the same operator in the C shell.
- 
- 
- File: gawk.info,  Node: TCP/IP Networking,  Next: Profiling,  Prev: Two-way 
I/O,  Up: Advanced Features
- 
- 12.4 Using `gawk' for Network Programming
- =========================================
- 
-      `EMISTERED':
-      A host is a host from coast to coast,
-      and no-one can talk to host that's close,
-      unless the host that isn't close
-      is busy hung or dead.
- 
-    In addition to being able to open a two-way pipeline to a coprocess
- on the same system (*note Two-way I/O::), it is possible to make a
- two-way connection to another process on another system across an IP
- network connection.
- 
-    You can think of this as just a _very long_ two-way pipeline to a
- coprocess.  The way `gawk' decides that you want to use TCP/IP
- networking is by recognizing special file names that begin with one of
- `/inet/', `/inet4/' or `/inet6'.
- 
-    The full syntax of the special file name is
- `/NET-TYPE/PROTOCOL/LOCAL-PORT/REMOTE-HOST/REMOTE-PORT'.  The
- components are:
- 
- NET-TYPE
-      Specifies the kind of Internet connection to make.  Use `/inet4/'
-      to force IPv4, and `/inet6/' to force IPv6.  Plain `/inet/' (which
-      used to be the only option) uses the system default, most likely
-      IPv4.
- 
- PROTOCOL
-      The protocol to use over IP.  This must be either `tcp', or `udp',
-      for a TCP or UDP IP connection, respectively.  The use of TCP is
-      recommended for most applications.
- 
- LOCAL-PORT
-      The local TCP or UDP port number to use.  Use a port number of `0'
-      when you want the system to pick a port. This is what you should do
-      when writing a TCP or UDP client.  You may also use a well-known
-      service name, such as `smtp' or `http', in which case `gawk'
-      attempts to determine the predefined port number using the C
-      `getaddrinfo()' function.
- 
- REMOTE-HOST
-      The IP address or fully-qualified domain name of the Internet host
-      to which you want to connect.
- 
- REMOTE-PORT
-      The TCP or UDP port number to use on the given REMOTE-HOST.
-      Again, use `0' if you don't care, or else a well-known service
-      name.
- 
-      NOTE: Failure in opening a two-way socket will result in a
-      non-fatal error being returned to the calling code. The value of
-      `ERRNO' indicates the error (*note Auto-set::).
- 
-    Consider the following very simple example:
- 
-      BEGIN {
-        Service = "/inet/tcp/0/localhost/daytime"
-        Service |& getline
-        print $0
-        close(Service)
-      }
- 
-    This program reads the current date and time from the local system's
- TCP `daytime' server.  It then prints the results and closes the
- connection.
- 
-    Because this topic is extensive, the use of `gawk' for TCP/IP
- programming is documented separately.  See *note (General
- Introduction)Top:: gawkinet, TCP/IP Internetworking with `gawk', for a
- much more complete introduction and discussion, as well as extensive
- examples.
- 
- 
- File: gawk.info,  Node: Profiling,  Prev: TCP/IP Networking,  Up: Advanced 
Features
- 
- 12.5 Profiling Your `awk' Programs
- ==================================
- 
- You may produce execution traces of your `awk' programs.  This is done
- by passing the option `--profile' to `gawk'.  When `gawk' has finished
- running, it creates a profile of your program in a file named
- `awkprof.out'. Because it is profiling, it also executes up to 45%
- slower than `gawk' normally does.
- 
-    As shown in the following example, the `--profile' option can be
- used to change the name of the file where `gawk' will write the profile:
- 
-      gawk --profile=myprog.prof -f myprog.awk data1 data2
- 
- In the above example, `gawk' places the profile in `myprog.prof'
- instead of in `awkprof.out'.
- 
-    Here is a sample session showing a simple `awk' program, its input
- data, and the results from running `gawk' with the `--profile' option.
- First, the `awk' program:
- 
-      BEGIN { print "First BEGIN rule" }
- 
-      END { print "First END rule" }
- 
-      /foo/ {
-          print "matched /foo/, gosh"
-          for (i = 1; i <= 3; i++)
-              sing()
-      }
- 
-      {
-          if (/foo/)
-              print "if is true"
-          else
-              print "else is true"
-      }
- 
-      BEGIN { print "Second BEGIN rule" }
- 
-      END { print "Second END rule" }
- 
-      function sing(    dummy)
-      {
-          print "I gotta be me!"
-      }
- 
-    Following is the input data:
- 
-      foo
-      bar
-      baz
-      foo
-      junk
- 
-    Here is the `awkprof.out' that results from running the `gawk'
- profiler on this program and data (this example also illustrates that
- `awk' programmers sometimes have to work late):
- 
-              # gawk profile, created Sun Aug 13 00:00:15 2000
- 
-              # BEGIN block(s)
- 
-              BEGIN {
-           1          print "First BEGIN rule"
-           1          print "Second BEGIN rule"
-              }
- 
-              # Rule(s)
- 
-           5  /foo/   { # 2
-           2          print "matched /foo/, gosh"
-           6          for (i = 1; i <= 3; i++) {
-           6                  sing()
-                      }
-              }
- 
-           5  {
-           5          if (/foo/) { # 2
-           2                  print "if is true"
-           3          } else {
-           3                  print "else is true"
-                      }
-              }
- 
-              # END block(s)
- 
-              END {
-           1          print "First END rule"
-           1          print "Second END rule"
-              }
- 
-              # Functions, listed alphabetically
- 
-           6  function sing(dummy)
-              {
-           6          print "I gotta be me!"
-              }
- 
-    This example illustrates many of the basic features of profiling
- output.  They are as follows:
- 
-    * The program is printed in the order `BEGIN' rule, `BEGINFILE' rule,
-      pattern/action rules, `ENDFILE' rule, `END' rule and functions,
-      listed alphabetically.  Multiple `BEGIN' and `END' rules are
-      merged together, as are multiple `BEGINFILE' and `ENDFILE' rules.
- 
-    * Pattern-action rules have two counts.  The first count, to the
-      left of the rule, shows how many times the rule's pattern was
-      _tested_.  The second count, to the right of the rule's opening
-      left brace in a comment, shows how many times the rule's action
-      was _executed_.  The difference between the two indicates how many
-      times the rule's pattern evaluated to false.
- 
-    * Similarly, the count for an `if'-`else' statement shows how many
-      times the condition was tested.  To the right of the opening left
-      brace for the `if''s body is a count showing how many times the
-      condition was true.  The count for the `else' indicates how many
-      times the test failed.
- 
-    * The count for a loop header (such as `for' or `while') shows how
-      many times the loop test was executed.  (Because of this, you
-      can't just look at the count on the first statement in a rule to
-      determine how many times the rule was executed.  If the first
-      statement is a loop, the count is misleading.)
- 
-    * For user-defined functions, the count next to the `function'
-      keyword indicates how many times the function was called.  The
-      counts next to the statements in the body show how many times
-      those statements were executed.
- 
-    * The layout uses "K&R" style with TABs.  Braces are used
-      everywhere, even when the body of an `if', `else', or loop is only
-      a single statement.
- 
-    * Parentheses are used only where needed, as indicated by the
-      structure of the program and the precedence rules.  For example,
-      `(3 + 5) * 4' means add three plus five, then multiply the total
-      by four.  However, `3 + 5 * 4' has no parentheses, and means `3 +
-      (5 * 4)'.
- 
-    * Parentheses are used around the arguments to `print' and `printf'
-      only when the `print' or `printf' statement is followed by a
-      redirection.  Similarly, if the target of a redirection isn't a
-      scalar, it gets parenthesized.
- 
-    * `gawk' supplies leading comments in front of the `BEGIN' and `END'
-      rules, the pattern/action rules, and the functions.
- 
- 
-    The profiled version of your program may not look exactly like what
- you typed when you wrote it.  This is because `gawk' creates the
- profiled version by "pretty printing" its internal representation of
- the program.  The advantage to this is that `gawk' can produce a
- standard representation.  The disadvantage is that all source-code
- comments are lost, as are the distinctions among multiple `BEGIN',
- `END', `BEGINFILE', and `ENDFILE' rules.  Also, things such as:
- 
-      /foo/
- 
- come out as:
- 
-      /foo/   {
-          print $0
-      }
- 
- which is correct, but possibly surprising.
- 
-    Besides creating profiles when a program has completed, `gawk' can
- produce a profile while it is running.  This is useful if your `awk'
- program goes into an infinite loop and you want to see what has been
- executed.  To use this feature, run `gawk' with the `--profile' option
- in the background:
- 
-      $ gawk --profile -f myprog &
-      [1] 13992
- 
- The shell prints a job number and process ID number; in this case,
- 13992.  Use the `kill' command to send the `USR1' signal to `gawk':
- 
-      $ kill -USR1 13992
- 
- As usual, the profiled version of the program is written to
- `awkprof.out', or to a different file if one specified with the
- `--profile' option.
- 
-    Along with the regular profile, as shown earlier, the profile
- includes a trace of any active functions:
- 
-      # Function Call Stack:
- 
-      #   3. baz
-      #   2. bar
-      #   1. foo
-      # -- main --
- 
-    You may send `gawk' the `USR1' signal as many times as you like.
- Each time, the profile and function call trace are appended to the
- output profile file.
- 
-    If you use the `HUP' signal instead of the `USR1' signal, `gawk'
- produces the profile and the function call trace and then exits.
- 
-    When `gawk' runs on MS-Windows systems, it uses the `INT' and `QUIT'
- signals for producing the profile and, in the case of the `INT' signal,
- `gawk' exits.  This is because these systems don't support the `kill'
- command, so the only signals you can deliver to a program are those
- generated by the keyboard.  The `INT' signal is generated by the
- `Ctrl-<C>' or `Ctrl-<BREAK>' key, while the `QUIT' signal is generated
- by the `Ctrl-<\>' key.
- 
-    Finally, `gawk' also accepts another option `--pretty-print'.  When
- called this way, `gawk' "pretty prints" the program into `awkprof.out',
- without any execution counts.
- 
- 
- File: gawk.info,  Node: Library Functions,  Next: Sample Programs,  Prev: 
Advanced Features,  Up: Top
- 
- 13 A Library of `awk' Functions
- *******************************
- 
- *note User-defined::, describes how to write your own `awk' functions.
- Writing functions is important, because it allows you to encapsulate
- algorithms and program tasks in a single place.  It simplifies
- programming, making program development more manageable, and making
- programs more readable.
- 
-    One valuable way to learn a new programming language is to _read_
- programs in that language.  To that end, this major node and *note
- Sample Programs::, provide a good-sized body of code for you to read,
- and hopefully, to learn from.
- 
-    This major node presents a library of useful `awk' functions.  Many
- of the sample programs presented later in this Info file use these
- functions.  The functions are presented here in a progression from
- simple to complex.
- 
-    *note Extract Program::, presents a program that you can use to
- extract the source code for these example library functions and
- programs from the Texinfo source for this Info file.  (This has already
- been done as part of the `gawk' distribution.)
- 
-    If you have written one or more useful, general-purpose `awk'
- functions and would like to contribute them to the `awk' user
- community, see *note How To Contribute::, for more information.
- 
-    The programs in this major node and in *note Sample Programs::,
- freely use features that are `gawk'-specific.  Rewriting these programs
- for different implementations of `awk' is pretty straightforward.
- 
-    * Diagnostic error messages are sent to `/dev/stderr'.  Use `| "cat
-      1>&2"' instead of `> "/dev/stderr"' if your system does not have a
-      `/dev/stderr', or if you cannot use `gawk'.
- 
-    * A number of programs use `nextfile' (*note Nextfile Statement::)
-      to skip any remaining input in the input file.
- 
-    * Finally, some of the programs choose to ignore upper- and lowercase
-      distinctions in their input. They do so by assigning one to
-      `IGNORECASE'.  You can achieve almost the same effect(1) by adding
-      the following rule to the beginning of the program:
- 
-           # ignore case
-           { $0 = tolower($0) }
- 
-      Also, verify that all regexp and string constants used in
-      comparisons use only lowercase letters.
- 
- * Menu:
- 
- * Library Names::               How to best name private global variables in
-                                 library functions.
- * General Functions::           Functions that are of general use.
- * Data File Management::        Functions for managing command-line data
-                                 files.
- * Getopt Function::             A function for processing command-line
-                                 arguments.
- * Passwd Functions::            Functions for getting user information.
- * Group Functions::             Functions for getting group information.
- * Walking Arrays::              A function to walk arrays of arrays.
- 
-    ---------- Footnotes ----------
- 
-    (1) The effects are not identical.  Output of the transformed record
- will be in all lowercase, while `IGNORECASE' preserves the original
- contents of the input record.
- 
- 
- File: gawk.info,  Node: Library Names,  Next: General Functions,  Up: Library 
Functions
- 
- 13.1 Naming Library Function Global Variables
- =============================================
- 
- Due to the way the `awk' language evolved, variables are either
- "global" (usable by the entire program) or "local" (usable just by a
- specific function).  There is no intermediate state analogous to
- `static' variables in C.
- 
-    Library functions often need to have global variables that they can
- use to preserve state information between calls to the function--for
- example, `getopt()''s variable `_opti' (*note Getopt Function::).  Such
- variables are called "private", since the only functions that need to
- use them are the ones in the library.
- 
-    When writing a library function, you should try to choose names for
- your private variables that will not conflict with any variables used by
- either another library function or a user's main program.  For example,
- a name like `i' or `j' is not a good choice, because user programs
- often use variable names like these for their own purposes.
- 
-    The example programs shown in this major node all start the names of
- their private variables with an underscore (`_').  Users generally
- don't use leading underscores in their variable names, so this
- convention immediately decreases the chances that the variable name
- will be accidentally shared with the user's program.
- 
-    In addition, several of the library functions use a prefix that helps
- indicate what function or set of functions use the variables--for
- example, `_pw_byname' in the user database routines (*note Passwd
- Functions::).  This convention is recommended, since it even further
- decreases the chance of inadvertent conflict among variable names.
- Note that this convention is used equally well for variable names and
- for private function names.(1)
- 
-    As a final note on variable naming, if a function makes global
- variables available for use by a main program, it is a good convention
- to start that variable's name with a capital letter--for example,
- `getopt()''s `Opterr' and `Optind' variables (*note Getopt Function::).
- The leading capital letter indicates that it is global, while the fact
- that the variable name is not all capital letters indicates that the
- variable is not one of `awk''s built-in variables, such as `FS'.
- 
-    It is also important that _all_ variables in library functions that
- do not need to save state are, in fact, declared local.(2) If this is
- not done, the variable could accidentally be used in the user's
- program, leading to bugs that are very difficult to track down:
- 
-      function lib_func(x, y,    l1, l2)
-      {
-          ...
-          USE VARIABLE some_var   # some_var should be local
-          ...                     # but is not by oversight
-      }
- 
-    A different convention, common in the Tcl community, is to use a
- single associative array to hold the values needed by the library
- function(s), or "package."  This significantly decreases the number of
- actual global names in use.  For example, the functions described in
- *note Passwd Functions::, might have used array elements
- `PW_data["inited"]', `PW_data["total"]', `PW_data["count"]', and
- `PW_data["awklib"]', instead of `_pw_inited', `_pw_awklib', `_pw_total',
- and `_pw_count'.
- 
-    The conventions presented in this minor node are exactly that:
- conventions. You are not required to write your programs this way--we
- merely recommend that you do so.
- 
-    ---------- Footnotes ----------
- 
-    (1) While all the library routines could have been rewritten to use
- this convention, this was not done, in order to show how our own `awk'
- programming style has evolved and to provide some basis for this
- discussion.
- 
-    (2) `gawk''s `--dump-variables' command-line option is useful for
- verifying this.
- 
- 
- File: gawk.info,  Node: General Functions,  Next: Data File Management,  
Prev: Library Names,  Up: Library Functions
- 
- 13.2 General Programming
- ========================
- 
- This minor node presents a number of functions that are of general
- programming use.
- 
- * Menu:
- 
- * Strtonum Function::           A replacement for the built-in
-                                 `strtonum()' function.
- * Assert Function::             A function for assertions in `awk'
-                                 programs.
- * Round Function::              A function for rounding if `sprintf()'
-                                 does not do it correctly.
- * Cliff Random Function::       The Cliff Random Number Generator.
- * Ordinal Functions::           Functions for using characters as numbers and
-                                 vice versa.
- * Join Function::               A function to join an array into a string.
- * Gettimeofday Function::       A function to get formatted times.
- 
- 
- File: gawk.info,  Node: Strtonum Function,  Next: Assert Function,  Up: 
General Functions
- 
- 13.2.1 Converting Strings To Numbers
- ------------------------------------
- 
- The `strtonum()' function (*note String Functions::) is a `gawk'
- extension.  The following function provides an implementation for other
- versions of `awk':
- 
-      # mystrtonum --- convert string to number
- 
-      function mystrtonum(str,        ret, chars, n, i, k, c)
-      {
-          if (str ~ /^0[0-7]*$/) {
-              # octal
-              n = length(str)
-              ret = 0
-              for (i = 1; i <= n; i++) {
-                  c = substr(str, i, 1)
-                  if ((k = index("01234567", c)) > 0)
-                      k-- # adjust for 1-basing in awk
- 
-                  ret = ret * 8 + k
-              }
-          } else if (str ~ /^0[xX][[:xdigit:]]+/) {
-              # hexadecimal
-              str = substr(str, 3)    # lop off leading 0x
-              n = length(str)
-              ret = 0
-              for (i = 1; i <= n; i++) {
-                  c = substr(str, i, 1)
-                  c = tolower(c)
-                  if ((k = index("0123456789", c)) > 0)
-                      k-- # adjust for 1-basing in awk
-                  else if ((k = index("abcdef", c)) > 0)
-                      k += 9
- 
-                  ret = ret * 16 + k
-              }
-          } else if (str ~ \
-        
/^[-+]?([0-9]+([.][0-9]*([Ee][0-9]+)?)?|([.][0-9]+([Ee][-+]?[0-9]+)?))$/) {
-              # decimal number, possibly floating point
-              ret = str + 0
-          } else
-              ret = "NOT-A-NUMBER"
- 
-          return ret
-      }
- 
-      # BEGIN {     # gawk test harness
-      #     a[1] = "25"
-      #     a[2] = ".31"
-      #     a[3] = "0123"
-      #     a[4] = "0xdeadBEEF"
-      #     a[5] = "123.45"
-      #     a[6] = "1.e3"
-      #     a[7] = "1.32"
-      #     a[7] = "1.32E2"
-      #
-      #     for (i = 1; i in a; i++)
-      #         print a[i], strtonum(a[i]), mystrtonum(a[i])
-      # }
- 
-    The function first looks for C-style octal numbers (base 8).  If the
- input string matches a regular expression describing octal numbers,
- then `mystrtonum()' loops through each character in the string.  It
- sets `k' to the index in `"01234567"' of the current octal digit.
- Since the return value is one-based, the `k--' adjusts `k' so it can be
- used in computing the return value.
- 
-    Similar logic applies to the code that checks for and converts a
- hexadecimal value, which starts with `0x' or `0X'.  The use of
- `tolower()' simplifies the computation for finding the correct numeric
- value for each hexadecimal digit.
- 
-    Finally, if the string matches the (rather complicated) regexp for a
- regular decimal integer or floating-point number, the computation `ret
- = str + 0' lets `awk' convert the value to a number.
- 
-    A commented-out test program is included, so that the function can
- be tested with `gawk' and the results compared to the built-in
- `strtonum()' function.
- 
- 
- File: gawk.info,  Node: Assert Function,  Next: Round Function,  Prev: 
Strtonum Function,  Up: General Functions
- 
- 13.2.2 Assertions
- -----------------
- 
- When writing large programs, it is often useful to know that a
- condition or set of conditions is true.  Before proceeding with a
- particular computation, you make a statement about what you believe to
- be the case.  Such a statement is known as an "assertion".  The C
- language provides an `<assert.h>' header file and corresponding
- `assert()' macro that the programmer can use to make assertions.  If an
- assertion fails, the `assert()' macro arranges to print a diagnostic
- message describing the condition that should have been true but was
- not, and then it kills the program.  In C, using `assert()' looks this:
- 
-      #include <assert.h>
- 
-      int myfunc(int a, double b)
-      {
-           assert(a <= 5 && b >= 17.1);
-           ...
-      }
- 
-    If the assertion fails, the program prints a message similar to this:
- 
-      prog.c:5: assertion failed: a <= 5 && b >= 17.1
- 
-    The C language makes it possible to turn the condition into a string
- for use in printing the diagnostic message.  This is not possible in
- `awk', so this `assert()' function also requires a string version of
- the condition that is being tested.  Following is the function:
- 
-      # assert --- assert that a condition is true. Otherwise exit.
- 
-      function assert(condition, string)
-      {
-          if (! condition) {
-              printf("%s:%d: assertion failed: %s\n",
-                  FILENAME, FNR, string) > "/dev/stderr"
-              _assert_exit = 1
-              exit 1
-          }
-      }
- 
-      END {
-          if (_assert_exit)
-              exit 1
-      }
- 
-    The `assert()' function tests the `condition' parameter. If it is
- false, it prints a message to standard error, using the `string'
- parameter to describe the failed condition.  It then sets the variable
- `_assert_exit' to one and executes the `exit' statement.  The `exit'
- statement jumps to the `END' rule. If the `END' rules finds
- `_assert_exit' to be true, it exits immediately.
- 
-    The purpose of the test in the `END' rule is to keep any other `END'
- rules from running.  When an assertion fails, the program should exit
- immediately.  If no assertions fail, then `_assert_exit' is still false
- when the `END' rule is run normally, and the rest of the program's
- `END' rules execute.  For all of this to work correctly, `assert.awk'
- must be the first source file read by `awk'.  The function can be used
- in a program in the following way:
- 
-      function myfunc(a, b)
-      {
-           assert(a <= 5 && b >= 17.1, "a <= 5 && b >= 17.1")
-           ...
-      }
- 
- If the assertion fails, you see a message similar to the following:
- 
-      mydata:1357: assertion failed: a <= 5 && b >= 17.1
- 
-    There is a small problem with this version of `assert()'.  An `END'
- rule is automatically added to the program calling `assert()'.
- Normally, if a program consists of just a `BEGIN' rule, the input files
- and/or standard input are not read. However, now that the program has
- an `END' rule, `awk' attempts to read the input data files or standard
- input (*note Using BEGIN/END::), most likely causing the program to
- hang as it waits for input.
- 
-    There is a simple workaround to this: make sure that such a `BEGIN'
- rule always ends with an `exit' statement.
- 
- 
- File: gawk.info,  Node: Round Function,  Next: Cliff Random Function,  Prev: 
Assert Function,  Up: General Functions
- 
- 13.2.3 Rounding Numbers
- -----------------------
- 
- The way `printf' and `sprintf()' (*note Printf::) perform rounding
- often depends upon the system's C `sprintf()' subroutine.  On many
- machines, `sprintf()' rounding is "unbiased," which means it doesn't
- always round a trailing `.5' up, contrary to naive expectations.  In
- unbiased rounding, `.5' rounds to even, rather than always up, so 1.5
- rounds to 2 but 4.5 rounds to 4.  This means that if you are using a
- format that does rounding (e.g., `"%.0f"'), you should check what your
- system does.  The following function does traditional rounding; it
- might be useful if your `awk''s `printf' does unbiased rounding:
- 
-      # round.awk --- do normal rounding
- 
-      function round(x,   ival, aval, fraction)
-      {
-         ival = int(x)    # integer part, int() truncates
- 
-         # see if fractional part
-         if (ival == x)   # no fraction
-            return ival   # ensure no decimals
- 
-         if (x < 0) {
-            aval = -x     # absolute value
-            ival = int(aval)
-            fraction = aval - ival
-            if (fraction >= .5)
-               return int(x) - 1   # -2.5 --> -3
-            else
-               return int(x)       # -2.3 --> -2
-         } else {
-            fraction = x - ival
-            if (fraction >= .5)
-               return ival + 1
-            else
-               return ival
-         }
-      }
- 
-      # test harness
-      { print $0, round($0) }
- 
- 
- File: gawk.info,  Node: Cliff Random Function,  Next: Ordinal Functions,  
Prev: Round Function,  Up: General Functions
- 
- 13.2.4 The Cliff Random Number Generator
- ----------------------------------------
- 
- The Cliff random number generator
- (http://mathworld.wolfram.com/CliffRandomNumberGenerator.html) is a
- very simple random number generator that "passes the noise sphere test
- for randomness by showing no structure."  It is easily programmed, in
- less than 10 lines of `awk' code:
- 
-      # cliff_rand.awk --- generate Cliff random numbers
- 
-      BEGIN { _cliff_seed = 0.1 }
- 
-      function cliff_rand()
-      {
-          _cliff_seed = (100 * log(_cliff_seed)) % 1
-          if (_cliff_seed < 0)
-              _cliff_seed = - _cliff_seed
-          return _cliff_seed
-      }
- 
-    This algorithm requires an initial "seed" of 0.1.  Each new value
- uses the current seed as input for the calculation.  If the built-in
- `rand()' function (*note Numeric Functions::) isn't random enough, you
- might try using this function instead.
- 
- 
- File: gawk.info,  Node: Ordinal Functions,  Next: Join Function,  Prev: Cliff 
Random Function,  Up: General Functions
- 
- 13.2.5 Translating Between Characters and Numbers
- -------------------------------------------------
- 
- One commercial implementation of `awk' supplies a built-in function,
- `ord()', which takes a character and returns the numeric value for that
- character in the machine's character set.  If the string passed to
- `ord()' has more than one character, only the first one is used.
- 
-    The inverse of this function is `chr()' (from the function of the
- same name in Pascal), which takes a number and returns the
- corresponding character.  Both functions are written very nicely in
- `awk'; there is no real reason to build them into the `awk' interpreter:
- 
-      # ord.awk --- do ord and chr
- 
-      # Global identifiers:
-      #    _ord_:        numerical values indexed by characters
-      #    _ord_init:    function to initialize _ord_
- 
-      BEGIN    { _ord_init() }
- 
-      function _ord_init(    low, high, i, t)
-      {
-          low = sprintf("%c", 7) # BEL is ascii 7
-          if (low == "\a") {    # regular ascii
-              low = 0
-              high = 127
-          } else if (sprintf("%c", 128 + 7) == "\a") {
-              # ascii, mark parity
-              low = 128
-              high = 255
-          } else {        # ebcdic(!)
-              low = 0
-              high = 255
-          }
- 
-          for (i = low; i <= high; i++) {
-              t = sprintf("%c", i)
-              _ord_[t] = i
-          }
-      }
- 
-    Some explanation of the numbers used by `chr' is worthwhile.  The
- most prominent character set in use today is ASCII.(1) Although an
- 8-bit byte can hold 256 distinct values (from 0 to 255), ASCII only
- defines characters that use the values from 0 to 127.(2) In the now
- distant past, at least one minicomputer manufacturer used ASCII, but
- with mark parity, meaning that the leftmost bit in the byte is always
- 1.  This means that on those systems, characters have numeric values
- from 128 to 255.  Finally, large mainframe systems use the EBCDIC
- character set, which uses all 256 values.  While there are other
- character sets in use on some older systems, they are not really worth
- worrying about:
- 
-      function ord(str,    c)
-      {
-          # only first character is of interest
-          c = substr(str, 1, 1)
-          return _ord_[c]
-      }
- 
-      function chr(c)
-      {
-          # force c to be numeric by adding 0
-          return sprintf("%c", c + 0)
-      }
- 
-      #### test code ####
-      # BEGIN    \
-      # {
-      #    for (;;) {
-      #        printf("enter a character: ")
-      #        if (getline var <= 0)
-      #            break
-      #        printf("ord(%s) = %d\n", var, ord(var))
-      #    }
-      # }
- 
-    An obvious improvement to these functions is to move the code for the
- `_ord_init' function into the body of the `BEGIN' rule.  It was written
- this way initially for ease of development.  There is a "test program"
- in a `BEGIN' rule, to test the function.  It is commented out for
- production use.
- 
-    ---------- Footnotes ----------
- 
-    (1) This is changing; many systems use Unicode, a very large
- character set that includes ASCII as a subset.  On systems with full
- Unicode support, a character can occupy up to 32 bits, making simple
- tests such as used here prohibitively expensive.
- 
-    (2) ASCII has been extended in many countries to use the values from
- 128 to 255 for country-specific characters.  If your  system uses these
- extensions, you can simplify `_ord_init' to loop from 0 to 255.
- 
- 
- File: gawk.info,  Node: Join Function,  Next: Gettimeofday Function,  Prev: 
Ordinal Functions,  Up: General Functions
- 
- 13.2.6 Merging an Array into a String
- -------------------------------------
- 
- When doing string processing, it is often useful to be able to join all
- the strings in an array into one long string.  The following function,
- `join()', accomplishes this task.  It is used later in several of the
- application programs (*note Sample Programs::).
- 
-    Good function design is important; this function needs to be general
- but it should also have a reasonable default behavior.  It is called
- with an array as well as the beginning and ending indices of the
- elements in the array to be merged.  This assumes that the array
- indices are numeric--a reasonable assumption since the array was likely
- created with `split()' (*note String Functions::):
- 
-      # join.awk --- join an array into a string
- 
-      function join(array, start, end, sep,    result, i)
-      {
-          if (sep == "")
-             sep = " "
-          else if (sep == SUBSEP) # magic value
-             sep = ""
-          result = array[start]
-          for (i = start + 1; i <= end; i++)
-              result = result sep array[i]
-          return result
-      }
- 
-    An optional additional argument is the separator to use when joining
- the strings back together.  If the caller supplies a nonempty value,
- `join()' uses it; if it is not supplied, it has a null value.  In this
- case, `join()' uses a single space as a default separator for the
- strings.  If the value is equal to `SUBSEP', then `join()' joins the
- strings with no separator between them.  `SUBSEP' serves as a "magic"
- value to indicate that there should be no separation between the
- component strings.(1)
- 
-    ---------- Footnotes ----------
- 
-    (1) It would be nice if `awk' had an assignment operator for
- concatenation.  The lack of an explicit operator for concatenation
- makes string operations more difficult than they really need to be.
- 
- 
- File: gawk.info,  Node: Gettimeofday Function,  Prev: Join Function,  Up: 
General Functions
- 
- 13.2.7 Managing the Time of Day
- -------------------------------
- 
- The `systime()' and `strftime()' functions described in *note Time
- Functions::, provide the minimum functionality necessary for dealing
- with the time of day in human readable form.  While `strftime()' is
- extensive, the control formats are not necessarily easy to remember or
- intuitively obvious when reading a program.
- 
-    The following function, `gettimeofday()', populates a user-supplied
- array with preformatted time information.  It returns a string with the
- current time formatted in the same way as the `date' utility:
- 
-      # gettimeofday.awk --- get the time of day in a usable format
- 
-      # Returns a string in the format of output of date(1)
-      # Populates the array argument time with individual values:
-      #    time["second"]       -- seconds (0 - 59)
-      #    time["minute"]       -- minutes (0 - 59)
-      #    time["hour"]         -- hours (0 - 23)
-      #    time["althour"]      -- hours (0 - 12)
-      #    time["monthday"]     -- day of month (1 - 31)
-      #    time["month"]        -- month of year (1 - 12)
-      #    time["monthname"]    -- name of the month
-      #    time["shortmonth"]   -- short name of the month
-      #    time["year"]         -- year modulo 100 (0 - 99)
-      #    time["fullyear"]     -- full year
-      #    time["weekday"]      -- day of week (Sunday = 0)
-      #    time["altweekday"]   -- day of week (Monday = 0)
-      #    time["dayname"]      -- name of weekday
-      #    time["shortdayname"] -- short name of weekday
-      #    time["yearday"]      -- day of year (0 - 365)
-      #    time["timezone"]     -- abbreviation of timezone name
-      #    time["ampm"]         -- AM or PM designation
-      #    time["weeknum"]      -- week number, Sunday first day
-      #    time["altweeknum"]   -- week number, Monday first day
- 
-      function gettimeofday(time,    ret, now, i)
-      {
-          # get time once, avoids unnecessary system calls
-          now = systime()
- 
-          # return date(1)-style output
-          ret = strftime("%a %b %e %H:%M:%S %Z %Y", now)
- 
-          # clear out target array
-          delete time
- 
-          # fill in values, force numeric values to be
-          # numeric by adding 0
-          time["second"]       = strftime("%S", now) + 0
-          time["minute"]       = strftime("%M", now) + 0
-          time["hour"]         = strftime("%H", now) + 0
-          time["althour"]      = strftime("%I", now) + 0
-          time["monthday"]     = strftime("%d", now) + 0
-          time["month"]        = strftime("%m", now) + 0
-          time["monthname"]    = strftime("%B", now)
-          time["shortmonth"]   = strftime("%b", now)
-          time["year"]         = strftime("%y", now) + 0
-          time["fullyear"]     = strftime("%Y", now) + 0
-          time["weekday"]      = strftime("%w", now) + 0
-          time["altweekday"]   = strftime("%u", now) + 0
-          time["dayname"]      = strftime("%A", now)
-          time["shortdayname"] = strftime("%a", now)
-          time["yearday"]      = strftime("%j", now) + 0
-          time["timezone"]     = strftime("%Z", now)
-          time["ampm"]         = strftime("%p", now)
-          time["weeknum"]      = strftime("%U", now) + 0
-          time["altweeknum"]   = strftime("%W", now) + 0
- 
-          return ret
-      }
- 
-    The string indices are easier to use and read than the various
- formats required by `strftime()'.  The `alarm' program presented in
- *note Alarm Program::, uses this function.  A more general design for
- the `gettimeofday()' function would have allowed the user to supply an
- optional timestamp value to use instead of the current time.
- 
- 
- File: gawk.info,  Node: Data File Management,  Next: Getopt Function,  Prev: 
General Functions,  Up: Library Functions
- 
- 13.3 Data File Management
- =========================
- 
- This minor node presents functions that are useful for managing
- command-line data files.
- 
- * Menu:
- 
- * Filetrans Function::          A function for handling data file transitions.
- * Rewind Function::             A function for rereading the current file.
- * File Checking::               Checking that data files are readable.
- * Empty Files::                 Checking for zero-length files.
- * Ignoring Assigns::            Treating assignments as file names.
- 
- 
- File: gawk.info,  Node: Filetrans Function,  Next: Rewind Function,  Up: Data 
File Management
- 
- 13.3.1 Noting Data File Boundaries
- ----------------------------------
- 
- The `BEGIN' and `END' rules are each executed exactly once at the
- beginning and end of your `awk' program, respectively (*note
- BEGIN/END::).  We (the `gawk' authors) once had a user who mistakenly
- thought that the `BEGIN' rule is executed at the beginning of each data
- file and the `END' rule is executed at the end of each data file.
- 
-    When informed that this was not the case, the user requested that we
- add new special patterns to `gawk', named `BEGIN_FILE' and `END_FILE',
- that would have the desired behavior.  He even supplied us the code to
- do so.
- 
-    Adding these special patterns to `gawk' wasn't necessary; the job
- can be done cleanly in `awk' itself, as illustrated by the following
- library program.  It arranges to call two user-supplied functions,
- `beginfile()' and `endfile()', at the beginning and end of each data
- file.  Besides solving the problem in only nine(!) lines of code, it
- does so _portably_; this works with any implementation of `awk':
- 
-      # transfile.awk
-      #
-      # Give the user a hook for filename transitions
-      #
-      # The user must supply functions beginfile() and endfile()
-      # that each take the name of the file being started or
-      # finished, respectively.
- 
-      FILENAME != _oldfilename \
-      {
-          if (_oldfilename != "")
-              endfile(_oldfilename)
-          _oldfilename = FILENAME
-          beginfile(FILENAME)
-      }
- 
-      END   { endfile(FILENAME) }
- 
-    This file must be loaded before the user's "main" program, so that
- the rule it supplies is executed first.
- 
-    This rule relies on `awk''s `FILENAME' variable that automatically
- changes for each new data file.  The current file name is saved in a
- private variable, `_oldfilename'.  If `FILENAME' does not equal
- `_oldfilename', then a new data file is being processed and it is
- necessary to call `endfile()' for the old file.  Because `endfile()'
- should only be called if a file has been processed, the program first
- checks to make sure that `_oldfilename' is not the null string.  The
- program then assigns the current file name to `_oldfilename' and calls
- `beginfile()' for the file.  Because, like all `awk' variables,
- `_oldfilename' is initialized to the null string, this rule executes
- correctly even for the first data file.
- 
-    The program also supplies an `END' rule to do the final processing
- for the last file.  Because this `END' rule comes before any `END' rules
- supplied in the "main" program, `endfile()' is called first.  Once
- again the value of multiple `BEGIN' and `END' rules should be clear.
- 
-    If the same data file occurs twice in a row on the command line, then
- `endfile()' and `beginfile()' are not executed at the end of the first
- pass and at the beginning of the second pass.  The following version
- solves the problem:
- 
-      # ftrans.awk --- handle data file transitions
-      #
-      # user supplies beginfile() and endfile() functions
- 
-      FNR == 1 {
-          if (_filename_ != "")
-              endfile(_filename_)
-          _filename_ = FILENAME
-          beginfile(FILENAME)
-      }
- 
-      END  { endfile(_filename_) }
- 
-    *note Wc Program::, shows how this library function can be used and
- how it simplifies writing the main program.
- 
- Advanced Notes: So Why Does `gawk' have `BEGINFILE' and `ENDFILE'?
- ------------------------------------------------------------------
- 
- You are probably wondering, if `beginfile()' and `endfile()' functions
- can do the job, why does `gawk' have `BEGINFILE' and `ENDFILE' patterns
- (*note BEGINFILE/ENDFILE::)?
- 
-    Good question.  Normally, if `awk' cannot open a file, this causes
- an immediate fatal error.  In this case, there is no way for a
- user-defined function to deal with the problem, since the mechanism for
- calling it relies on the file being open and at the first record.  Thus,
- the main reason for `BEGINFILE' is to give you a "hook" to catch files
- that cannot be processed.  `ENDFILE' exists for symmetry, and because
- it provides an easy way to do per-file cleanup processing.
- 
- 
- File: gawk.info,  Node: Rewind Function,  Next: File Checking,  Prev: 
Filetrans Function,  Up: Data File Management
- 
- 13.3.2 Rereading the Current File
- ---------------------------------
- 
- Another request for a new built-in function was for a `rewind()'
- function that would make it possible to reread the current file.  The
- requesting user didn't want to have to use `getline' (*note Getline::)
- inside a loop.
- 
-    However, as long as you are not in the `END' rule, it is quite easy
- to arrange to immediately close the current input file and then start
- over with it from the top.  For lack of a better name, we'll call it
- `rewind()':
- 
-      # rewind.awk --- rewind the current file and start over
- 
-      function rewind(    i)
-      {
-          # shift remaining arguments up
-          for (i = ARGC; i > ARGIND; i--)
-              ARGV[i] = ARGV[i-1]
- 
-          # make sure gawk knows to keep going
-          ARGC++
- 
-          # make current file next to get done
-          ARGV[ARGIND+1] = FILENAME
- 
-          # do it
-          nextfile
-      }
- 
-    This code relies on the `ARGIND' variable (*note Auto-set::), which
- is specific to `gawk'.  If you are not using `gawk', you can use ideas
- presented in *note Filetrans Function::, to either update `ARGIND' on
- your own or modify this code as appropriate.
- 
-    The `rewind()' function also relies on the `nextfile' keyword (*note
- Nextfile Statement::).
- 
- 
- File: gawk.info,  Node: File Checking,  Next: Empty Files,  Prev: Rewind 
Function,  Up: Data File Management
- 
- 13.3.3 Checking for Readable Data Files
- ---------------------------------------
- 
- Normally, if you give `awk' a data file that isn't readable, it stops
- with a fatal error.  There are times when you might want to just ignore
- such files and keep going.  You can do this by prepending the following
- program to your `awk' program:
- 
-      # readable.awk --- library file to skip over unreadable files
- 
-      BEGIN {
-          for (i = 1; i < ARGC; i++) {
-              if (ARGV[i] ~ /^[[:alpha:]_][[:alnum:]_]*=.*/ \
-                  || ARGV[i] == "-" || ARGV[i] == "/dev/stdin")
-                  continue    # assignment or standard input
-              else if ((getline junk < ARGV[i]) < 0) # unreadable
-                  delete ARGV[i]
-              else
-                  close(ARGV[i])
-          }
-      }
- 
-    This works, because the `getline' won't be fatal.  Removing the
- element from `ARGV' with `delete' skips the file (since it's no longer
- in the list).  See also *note ARGC and ARGV::.
- 
- 
- File: gawk.info,  Node: Empty Files,  Next: Ignoring Assigns,  Prev: File 
Checking,  Up: Data File Management
- 
- 13.3.4 Checking For Zero-length Files
- -------------------------------------
- 
- All known `awk' implementations silently skip over zero-length files.
- This is a by-product of `awk''s implicit
- read-a-record-and-match-against-the-rules loop: when `awk' tries to
- read a record from an empty file, it immediately receives an end of
- file indication, closes the file, and proceeds on to the next
- command-line data file, _without_ executing any user-level `awk'
- program code.
- 
-    Using `gawk''s `ARGIND' variable (*note Built-in Variables::), it is
- possible to detect when an empty data file has been skipped.  Similar
- to the library file presented in *note Filetrans Function::, the
- following library file calls a function named `zerofile()' that the
- user must provide.  The arguments passed are the file name and the
- position in `ARGV' where it was found:
- 
-      # zerofile.awk --- library file to process empty input files
- 
-      BEGIN { Argind = 0 }
- 
-      ARGIND > Argind + 1 {
-          for (Argind++; Argind < ARGIND; Argind++)
-              zerofile(ARGV[Argind], Argind)
-      }
- 
-      ARGIND != Argind { Argind = ARGIND }
- 
-      END {
-          if (ARGIND > Argind)
-              for (Argind++; Argind <= ARGIND; Argind++)
-                  zerofile(ARGV[Argind], Argind)
-      }
- 
-    The user-level variable `Argind' allows the `awk' program to track
- its progress through `ARGV'.  Whenever the program detects that
- `ARGIND' is greater than `Argind + 1', it means that one or more empty
- files were skipped.  The action then calls `zerofile()' for each such
- file, incrementing `Argind' along the way.
- 
-    The `Argind != ARGIND' rule simply keeps `Argind' up to date in the
- normal case.
- 
-    Finally, the `END' rule catches the case of any empty files at the
- end of the command-line arguments.  Note that the test in the condition
- of the `for' loop uses the `<=' operator, not `<'.
- 
-    As an exercise, you might consider whether this same problem can be
- solved without relying on `gawk''s `ARGIND' variable.
- 
-    As a second exercise, revise this code to handle the case where an
- intervening value in `ARGV' is a variable assignment.
- 
- 
- File: gawk.info,  Node: Ignoring Assigns,  Prev: Empty Files,  Up: Data File 
Management
- 
- 13.3.5 Treating Assignments as File Names
- -----------------------------------------
- 
- Occasionally, you might not want `awk' to process command-line variable
- assignments (*note Assignment Options::).  In particular, if you have a
- file name that contain an `=' character, `awk' treats the file name as
- an assignment, and does not process it.
- 
-    Some users have suggested an additional command-line option for
- `gawk' to disable command-line assignments.  However, some simple
- programming with a library file does the trick:
- 
-      # noassign.awk --- library file to avoid the need for a
-      # special option that disables command-line assignments
- 
-      function disable_assigns(argc, argv,    i)
-      {
-          for (i = 1; i < argc; i++)
-              if (argv[i] ~ /^[[:alpha:]_][[:alnum:]_]*=.*/)
-                  argv[i] = ("./" argv[i])
-      }
- 
-      BEGIN {
-          if (No_command_assign)
-              disable_assigns(ARGC, ARGV)
-      }
- 
-    You then run your program this way:
- 
-      awk -v No_command_assign=1 -f noassign.awk -f yourprog.awk *
- 
-    The function works by looping through the arguments.  It prepends
- `./' to any argument that matches the form of a variable assignment,
- turning that argument into a file name.
- 
-    The use of `No_command_assign' allows you to disable command-line
- assignments at invocation time, by giving the variable a true value.
- When not set, it is initially zero (i.e., false), so the command-line
- arguments are left alone.
- 
- 
- File: gawk.info,  Node: Getopt Function,  Next: Passwd Functions,  Prev: Data 
File Management,  Up: Library Functions
- 
- 13.4 Processing Command-Line Options
- ====================================
- 
- Most utilities on POSIX compatible systems take options on the command
- line that can be used to change the way a program behaves.  `awk' is an
- example of such a program (*note Options::).  Often, options take
- "arguments"; i.e., data that the program needs to correctly obey the
- command-line option.  For example, `awk''s `-F' option requires a
- string to use as the field separator.  The first occurrence on the
- command line of either `--' or a string that does not begin with `-'
- ends the options.
- 
-    Modern Unix systems provide a C function named `getopt()' for
- processing command-line arguments.  The programmer provides a string
- describing the one-letter options. If an option requires an argument,
- it is followed in the string with a colon.  `getopt()' is also passed
- the count and values of the command-line arguments and is called in a
- loop.  `getopt()' processes the command-line arguments for option
- letters.  Each time around the loop, it returns a single character
- representing the next option letter that it finds, or `?' if it finds
- an invalid option.  When it returns -1, there are no options left on
- the command line.
- 
-    When using `getopt()', options that do not take arguments can be
- grouped together.  Furthermore, options that take arguments require
- that the argument be present.  The argument can immediately follow the
- option letter, or it can be a separate command-line argument.
- 
-    Given a hypothetical program that takes three command-line options,
- `-a', `-b', and `-c', where `-b' requires an argument, all of the
- following are valid ways of invoking the program:
- 
-      prog -a -b foo -c data1 data2 data3
-      prog -ac -bfoo -- data1 data2 data3
-      prog -acbfoo data1 data2 data3
- 
-    Notice that when the argument is grouped with its option, the rest of
- the argument is considered to be the option's argument.  In this
- example, `-acbfoo' indicates that all of the `-a', `-b', and `-c'
- options were supplied, and that `foo' is the argument to the `-b'
- option.
- 
-    `getopt()' provides four external variables that the programmer can
- use:
- 
- `optind'
-      The index in the argument value array (`argv') where the first
-      nonoption command-line argument can be found.
- 
- `optarg'
-      The string value of the argument to an option.
- 
- `opterr'
-      Usually `getopt()' prints an error message when it finds an invalid
-      option.  Setting `opterr' to zero disables this feature.  (An
-      application might want to print its own error message.)
- 
- `optopt'
-      The letter representing the command-line option.
- 
-    The following C fragment shows how `getopt()' might process
- command-line arguments for `awk':
- 
-      int
-      main(int argc, char *argv[])
-      {
-          ...
-          /* print our own message */
-          opterr = 0;
-          while ((c = getopt(argc, argv, "v:f:F:W:")) != -1) {
-              switch (c) {
-              case 'f':    /* file */
-                  ...
-                  break;
-              case 'F':    /* field separator */
-                  ...
-                  break;
-              case 'v':    /* variable assignment */
-                  ...
-                  break;
-              case 'W':    /* extension */
-                  ...
-                  break;
-              case '?':
-              default:
-                  usage();
-                  break;
-              }
-          }
-          ...
-      }
- 
-    As a side point, `gawk' actually uses the GNU `getopt_long()'
- function to process both normal and GNU-style long options (*note
- Options::).
- 
-    The abstraction provided by `getopt()' is very useful and is quite
- handy in `awk' programs as well.  Following is an `awk' version of
- `getopt()'.  This function highlights one of the greatest weaknesses in
- `awk', which is that it is very poor at manipulating single characters.
- Repeated calls to `substr()' are necessary for accessing individual
- characters (*note String Functions::).(1)
- 
-    The discussion that follows walks through the code a bit at a time:
- 
-      # getopt.awk --- Do C library getopt(3) function in awk
- 
-      # External variables:
-      #    Optind -- index in ARGV of first nonoption argument
-      #    Optarg -- string value of argument to current option
-      #    Opterr -- if nonzero, print our own diagnostic
-      #    Optopt -- current option letter
- 
-      # Returns:
-      #    -1     at end of options
-      #    "?"    for unrecognized option
-      #    <c>    a character representing the current option
- 
-      # Private Data:
-      #    _opti  -- index in multi-flag option, e.g., -abc
- 
-    The function starts out with comments presenting a list of the
- global variables it uses, what the return values are, what they mean,
- and any global variables that are "private" to this library function.
- Such documentation is essential for any program, and particularly for
- library functions.
- 
-    The `getopt()' function first checks that it was indeed called with
- a string of options (the `options' parameter).  If `options' has a zero
- length, `getopt()' immediately returns -1:
- 
-      function getopt(argc, argv, options,    thisopt, i)
-      {
-          if (length(options) == 0)    # no options given
-              return -1
- 
-          if (argv[Optind] == "--") {  # all done
-              Optind++
-              _opti = 0
-              return -1
-          } else if (argv[Optind] !~ /^-[^:[:space:]]/) {
-              _opti = 0
-              return -1
-          }
- 
-    The next thing to check for is the end of the options.  A `--' ends
- the command-line options, as does any command-line argument that does
- not begin with a `-'.  `Optind' is used to step through the array of
- command-line arguments; it retains its value across calls to
- `getopt()', because it is a global variable.
- 
-    The regular expression that is used, `/^-[^:[:space:]/', checks for
- a `-' followed by anything that is not whitespace and not a colon.  If
- the current command-line argument does not match this pattern, it is
- not an option, and it ends option processing. Continuing on:
- 
-          if (_opti == 0)
-              _opti = 2
-          thisopt = substr(argv[Optind], _opti, 1)
-          Optopt = thisopt
-          i = index(options, thisopt)
-          if (i == 0) {
-              if (Opterr)
-                  printf("%c -- invalid option\n",
-                                        thisopt) > "/dev/stderr"
-              if (_opti >= length(argv[Optind])) {
-                  Optind++
-                  _opti = 0
-              } else
-                  _opti++
-              return "?"
-          }
- 
-    The `_opti' variable tracks the position in the current command-line
- argument (`argv[Optind]').  If multiple options are grouped together
- with one `-' (e.g., `-abx'), it is necessary to return them to the user
- one at a time.
- 
-    If `_opti' is equal to zero, it is set to two, which is the index in
- the string of the next character to look at (we skip the `-', which is
- at position one).  The variable `thisopt' holds the character, obtained
- with `substr()'.  It is saved in `Optopt' for the main program to use.
- 
-    If `thisopt' is not in the `options' string, then it is an invalid
- option.  If `Opterr' is nonzero, `getopt()' prints an error message on
- the standard error that is similar to the message from the C version of
- `getopt()'.
- 
-    Because the option is invalid, it is necessary to skip it and move
- on to the next option character.  If `_opti' is greater than or equal
- to the length of the current command-line argument, it is necessary to
- move on to the next argument, so `Optind' is incremented and `_opti' is
- reset to zero. Otherwise, `Optind' is left alone and `_opti' is merely
- incremented.
- 
-    In any case, because the option is invalid, `getopt()' returns `"?"'.
- The main program can examine `Optopt' if it needs to know what the
- invalid option letter actually is. Continuing on:
- 
-          if (substr(options, i + 1, 1) == ":") {
-              # get option argument
-              if (length(substr(argv[Optind], _opti + 1)) > 0)
-                  Optarg = substr(argv[Optind], _opti + 1)
-              else
-                  Optarg = argv[++Optind]
-              _opti = 0
-          } else
-              Optarg = ""
- 
-    If the option requires an argument, the option letter is followed by
- a colon in the `options' string.  If there are remaining characters in
- the current command-line argument (`argv[Optind]'), then the rest of
- that string is assigned to `Optarg'.  Otherwise, the next command-line
- argument is used (`-xFOO' versus `-x FOO'). In either case, `_opti' is
- reset to zero, because there are no more characters left to examine in
- the current command-line argument. Continuing:
- 
-          if (_opti == 0 || _opti >= length(argv[Optind])) {
-              Optind++
-              _opti = 0
-          } else
-              _opti++
-          return thisopt
-      }
- 
-    Finally, if `_opti' is either zero or greater than the length of the
- current command-line argument, it means this element in `argv' is
- through being processed, so `Optind' is incremented to point to the
- next element in `argv'.  If neither condition is true, then only
- `_opti' is incremented, so that the next option letter can be processed
- on the next call to `getopt()'.
- 
-    The `BEGIN' rule initializes both `Opterr' and `Optind' to one.
- `Opterr' is set to one, since the default behavior is for `getopt()' to
- print a diagnostic message upon seeing an invalid option.  `Optind' is
- set to one, since there's no reason to look at the program name, which
- is in `ARGV[0]':
- 
-      BEGIN {
-          Opterr = 1    # default is to diagnose
-          Optind = 1    # skip ARGV[0]
- 
-          # test program
-          if (_getopt_test) {
-              while ((_go_c = getopt(ARGC, ARGV, "ab:cd")) != -1)
-                  printf("c = <%c>, optarg = <%s>\n",
-                                             _go_c, Optarg)
-              printf("non-option arguments:\n")
-              for (; Optind < ARGC; Optind++)
-                  printf("\tARGV[%d] = <%s>\n",
-                                          Optind, ARGV[Optind])
-          }
-      }
- 
-    The rest of the `BEGIN' rule is a simple test program.  Here is the
- result of two sample runs of the test program:
- 
-      $ awk -f getopt.awk -v _getopt_test=1 -- -a -cbARG bax -x
-      -| c = <a>, optarg = <>
-      -| c = <c>, optarg = <>
-      -| c = <b>, optarg = <ARG>
-      -| non-option arguments:
-      -|         ARGV[3] = <bax>
-      -|         ARGV[4] = <-x>
- 
-      $ awk -f getopt.awk -v _getopt_test=1 -- -a -x -- xyz abc
-      -| c = <a>, optarg = <>
-      error--> x -- invalid option
-      -| c = <?>, optarg = <>
-      -| non-option arguments:
-      -|         ARGV[4] = <xyz>
-      -|         ARGV[5] = <abc>
- 
-    In both runs, the first `--' terminates the arguments to `awk', so
- that it does not try to interpret the `-a', etc., as its own options.
- 
-      NOTE: After `getopt()' is through, it is the responsibility of the
-      user level code to clear out all the elements of `ARGV' from 1 to
-      `Optind', so that `awk' does not try to process the command-line
-      options as file names.
- 
-    Several of the sample programs presented in *note Sample Programs::,
- use `getopt()' to process their arguments.
- 
-    ---------- Footnotes ----------
- 
-    (1) This function was written before `gawk' acquired the ability to
- split strings into single characters using `""' as the separator.  We
- have left it alone, since using `substr()' is more portable.
- 
- 
- File: gawk.info,  Node: Passwd Functions,  Next: Group Functions,  Prev: 
Getopt Function,  Up: Library Functions
- 
- 13.5 Reading the User Database
- ==============================
- 
- The `PROCINFO' array (*note Built-in Variables::) provides access to
- the current user's real and effective user and group ID numbers, and if
- available, the user's supplementary group set.  However, because these
- are numbers, they do not provide very useful information to the average
- user.  There needs to be some way to find the user information
- associated with the user and group ID numbers.  This minor node
- presents a suite of functions for retrieving information from the user
- database.  *Note Group Functions::, for a similar suite that retrieves
- information from the group database.
- 
-    The POSIX standard does not define the file where user information is
- kept.  Instead, it provides the `<pwd.h>' header file and several C
- language subroutines for obtaining user information.  The primary
- function is `getpwent()', for "get password entry."  The "password"
- comes from the original user database file, `/etc/passwd', which stores
- user information, along with the encrypted passwords (hence the name).
- 
-    While an `awk' program could simply read `/etc/passwd' directly,
- this file may not contain complete information about the system's set
- of users.(1) To be sure you are able to produce a readable and complete
- version of the user database, it is necessary to write a small C
- program that calls `getpwent()'.  `getpwent()' is defined as returning
- a pointer to a `struct passwd'.  Each time it is called, it returns the
- next entry in the database.  When there are no more entries, it returns
- `NULL', the null pointer.  When this happens, the C program should call
- `endpwent()' to close the database.  Following is `pwcat', a C program
- that "cats" the password database:
- 
-      /*
-       * pwcat.c
-       *
-       * Generate a printable version of the password database
-       */
-      #include <stdio.h>
-      #include <pwd.h>
- 
-      int
-      main(int argc, char **argv)
-      {
-          struct passwd *p;
- 
-          while ((p = getpwent()) != NULL)
-              printf("%s:%s:%ld:%ld:%s:%s:%s\n",
-                  p->pw_name, p->pw_passwd, (long) p->pw_uid,
-                  (long) p->pw_gid, p->pw_gecos, p->pw_dir, p->pw_shell);
- 
-          endpwent();
-          return 0;
-      }
- 
-    If you don't understand C, don't worry about it.  The output from
- `pwcat' is the user database, in the traditional `/etc/passwd' format
- of colon-separated fields.  The fields are:
- 
- Login name
-      The user's login name.
- 
- Encrypted password
-      The user's encrypted password.  This may not be available on some
-      systems.
- 
- User-ID
-      The user's numeric user ID number.  (On some systems it's a C
-      `long', and not an `int'.  Thus we cast it to `long' for all
-      cases.)
- 
- Group-ID
-      The user's numeric group ID number.  (Similar comments about
-      `long' vs. `int' apply here.)
- 
- Full name
-      The user's full name, and perhaps other information associated
-      with the user.
- 
- Home directory
-      The user's login (or "home") directory (familiar to shell
-      programmers as `$HOME').
- 
- Login shell
-      The program that is run when the user logs in.  This is usually a
-      shell, such as Bash.
- 
-    A few lines representative of `pwcat''s output are as follows:
- 
-      $ pwcat
-      -| root:3Ov02d5VaUPB6:0:1:Operator:/:/bin/sh
-      -| nobody:*:65534:65534::/:
-      -| daemon:*:1:1::/:
-      -| sys:*:2:2::/:/bin/csh
-      -| bin:*:3:3::/bin:
-      -| arnold:xyzzy:2076:10:Arnold Robbins:/home/arnold:/bin/sh
-      -| miriam:yxaay:112:10:Miriam Robbins:/home/miriam:/bin/sh
-      -| andy:abcca2:113:10:Andy Jacobs:/home/andy:/bin/sh
-      ...
- 
-    With that introduction, following is a group of functions for
- getting user information.  There are several functions here,
- corresponding to the C functions of the same names:
- 
-      # passwd.awk --- access password file information
- 
-      BEGIN {
-          # tailor this to suit your system
-          _pw_awklib = "/usr/local/libexec/awk/"
-      }
- 
-      function _pw_init(    oldfs, oldrs, olddol0, pwcat, using_fw, using_fpat)
-      {
-          if (_pw_inited)
-              return
- 
-          oldfs = FS
-          oldrs = RS
-          olddol0 = $0
-          using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
-          using_fpat = (PROCINFO["FS"] == "FPAT")
-          FS = ":"
-          RS = "\n"
- 
-          pwcat = _pw_awklib "pwcat"
-          while ((pwcat | getline) > 0) {
-              _pw_byname[$1] = $0
-              _pw_byuid[$3] = $0
-              _pw_bycount[++_pw_total] = $0
-          }
-          close(pwcat)
-          _pw_count = 0
-          _pw_inited = 1
-          FS = oldfs
-          if (using_fw)
-              FIELDWIDTHS = FIELDWIDTHS
-          else if (using_fpat)
-              FPAT = FPAT
-          RS = oldrs
-          $0 = olddol0
-      }
- 
-    The `BEGIN' rule sets a private variable to the directory where
- `pwcat' is stored.  Because it is used to help out an `awk' library
- routine, we have chosen to put it in `/usr/local/libexec/awk'; however,
- you might want it to be in a different directory on your system.
- 
-    The function `_pw_init()' keeps three copies of the user information
- in three associative arrays.  The arrays are indexed by username
- (`_pw_byname'), by user ID number (`_pw_byuid'), and by order of
- occurrence (`_pw_bycount').  The variable `_pw_inited' is used for
- efficiency, since `_pw_init()' needs to be called only once.
- 
-    Because this function uses `getline' to read information from
- `pwcat', it first saves the values of `FS', `RS', and `$0'.  It notes
- in the variable `using_fw' whether field splitting with `FIELDWIDTHS'
- is in effect or not.  Doing so is necessary, since these functions
- could be called from anywhere within a user's program, and the user may
- have his or her own way of splitting records and fields.
- 
-    The `using_fw' variable checks `PROCINFO["FS"]', which is
- `"FIELDWIDTHS"' if field splitting is being done with `FIELDWIDTHS'.
- This makes it possible to restore the correct field-splitting mechanism
- later.  The test can only be true for `gawk'.  It is false if using
- `FS' or `FPAT', or on some other `awk' implementation.
- 
-    The code that checks for using `FPAT', using `using_fpat' and
- `PROCINFO["FS"]' is similar.
- 
-    The main part of the function uses a loop to read database lines,
- split the line into fields, and then store the line into each array as
- necessary.  When the loop is done, `_pw_init()' cleans up by closing
- the pipeline, setting `_pw_inited' to one, and restoring `FS' (and
- `FIELDWIDTHS' or `FPAT' if necessary), `RS', and `$0'.  The use of
- `_pw_count' is explained shortly.
- 
-    The `getpwnam()' function takes a username as a string argument. If
- that user is in the database, it returns the appropriate line.
- Otherwise, it relies on the array reference to a nonexistent element to
- create the element with the null string as its value:
- 
-      function getpwnam(name)
-      {
-          _pw_init()
-          return _pw_byname[name]
-      }
- 
-    Similarly, the `getpwuid' function takes a user ID number argument.
- If that user number is in the database, it returns the appropriate
- line. Otherwise, it returns the null string:
- 
-      function getpwuid(uid)
-      {
-          _pw_init()
-          return _pw_byuid[uid]
-      }
- 
-    The `getpwent()' function simply steps through the database, one
- entry at a time.  It uses `_pw_count' to track its current position in
- the `_pw_bycount' array:
- 
-      function getpwent()
-      {
-          _pw_init()
-          if (_pw_count < _pw_total)
-              return _pw_bycount[++_pw_count]
-          return ""
-      }
- 
-    The `endpwent()' function resets `_pw_count' to zero, so that
- subsequent calls to `getpwent()' start over again:
- 
-      function endpwent()
-      {
-          _pw_count = 0
-      }
- 
-    A conscious design decision in this suite is that each subroutine
- calls `_pw_init()' to initialize the database arrays.  The overhead of
- running a separate process to generate the user database, and the I/O
- to scan it, are only incurred if the user's main program actually calls
- one of these functions.  If this library file is loaded along with a
- user's program, but none of the routines are ever called, then there is
- no extra runtime overhead.  (The alternative is move the body of
- `_pw_init()' into a `BEGIN' rule, which always runs `pwcat'.  This
- simplifies the code but runs an extra process that may never be needed.)
- 
-    In turn, calling `_pw_init()' is not too expensive, because the
- `_pw_inited' variable keeps the program from reading the data more than
- once.  If you are worried about squeezing every last cycle out of your
- `awk' program, the check of `_pw_inited' could be moved out of
- `_pw_init()' and duplicated in all the other functions.  In practice,
- this is not necessary, since most `awk' programs are I/O-bound, and
- such a change would clutter up the code.
- 
-    The `id' program in *note Id Program::, uses these functions.
- 
-    ---------- Footnotes ----------
- 
-    (1) It is often the case that password information is stored in a
- network database.
- 
- 
- File: gawk.info,  Node: Group Functions,  Next: Walking Arrays,  Prev: Passwd 
Functions,  Up: Library Functions
- 
- 13.6 Reading the Group Database
- ===============================
- 
- Much of the discussion presented in *note Passwd Functions::, applies
- to the group database as well.  Although there has traditionally been a
- well-known file (`/etc/group') in a well-known format, the POSIX
- standard only provides a set of C library routines (`<grp.h>' and
- `getgrent()') for accessing the information.  Even though this file may
- exist, it may not have complete information.  Therefore, as with the
- user database, it is necessary to have a small C program that generates
- the group database as its output.  `grcat', a C program that "cats" the
- group database, is as follows:
- 
-      /*
-       * grcat.c
-       *
-       * Generate a printable version of the group database
-       */
-      #include <stdio.h>
-      #include <grp.h>
- 
-      int
-      main(int argc, char **argv)
-      {
-          struct group *g;
-          int i;
- 
-          while ((g = getgrent()) != NULL) {
-              printf("%s:%s:%ld:", g->gr_name, g->gr_passwd,
-                                           (long) g->gr_gid);
-              for (i = 0; g->gr_mem[i] != NULL; i++) {
-                  printf("%s", g->gr_mem[i]);
-                  if (g->gr_mem[i+1] != NULL)
-                      putchar(',');
-              }
-              putchar('\n');
-          }
-          endgrent();
-          return 0;
-      }
- 
-    Each line in the group database represents one group.  The fields are
- separated with colons and represent the following information:
- 
- Group Name
-      The group's name.
- 
- Group Password
-      The group's encrypted password. In practice, this field is never
-      used; it is usually empty or set to `*'.
- 
- Group ID Number
-      The group's numeric group ID number; this number must be unique
-      within the file.  (On some systems it's a C `long', and not an
-      `int'.  Thus we cast it to `long' for all cases.)
- 
- Group Member List
-      A comma-separated list of user names.  These users are members of
-      the group.  Modern Unix systems allow users to be members of
-      several groups simultaneously.  If your system does, then there
-      are elements `"group1"' through `"groupN"' in `PROCINFO' for those
-      group ID numbers.  (Note that `PROCINFO' is a `gawk' extension;
-      *note Built-in Variables::.)
- 
-    Here is what running `grcat' might produce:
- 
-      $ grcat
-      -| wheel:*:0:arnold
-      -| nogroup:*:65534:
-      -| daemon:*:1:
-      -| kmem:*:2:
-      -| staff:*:10:arnold,miriam,andy
-      -| other:*:20:
-      ...
- 
-    Here are the functions for obtaining information from the group
- database.  There are several, modeled after the C library functions of
- the same names:
- 
-      # group.awk --- functions for dealing with the group file
- 
-      BEGIN    \
-      {
-          # Change to suit your system
-          _gr_awklib = "/usr/local/libexec/awk/"
-      }
- 
-      function _gr_init(    oldfs, oldrs, olddol0, grcat,
-                                   using_fw, using_fpat, n, a, i)
-      {
-          if (_gr_inited)
-              return
- 
-          oldfs = FS
-          oldrs = RS
-          olddol0 = $0
-          using_fw = (PROCINFO["FS"] == "FIELDWIDTHS")
-          using_fpat = (PROCINFO["FS"] == "FPAT")
-          FS = ":"
-          RS = "\n"
- 
-          grcat = _gr_awklib "grcat"
-          while ((grcat | getline) > 0) {
-              if ($1 in _gr_byname)
-                  _gr_byname[$1] = _gr_byname[$1] "," $4
-              else
-                  _gr_byname[$1] = $0
-              if ($3 in _gr_bygid)
-                  _gr_bygid[$3] = _gr_bygid[$3] "," $4
-              else
-                  _gr_bygid[$3] = $0
- 
-              n = split($4, a, "[ \t]*,[ \t]*")
-              for (i = 1; i <= n; i++)
-                  if (a[i] in _gr_groupsbyuser)
-                      _gr_groupsbyuser[a[i]] = \
-                          _gr_groupsbyuser[a[i]] " " $1
-                  else
-                      _gr_groupsbyuser[a[i]] = $1
- 
-              _gr_bycount[++_gr_count] = $0
-          }
-          close(grcat)
-          _gr_count = 0
-          _gr_inited++
-          FS = oldfs
-          if (using_fw)
-              FIELDWIDTHS = FIELDWIDTHS
-          else if (using_fpat)
-              FPAT = FPAT
-          RS = oldrs
-          $0 = olddol0
-      }
- 
-    The `BEGIN' rule sets a private variable to the directory where
- `grcat' is stored.  Because it is used to help out an `awk' library
- routine, we have chosen to put it in `/usr/local/libexec/awk'.  You
- might want it to be in a different directory on your system.
- 
-    These routines follow the same general outline as the user database
- routines (*note Passwd Functions::).  The `_gr_inited' variable is used
- to ensure that the database is scanned no more than once.  The
- `_gr_init()' function first saves `FS', `RS', and `$0', and then sets
- `FS' and `RS' to the correct values for scanning the group information.
- It also takes care to note whether `FIELDWIDTHS' or `FPAT' is being
- used, and to restore the appropriate field splitting mechanism.
- 
-    The group information is stored is several associative arrays.  The
- arrays are indexed by group name (`_gr_byname'), by group ID number
- (`_gr_bygid'), and by position in the database (`_gr_bycount').  There
- is an additional array indexed by user name (`_gr_groupsbyuser'), which
- is a space-separated list of groups to which each user belongs.
- 
-    Unlike the user database, it is possible to have multiple records in
- the database for the same group.  This is common when a group has a
- large number of members.  A pair of such entries might look like the
- following:
- 
-      tvpeople:*:101:johnny,jay,arsenio
-      tvpeople:*:101:david,conan,tom,joan
- 
-    For this reason, `_gr_init()' looks to see if a group name or group
- ID number is already seen.  If it is, then the user names are simply
- concatenated onto the previous list of users.  (There is actually a
- subtle problem with the code just presented.  Suppose that the first
- time there were no names. This code adds the names with a leading
- comma. It also doesn't check that there is a `$4'.)
- 
-    Finally, `_gr_init()' closes the pipeline to `grcat', restores `FS'
- (and `FIELDWIDTHS' or `FPAT' if necessary), `RS', and `$0', initializes
- `_gr_count' to zero (it is used later), and makes `_gr_inited' nonzero.
- 
-    The `getgrnam()' function takes a group name as its argument, and if
- that group exists, it is returned.  Otherwise, it relies on the array
- reference to a nonexistent element to create the element with the null
- string as its value:
- 
-      function getgrnam(group)
-      {
-          _gr_init()
-          return _gr_byname[group]
-      }
- 
-    The `getgrgid()' function is similar; it takes a numeric group ID and
- looks up the information associated with that group ID:
- 
-      function getgrgid(gid)
-      {
-          _gr_init()
-          return _gr_bygid[gid]
-      }
- 
-    The `getgruser()' function does not have a C counterpart. It takes a
- user name and returns the list of groups that have the user as a member:
- 
-      function getgruser(user)
-      {
-          _gr_init()
-          return _gr_groupsbyuser[user]
-      }
- 
-    The `getgrent()' function steps through the database one entry at a
- time.  It uses `_gr_count' to track its position in the list:
- 
-      function getgrent()
-      {
-          _gr_init()
-          if (++_gr_count in _gr_bycount)
-              return _gr_bycount[_gr_count]
-          return ""
-      }
- 
-    The `endgrent()' function resets `_gr_count' to zero so that
- `getgrent()' can start over again:
- 
-      function endgrent()
-      {
-          _gr_count = 0
-      }
- 
-    As with the user database routines, each function calls `_gr_init()'
- to initialize the arrays.  Doing so only incurs the extra overhead of
- running `grcat' if these functions are used (as opposed to moving the
- body of `_gr_init()' into a `BEGIN' rule).
- 
-    Most of the work is in scanning the database and building the various
- associative arrays.  The functions that the user calls are themselves
- very simple, relying on `awk''s associative arrays to do work.
- 
-    The `id' program in *note Id Program::, uses these functions.
- 
- 
- File: gawk.info,  Node: Walking Arrays,  Prev: Group Functions,  Up: Library 
Functions
- 
- 13.7 Traversing Arrays of Arrays
- ================================
- 
- *note Arrays of Arrays::, described how `gawk' provides arrays of
- arrays.  In particular, any element of an array may be either a scalar,
- or another array. The `isarray()' function (*note Type Functions::)
- lets you distinguish an array from a scalar.  The following function,
- `walk_array()', recursively traverses an array, printing each element's
- indices and value.  You call it with the array and a string
- representing the name of the array:
- 
-      function walk_array(arr, name,      i)
-      {
-          for (i in arr) {
-              if (isarray(arr[i]))
-                  walk_array(arr[i], (name "[" i "]"))
-              else
-                  printf("%s[%s] = %s\n", name, i, arr[i])
-          }
-      }
- 
- It works by looping over each element of the array. If any given
- element is itself an array, the function calls itself recursively,
- passing the subarray and a new string representing the current index.
- Otherwise, the function simply prints the element's name, index, and
- value.  Here is a main program to demonstrate:
- 
-      BEGIN {
-          a[1] = 1
-          a[2][1] = 21
-          a[2][2] = 22
-          a[3] = 3
-          a[4][1][1] = 411
-          a[4][2] = 42
- 
-          walk_array(a, "a")
-      }
- 
-    When run, the program produces the following output:
- 
-      $ gawk -f walk_array.awk
-      -| a[4][1][1] = 411
-      -| a[4][2] = 42
-      -| a[1] = 1
-      -| a[2][1] = 21
-      -| a[2][2] = 22
-      -| a[3] = 3
- 
- 
- File: gawk.info,  Node: Sample Programs,  Next: Debugger,  Prev: Library 
Functions,  Up: Top
- 
- 14 Practical `awk' Programs
- ***************************
- 
- *note Library Functions::, presents the idea that reading programs in a
- language contributes to learning that language.  This major node
- continues that theme, presenting a potpourri of `awk' programs for your
- reading enjoyment.
- 
-    Many of these programs use library functions presented in *note
- Library Functions::.
- 
- * Menu:
- 
- * Running Examples::            How to run these examples.
- * Clones::                      Clones of common utilities.
- * Miscellaneous Programs::      Some interesting `awk' programs.
- 
- 
- File: gawk.info,  Node: Running Examples,  Next: Clones,  Up: Sample Programs
- 
- 14.1 Running the Example Programs
- =================================
- 
- To run a given program, you would typically do something like this:
- 
-      awk -f PROGRAM -- OPTIONS FILES
- 
- Here, PROGRAM is the name of the `awk' program (such as `cut.awk'),
- OPTIONS are any command-line options for the program that start with a
- `-', and FILES are the actual data files.
- 
-    If your system supports the `#!' executable interpreter mechanism
- (*note Executable Scripts::), you can instead run your program directly:
- 
-      cut.awk -c1-8 myfiles > results
- 
-    If your `awk' is not `gawk', you may instead need to use this:
- 
-      cut.awk -- -c1-8 myfiles > results
- 
- 
- File: gawk.info,  Node: Clones,  Next: Miscellaneous Programs,  Prev: Running 
Examples,  Up: Sample Programs
- 
- 14.2 Reinventing Wheels for Fun and Profit
- ==========================================
- 
- This minor node presents a number of POSIX utilities implemented in
- `awk'.  Reinventing these programs in `awk' is often enjoyable, because
- the algorithms can be very clearly expressed, and the code is usually
- very concise and simple.  This is true because `awk' does so much for
- you.
- 
-    It should be noted that these programs are not necessarily intended
- to replace the installed versions on your system.  Nor may all of these
- programs be fully compliant with the most recent POSIX standard.  This
- is not a problem; their purpose is to illustrate `awk' language
- programming for "real world" tasks.
- 
-    The programs are presented in alphabetical order.
- 
- * Menu:
- 
- * Cut Program::                 The `cut' utility.
- * Egrep Program::               The `egrep' utility.
- * Id Program::                  The `id' utility.
- * Split Program::               The `split' utility.
- * Tee Program::                 The `tee' utility.
- * Uniq Program::                The `uniq' utility.
- * Wc Program::                  The `wc' utility.
- 
- 
- File: gawk.info,  Node: Cut Program,  Next: Egrep Program,  Up: Clones
- 
- 14.2.1 Cutting out Fields and Columns
- -------------------------------------
- 
- The `cut' utility selects, or "cuts," characters or fields from its
- standard input and sends them to its standard output.  Fields are
- separated by TABs by default, but you may supply a command-line option
- to change the field "delimiter" (i.e., the field-separator character).
- `cut''s definition of fields is less general than `awk''s.
- 
-    A common use of `cut' might be to pull out just the login name of
- logged-on users from the output of `who'.  For example, the following
- pipeline generates a sorted, unique list of the logged-on users:
- 
-      who | cut -c1-8 | sort | uniq
- 
-    The options for `cut' are:
- 
- `-c LIST'
-      Use LIST as the list of characters to cut out.  Items within the
-      list may be separated by commas, and ranges of characters can be
-      separated with dashes.  The list `1-8,15,22-35' specifies
-      characters 1 through 8, 15, and 22 through 35.
- 
- `-f LIST'
-      Use LIST as the list of fields to cut out.
- 
- `-d DELIM'
-      Use DELIM as the field-separator character instead of the TAB
-      character.
- 
- `-s'
-      Suppress printing of lines that do not contain the field delimiter.
- 
-    The `awk' implementation of `cut' uses the `getopt()' library
- function (*note Getopt Function::) and the `join()' library function
- (*note Join Function::).
- 
-    The program begins with a comment describing the options, the library
- functions needed, and a `usage()' function that prints out a usage
- message and exits.  `usage()' is called if invalid arguments are
- supplied:
- 
-      # cut.awk --- implement cut in awk
- 
-      # Options:
-      #    -f list     Cut fields
-      #    -d c        Field delimiter character
-      #    -c list     Cut characters
-      #
-      #    -s          Suppress lines without the delimiter
-      #
-      # Requires getopt() and join() library functions
- 
-      function usage(    e1, e2)
-      {
-          e1 = "usage: cut [-f list] [-d c] [-s] [files...]"
-          e2 = "usage: cut [-c list] [files...]"
-          print e1 > "/dev/stderr"
-          print e2 > "/dev/stderr"
-          exit 1
-      }
- 
- The variables `e1' and `e2' are used so that the function fits nicely
- on the screen.
- 
-    Next comes a `BEGIN' rule that parses the command-line options.  It
- sets `FS' to a single TAB character, because that is `cut''s default
- field separator. The rule then sets the output field separator to be the
- same as the input field separator.  A loop using `getopt()' steps
- through the command-line options.  Exactly one of the variables
- `by_fields' or `by_chars' is set to true, to indicate that processing
- should be done by fields or by characters, respectively.  When cutting
- by characters, the output field separator is set to the null string:
- 
-      BEGIN    \
-      {
-          FS = "\t"    # default
-          OFS = FS
-          while ((c = getopt(ARGC, ARGV, "sf:c:d:")) != -1) {
-              if (c == "f") {
-                  by_fields = 1
-                  fieldlist = Optarg
-              } else if (c == "c") {
-                  by_chars = 1
-                  fieldlist = Optarg
-                  OFS = ""
-              } else if (c == "d") {
-                  if (length(Optarg) > 1) {
-                      printf("Using first character of %s" \
-                             " for delimiter\n", Optarg) > "/dev/stderr"
-                      Optarg = substr(Optarg, 1, 1)
-                  }
-                  FS = Optarg
-                  OFS = FS
-                  if (FS == " ")    # defeat awk semantics
-                      FS = "[ ]"
-              } else if (c == "s")
-                  suppress++
-              else
-                  usage()
-          }
- 
-          # Clear out options
-          for (i = 1; i < Optind; i++)
-              ARGV[i] = ""
- 
-    The code must take special care when the field delimiter is a space.
- Using a single space (`" "') for the value of `FS' is incorrect--`awk'
- would separate fields with runs of spaces, TABs, and/or newlines, and
- we want them to be separated with individual spaces.  Also remember
- that after `getopt()' is through (as described in *note Getopt
- Function::), we have to clear out all the elements of `ARGV' from 1 to
- `Optind', so that `awk' does not try to process the command-line options
- as file names.
- 
-    After dealing with the command-line options, the program verifies
- that the options make sense.  Only one or the other of `-c' and `-f'
- should be used, and both require a field list.  Then the program calls
- either `set_fieldlist()' or `set_charlist()' to pull apart the list of
- fields or characters:
- 
-          if (by_fields && by_chars)
-              usage()
- 
-          if (by_fields == 0 && by_chars == 0)
-              by_fields = 1    # default
- 
-          if (fieldlist == "") {
-              print "cut: needs list for -c or -f" > "/dev/stderr"
-              exit 1
-          }
- 
-          if (by_fields)
-              set_fieldlist()
-          else
-              set_charlist()
-      }
- 
-    `set_fieldlist()' splits the field list apart at the commas into an
- array.  Then, for each element of the array, it looks to see if the
- element is actually a range, and if so, splits it apart.  The function
- checks the range to make sure that the first number is smaller than the
- second.  Each number in the list is added to the `flist' array, which
- simply lists the fields that will be printed.  Normal field splitting
- is used.  The program lets `awk' handle the job of doing the field
- splitting:
- 
-      function set_fieldlist(        n, m, i, j, k, f, g)
-      {
-          n = split(fieldlist, f, ",")
-          j = 1    # index in flist
-          for (i = 1; i <= n; i++) {
-              if (index(f[i], "-") != 0) { # a range
-                  m = split(f[i], g, "-")
-                  if (m != 2 || g[1] >= g[2]) {
-                      printf("bad field list: %s\n",
-                                        f[i]) > "/dev/stderr"
-                      exit 1
-                  }
-                  for (k = g[1]; k <= g[2]; k++)
-                      flist[j++] = k
-              } else
-                  flist[j++] = f[i]
-          }
-          nfields = j - 1
-      }
- 
-    The `set_charlist()' function is more complicated than
- `set_fieldlist()'.  The idea here is to use `gawk''s `FIELDWIDTHS'
- variable (*note Constant Size::), which describes constant-width input.
- When using a character list, that is exactly what we have.
- 
-    Setting up `FIELDWIDTHS' is more complicated than simply listing the
- fields that need to be printed.  We have to keep track of the fields to
- print and also the intervening characters that have to be skipped.  For
- example, suppose you wanted characters 1 through 8, 15, and 22 through
- 35.  You would use `-c 1-8,15,22-35'.  The necessary value for
- `FIELDWIDTHS' is `"8 6 1 6 14"'.  This yields five fields, and the
- fields to print are `$1', `$3', and `$5'.  The intermediate fields are
- "filler", which is stuff in between the desired data.  `flist' lists
- the fields to print, and `t' tracks the complete field list, including
- filler fields:
- 
-      function set_charlist(    field, i, j, f, g, t,
-                                filler, last, len)
-      {
-          field = 1   # count total fields
-          n = split(fieldlist, f, ",")
-          j = 1       # index in flist
-          for (i = 1; i <= n; i++) {
-              if (index(f[i], "-") != 0) { # range
-                  m = split(f[i], g, "-")
-                  if (m != 2 || g[1] >= g[2]) {
-                      printf("bad character list: %s\n",
-                                     f[i]) > "/dev/stderr"
-                      exit 1
-                  }
-                  len = g[2] - g[1] + 1
-                  if (g[1] > 1)  # compute length of filler
-                      filler = g[1] - last - 1
-                  else
-                      filler = 0
-                  if (filler)
-                      t[field++] = filler
-                  t[field++] = len  # length of field
-                  last = g[2]
-                  flist[j++] = field - 1
-              } else {
-                  if (f[i] > 1)
-                      filler = f[i] - last - 1
-                  else
-                      filler = 0
-                  if (filler)
-                      t[field++] = filler
-                  t[field++] = 1
-                  last = f[i]
-                  flist[j++] = field - 1
-              }
-          }
-          FIELDWIDTHS = join(t, 1, field - 1)
-          nfields = j - 1
-      }
- 
-    Next is the rule that actually processes the data.  If the `-s'
- option is given, then `suppress' is true.  The first `if' statement
- makes sure that the input record does have the field separator.  If
- `cut' is processing fields, `suppress' is true, and the field separator
- character is not in the record, then the record is skipped.
- 
-    If the record is valid, then `gawk' has split the data into fields,
- either using the character in `FS' or using fixed-length fields and
- `FIELDWIDTHS'.  The loop goes through the list of fields that should be
- printed.  The corresponding field is printed if it contains data.  If
- the next field also has data, then the separator character is written
- out between the fields:
- 
-      {
-          if (by_fields && suppress && index($0, FS) != 0)
-              next
- 
-          for (i = 1; i <= nfields; i++) {
-              if ($flist[i] != "") {
-                  printf "%s", $flist[i]
-                  if (i < nfields && $flist[i+1] != "")
-                      printf "%s", OFS
-              }
-          }
-          print ""
-      }
- 
-    This version of `cut' relies on `gawk''s `FIELDWIDTHS' variable to
- do the character-based cutting.  While it is possible in other `awk'
- implementations to use `substr()' (*note String Functions::), it is
- also extremely painful.  The `FIELDWIDTHS' variable supplies an elegant
- solution to the problem of picking the input line apart by characters.
- 
- 
- File: gawk.info,  Node: Egrep Program,  Next: Id Program,  Prev: Cut Program, 
 Up: Clones
- 
- 14.2.2 Searching for Regular Expressions in Files
- -------------------------------------------------
- 
- The `egrep' utility searches files for patterns.  It uses regular
- expressions that are almost identical to those available in `awk'
- (*note Regexp::).  You invoke it as follows:
- 
-      egrep [ OPTIONS ] 'PATTERN' FILES ...
- 
-    The PATTERN is a regular expression.  In typical usage, the regular
- expression is quoted to prevent the shell from expanding any of the
- special characters as file name wildcards.  Normally, `egrep' prints
- the lines that matched.  If multiple file names are provided on the
- command line, each output line is preceded by the name of the file and
- a colon.
- 
-    The options to `egrep' are as follows:
- 
- `-c'
-      Print out a count of the lines that matched the pattern, instead
-      of the lines themselves.
- 
- `-s'
-      Be silent.  No output is produced and the exit value indicates
-      whether the pattern was matched.
- 
- `-v'
-      Invert the sense of the test. `egrep' prints the lines that do
-      _not_ match the pattern and exits successfully if the pattern is
-      not matched.
- 
- `-i'
-      Ignore case distinctions in both the pattern and the input data.
- 
- `-l'
-      Only print (list) the names of the files that matched, not the
-      lines that matched.
- 
- `-e PATTERN'
-      Use PATTERN as the regexp to match.  The purpose of the `-e'
-      option is to allow patterns that start with a `-'.
- 
-    This version uses the `getopt()' library function (*note Getopt
- Function::) and the file transition library program (*note Filetrans
- Function::).
- 
-    The program begins with a descriptive comment and then a `BEGIN' rule
- that processes the command-line arguments with `getopt()'.  The `-i'
- (ignore case) option is particularly easy with `gawk'; we just use the
- `IGNORECASE' built-in variable (*note Built-in Variables::):
- 
-      # egrep.awk --- simulate egrep in awk
-      #
-      # Options:
-      #    -c    count of lines
-      #    -s    silent - use exit value
-      #    -v    invert test, success if no match
-      #    -i    ignore case
-      #    -l    print filenames only
-      #    -e    argument is pattern
-      #
-      # Requires getopt and file transition library functions
- 
-      BEGIN {
-          while ((c = getopt(ARGC, ARGV, "ce:svil")) != -1) {
-              if (c == "c")
-                  count_only++
-              else if (c == "s")
-                  no_print++
-              else if (c == "v")
-                  invert++
-              else if (c == "i")
-                  IGNORECASE = 1
-              else if (c == "l")
-                  filenames_only++
-              else if (c == "e")
-                  pattern = Optarg
-              else
-                  usage()
-          }
- 
-    Next comes the code that handles the `egrep'-specific behavior. If no
- pattern is supplied with `-e', the first nonoption on the command line
- is used.  The `awk' command-line arguments up to `ARGV[Optind]' are
- cleared, so that `awk' won't try to process them as files.  If no files
- are specified, the standard input is used, and if multiple files are
- specified, we make sure to note this so that the file names can precede
- the matched lines in the output:
- 
-          if (pattern == "")
-              pattern = ARGV[Optind++]
- 
-          for (i = 1; i < Optind; i++)
-              ARGV[i] = ""
-          if (Optind >= ARGC) {
-              ARGV[1] = "-"
-              ARGC = 2
-          } else if (ARGC - Optind > 1)
-              do_filenames++
- 
-      #    if (IGNORECASE)
-      #        pattern = tolower(pattern)
-      }
- 
-    The last two lines are commented out, since they are not needed in
- `gawk'.  They should be uncommented if you have to use another version
- of `awk'.
- 
-    The next set of lines should be uncommented if you are not using
- `gawk'.  This rule translates all the characters in the input line into
- lowercase if the `-i' option is specified.(1) The rule is commented out
- since it is not necessary with `gawk':
- 
-      #{
-      #    if (IGNORECASE)
-      #        $0 = tolower($0)
-      #}
- 
-    The `beginfile()' function is called by the rule in `ftrans.awk'
- when each new file is processed.  In this case, it is very simple; all
- it does is initialize a variable `fcount' to zero. `fcount' tracks how
- many lines in the current file matched the pattern.  Naming the
- parameter `junk' shows we know that `beginfile()' is called with a
- parameter, but that we're not interested in its value:
- 
-      function beginfile(junk)
-      {
-          fcount = 0
-      }
- 
-    The `endfile()' function is called after each file has been
- processed.  It affects the output only when the user wants a count of
- the number of lines that matched.  `no_print' is true only if the exit
- status is desired.  `count_only' is true if line counts are desired.
- `egrep' therefore only prints line counts if printing and counting are
- enabled.  The output format must be adjusted depending upon the number
- of files to process.  Finally, `fcount' is added to `total', so that we
- know the total number of lines that matched the pattern:
- 
-      function endfile(file)
-      {
-          if (! no_print && count_only) {
-              if (do_filenames)
-                  print file ":" fcount
-              else
-                  print fcount
-          }
- 
-          total += fcount
-      }
- 
-    The following rule does most of the work of matching lines. The
- variable `matches' is true if the line matched the pattern. If the user
- wants lines that did not match, the sense of `matches' is inverted
- using the `!' operator. `fcount' is incremented with the value of
- `matches', which is either one or zero, depending upon a successful or
- unsuccessful match.  If the line does not match, the `next' statement
- just moves on to the next record.
- 
-    A number of additional tests are made, but they are only done if we
- are not counting lines.  First, if the user only wants exit status
- (`no_print' is true), then it is enough to know that _one_ line in this
- file matched, and we can skip on to the next file with `nextfile'.
- Similarly, if we are only printing file names, we can print the file
- name, and then skip to the next file with `nextfile'.  Finally, each
- line is printed, with a leading file name and colon if necessary:
- 
-      {
-          matches = ($0 ~ pattern)
-          if (invert)
-              matches = ! matches
- 
-          fcount += matches    # 1 or 0
- 
-          if (! matches)
-              next
- 
-          if (! count_only) {
-              if (no_print)
-                  nextfile
- 
-              if (filenames_only) {
-                  print FILENAME
-                  nextfile
-              }
- 
-              if (do_filenames)
-                  print FILENAME ":" $0
-              else
-                  print
-          }
-      }
- 
-    The `END' rule takes care of producing the correct exit status. If
- there are no matches, the exit status is one; otherwise it is zero:
- 
-      END    \
-      {
-          if (total == 0)
-              exit 1
-          exit 0
-      }
- 
-    The `usage()' function prints a usage message in case of invalid
- options, and then exits:
- 
-      function usage(    e)
-      {
-          e = "Usage: egrep [-csvil] [-e pat] [files ...]"
-          e = e "\n\tegrep [-csvil] pat [files ...]"
-          print e > "/dev/stderr"
-          exit 1
-      }
- 
-    The variable `e' is used so that the function fits nicely on the
- printed page.
- 
-    Just a note on programming style: you may have noticed that the `END'
- rule uses backslash continuation, with the open brace on a line by
- itself.  This is so that it more closely resembles the way functions
- are written.  Many of the examples in this major node use this style.
- You can decide for yourself if you like writing your `BEGIN' and `END'
- rules this way or not.
- 
-    ---------- Footnotes ----------
- 
-    (1) It also introduces a subtle bug; if a match happens, we output
- the translated line, not the original.
- 
- 
- File: gawk.info,  Node: Id Program,  Next: Split Program,  Prev: Egrep 
Program,  Up: Clones
- 
- 14.2.3 Printing out User Information
- ------------------------------------
- 
- The `id' utility lists a user's real and effective user ID numbers,
- real and effective group ID numbers, and the user's group set, if any.
- `id' only prints the effective user ID and group ID if they are
- different from the real ones.  If possible, `id' also supplies the
- corresponding user and group names.  The output might look like this:
- 
-      $ id
-      -| uid=500(arnold) gid=500(arnold) groups=6(disk),7(lp),19(floppy)
- 
-    This information is part of what is provided by `gawk''s `PROCINFO'
- array (*note Built-in Variables::).  However, the `id' utility provides
- a more palatable output than just individual numbers.
- 
-    Here is a simple version of `id' written in `awk'.  It uses the user
- database library functions (*note Passwd Functions::) and the group
- database library functions (*note Group Functions::):
- 
-    The program is fairly straightforward.  All the work is done in the
- `BEGIN' rule.  The user and group ID numbers are obtained from
- `PROCINFO'.  The code is repetitive.  The entry in the user database
- for the real user ID number is split into parts at the `:'. The name is
- the first field.  Similar code is used for the effective user ID number
- and the group numbers:
- 
-      # id.awk --- implement id in awk
-      #
-      # Requires user and group library functions
-      # output is:
-      # uid=12(foo) euid=34(bar) gid=3(baz) \
-      #             egid=5(blat) groups=9(nine),2(two),1(one)
- 
-      BEGIN    \
-      {
-          uid = PROCINFO["uid"]
-          euid = PROCINFO["euid"]
-          gid = PROCINFO["gid"]
-          egid = PROCINFO["egid"]
- 
-          printf("uid=%d", uid)
-          pw = getpwuid(uid)
-          if (pw != "") {
-              split(pw, a, ":")
-              printf("(%s)", a[1])
-          }
- 
-          if (euid != uid) {
-              printf(" euid=%d", euid)
-              pw = getpwuid(euid)
-              if (pw != "") {
-                  split(pw, a, ":")
-                  printf("(%s)", a[1])
-              }
-          }
- 
-          printf(" gid=%d", gid)
-          pw = getgrgid(gid)
-          if (pw != "") {
-              split(pw, a, ":")
-              printf("(%s)", a[1])
-          }
- 
-          if (egid != gid) {
-              printf(" egid=%d", egid)
-              pw = getgrgid(egid)
-              if (pw != "") {
-                  split(pw, a, ":")
-                  printf("(%s)", a[1])
-              }
-          }
- 
-          for (i = 1; ("group" i) in PROCINFO; i++) {
-              if (i == 1)
-                  printf(" groups=")
-              group = PROCINFO["group" i]
-              printf("%d", group)
-              pw = getgrgid(group)
-              if (pw != "") {
-                  split(pw, a, ":")
-                  printf("(%s)", a[1])
-              }
-              if (("group" (i+1)) in PROCINFO)
-                  printf(",")
-          }
- 
-          print ""
-      }
- 
-    The test in the `for' loop is worth noting.  Any supplementary
- groups in the `PROCINFO' array have the indices `"group1"' through
- `"groupN"' for some N, i.e., the total number of supplementary groups.
- However, we don't know in advance how many of these groups there are.
- 
-    This loop works by starting at one, concatenating the value with
- `"group"', and then using `in' to see if that value is in the array.
- Eventually, `i' is incremented past the last group in the array and the
- loop exits.
- 
-    The loop is also correct if there are _no_ supplementary groups;
- then the condition is false the first time it's tested, and the loop
- body never executes.
- 
- 
- File: gawk.info,  Node: Split Program,  Next: Tee Program,  Prev: Id Program, 
 Up: Clones
- 
- 14.2.4 Splitting a Large File into Pieces
- -----------------------------------------
- 
- The `split' program splits large text files into smaller pieces.  Usage
- is as follows:(1)
- 
-      split [-COUNT] file [ PREFIX ]
- 
-    By default, the output files are named `xaa', `xab', and so on. Each
- file has 1000 lines in it, with the likely exception of the last file.
- To change the number of lines in each file, supply a number on the
- command line preceded with a minus; e.g., `-500' for files with 500
- lines in them instead of 1000.  To change the name of the output files
- to something like `myfileaa', `myfileab', and so on, supply an
- additional argument that specifies the file name prefix.
- 
-    Here is a version of `split' in `awk'. It uses the `ord()' and
- `chr()' functions presented in *note Ordinal Functions::.
- 
-    The program first sets its defaults, and then tests to make sure
- there are not too many arguments.  It then looks at each argument in
- turn.  The first argument could be a minus sign followed by a number.
- If it is, this happens to look like a negative number, so it is made
- positive, and that is the count of lines.  The data file name is
- skipped over and the final argument is used as the prefix for the
- output file names:
- 
-      # split.awk --- do split in awk
-      #
-      # Requires ord() and chr() library functions
-      # usage: split [-num] [file] [outname]
- 
-      BEGIN {
-          outfile = "x"    # default
-          count = 1000
-          if (ARGC > 4)
-              usage()
- 
-          i = 1
-          if (ARGV[i] ~ /^-[[:digit:]]+$/) {
-              count = -ARGV[i]
-              ARGV[i] = ""
-              i++
-          }
-          # test argv in case reading from stdin instead of file
-          if (i in ARGV)
-              i++    # skip data file name
-          if (i in ARGV) {
-              outfile = ARGV[i]
-              ARGV[i] = ""
-          }
- 
-          s1 = s2 = "a"
-          out = (outfile s1 s2)
-      }
- 
-    The next rule does most of the work. `tcount' (temporary count)
- tracks how many lines have been printed to the output file so far. If
- it is greater than `count', it is time to close the current file and
- start a new one.  `s1' and `s2' track the current suffixes for the file
- name. If they are both `z', the file is just too big.  Otherwise, `s1'
- moves to the next letter in the alphabet and `s2' starts over again at
- `a':
- 
-      {
-          if (++tcount > count) {
-              close(out)
-              if (s2 == "z") {
-                  if (s1 == "z") {
-                      printf("split: %s is too large to split\n",
-                             FILENAME) > "/dev/stderr"
-                      exit 1
-                  }
-                  s1 = chr(ord(s1) + 1)
-                  s2 = "a"
-              }
-              else
-                  s2 = chr(ord(s2) + 1)
-              out = (outfile s1 s2)
-              tcount = 1
-          }
-          print > out
-      }
- 
- The `usage()' function simply prints an error message and exits:
- 
-      function usage(   e)
-      {
-          e = "usage: split [-num] [file] [outname]"
-          print e > "/dev/stderr"
-          exit 1
-      }
- 
- The variable `e' is used so that the function fits nicely on the screen.
- 
-    This program is a bit sloppy; it relies on `awk' to automatically
- close the last file instead of doing it in an `END' rule.  It also
- assumes that letters are contiguous in the character set, which isn't
- true for EBCDIC systems.
- 
-    ---------- Footnotes ----------
- 
-    (1) This is the traditional usage. The POSIX usage is different, but
- not relevant for what the program aims to demonstrate.
- 
- 
- File: gawk.info,  Node: Tee Program,  Next: Uniq Program,  Prev: Split 
Program,  Up: Clones
- 
- 14.2.5 Duplicating Output into Multiple Files
- ---------------------------------------------
- 
- The `tee' program is known as a "pipe fitting."  `tee' copies its
- standard input to its standard output and also duplicates it to the
- files named on the command line.  Its usage is as follows:
- 
-      tee [-a] file ...
- 
-    The `-a' option tells `tee' to append to the named files, instead of
- truncating them and starting over.
- 
-    The `BEGIN' rule first makes a copy of all the command-line arguments
- into an array named `copy'.  `ARGV[0]' is not copied, since it is not
- needed.  `tee' cannot use `ARGV' directly, since `awk' attempts to
- process each file name in `ARGV' as input data.
- 
-    If the first argument is `-a', then the flag variable `append' is
- set to true, and both `ARGV[1]' and `copy[1]' are deleted. If `ARGC' is
- less than two, then no file names were supplied and `tee' prints a
- usage message and exits.  Finally, `awk' is forced to read the standard
- input by setting `ARGV[1]' to `"-"' and `ARGC' to two:
- 
-      # tee.awk --- tee in awk
-      #
-      # Copy standard input to all named output files.
-      # Append content if -a option is supplied.
-      #
-      BEGIN    \
-      {
-          for (i = 1; i < ARGC; i++)
-              copy[i] = ARGV[i]
- 
-          if (ARGV[1] == "-a") {
-              append = 1
-              delete ARGV[1]
-              delete copy[1]
-              ARGC--
-          }
-          if (ARGC < 2) {
-              print "usage: tee [-a] file ..." > "/dev/stderr"
-              exit 1
-          }
-          ARGV[1] = "-"
-          ARGC = 2
-      }
- 
-    The following single rule does all the work.  Since there is no
- pattern, it is executed for each line of input.  The body of the rule
- simply prints the line into each file on the command line, and then to
- the standard output:
- 
-      {
-          # moving the if outside the loop makes it run faster
-          if (append)
-              for (i in copy)
-                  print >> copy[i]
-          else
-              for (i in copy)
-                  print > copy[i]
-          print
-      }
- 
- It is also possible to write the loop this way:
- 
-      for (i in copy)
-          if (append)
-              print >> copy[i]
-          else
-              print > copy[i]
- 
- This is more concise but it is also less efficient.  The `if' is tested
- for each record and for each output file.  By duplicating the loop
- body, the `if' is only tested once for each input record.  If there are
- N input records and M output files, the first method only executes N
- `if' statements, while the second executes N`*'M `if' statements.
- 
-    Finally, the `END' rule cleans up by closing all the output files:
- 
-      END    \
-      {
-          for (i in copy)
-              close(copy[i])
-      }
- 
- 
- File: gawk.info,  Node: Uniq Program,  Next: Wc Program,  Prev: Tee Program,  
Up: Clones
- 
- 14.2.6 Printing Nonduplicated Lines of Text
- -------------------------------------------
- 
- The `uniq' utility reads sorted lines of data on its standard input,
- and by default removes duplicate lines.  In other words, it only prints
- unique lines--hence the name.  `uniq' has a number of options. The
- usage is as follows:
- 
-      uniq [-udc [-N]] [+N] [ INPUT FILE [ OUTPUT FILE ]]
- 
-    The options for `uniq' are:
- 
- `-d'
-      Print only repeated lines.
- 
- `-u'
-      Print only nonrepeated lines.
- 
- `-c'
-      Count lines. This option overrides `-d' and `-u'.  Both repeated
-      and nonrepeated lines are counted.
- 
- `-N'
-      Skip N fields before comparing lines.  The definition of fields is
-      similar to `awk''s default: nonwhitespace characters separated by
-      runs of spaces and/or TABs.
- 
- `+N'
-      Skip N characters before comparing lines.  Any fields specified
-      with `-N' are skipped first.
- 
- `INPUT FILE'
-      Data is read from the input file named on the command line,
-      instead of from the standard input.
- 
- `OUTPUT FILE'
-      The generated output is sent to the named output file, instead of
-      to the standard output.
- 
-    Normally `uniq' behaves as if both the `-d' and `-u' options are
- provided.
- 
-    `uniq' uses the `getopt()' library function (*note Getopt Function::)
- and the `join()' library function (*note Join Function::).
- 
-    The program begins with a `usage()' function and then a brief
- outline of the options and their meanings in comments.  The `BEGIN'
- rule deals with the command-line arguments and options. It uses a trick
- to get `getopt()' to handle options of the form `-25', treating such an
- option as the option letter `2' with an argument of `5'. If indeed two
- or more digits are supplied (`Optarg' looks like a number), `Optarg' is
- concatenated with the option digit and then the result is added to zero
- to make it into a number.  If there is only one digit in the option,
- then `Optarg' is not needed. In this case, `Optind' must be decremented
- so that `getopt()' processes it next time.  This code is admittedly a
- bit tricky.
- 
-    If no options are supplied, then the default is taken, to print both
- repeated and nonrepeated lines.  The output file, if provided, is
- assigned to `outputfile'.  Early on, `outputfile' is initialized to the
- standard output, `/dev/stdout':
- 
-      # uniq.awk --- do uniq in awk
-      #
-      # Requires getopt() and join() library functions
- 
-      function usage(    e)
-      {
-          e = "Usage: uniq [-udc [-n]] [+n] [ in [ out ]]"
-          print e > "/dev/stderr"
-          exit 1
-      }
- 
-      # -c    count lines. overrides -d and -u
-      # -d    only repeated lines
-      # -u    only nonrepeated lines
-      # -n    skip n fields
-      # +n    skip n characters, skip fields first
- 
-      BEGIN   \
-      {
-          count = 1
-          outputfile = "/dev/stdout"
-          opts = "udc0:1:2:3:4:5:6:7:8:9:"
-          while ((c = getopt(ARGC, ARGV, opts)) != -1) {
-              if (c == "u")
-                  non_repeated_only++
-              else if (c == "d")
-                  repeated_only++
-              else if (c == "c")
-                  do_count++
-              else if (index("0123456789", c) != 0) {
-                  # getopt requires args to options
-                  # this messes us up for things like -5
-                  if (Optarg ~ /^[[:digit:]]+$/)
-                      fcount = (c Optarg) + 0
-                  else {
-                      fcount = c + 0
-                      Optind--
-                  }
-              } else
-                  usage()
-          }
- 
-          if (ARGV[Optind] ~ /^\+[[:digit:]]+$/) {
-              charcount = substr(ARGV[Optind], 2) + 0
-              Optind++
-          }
- 
-          for (i = 1; i < Optind; i++)
-              ARGV[i] = ""
- 
-          if (repeated_only == 0 && non_repeated_only == 0)
-              repeated_only = non_repeated_only = 1
- 
-          if (ARGC - Optind == 2) {
-              outputfile = ARGV[ARGC - 1]
-              ARGV[ARGC - 1] = ""
-          }
-      }
- 
-    The following function, `are_equal()', compares the current line,
- `$0', to the previous line, `last'.  It handles skipping fields and
- characters.  If no field count and no character count are specified,
- `are_equal()' simply returns one or zero depending upon the result of a
- simple string comparison of `last' and `$0'.  Otherwise, things get more
- complicated.  If fields have to be skipped, each line is broken into an
- array using `split()' (*note String Functions::); the desired fields
- are then joined back into a line using `join()'.  The joined lines are
- stored in `clast' and `cline'.  If no fields are skipped, `clast' and
- `cline' are set to `last' and `$0', respectively.  Finally, if
- characters are skipped, `substr()' is used to strip off the leading
- `charcount' characters in `clast' and `cline'.  The two strings are
- then compared and `are_equal()' returns the result:
- 
-      function are_equal(    n, m, clast, cline, alast, aline)
-      {
-          if (fcount == 0 && charcount == 0)
-              return (last == $0)
- 
-          if (fcount > 0) {
-              n = split(last, alast)
-              m = split($0, aline)
-              clast = join(alast, fcount+1, n)
-              cline = join(aline, fcount+1, m)
-          } else {
-              clast = last
-              cline = $0
-          }
-          if (charcount) {
-              clast = substr(clast, charcount + 1)
-              cline = substr(cline, charcount + 1)
-          }
- 
-          return (clast == cline)
-      }
- 
-    The following two rules are the body of the program.  The first one
- is executed only for the very first line of data.  It sets `last' equal
- to `$0', so that subsequent lines of text have something to be compared
- to.
- 
-    The second rule does the work. The variable `equal' is one or zero,
- depending upon the results of `are_equal()''s comparison. If `uniq' is
- counting repeated lines, and the lines are equal, then it increments
- the `count' variable.  Otherwise, it prints the line and resets `count',
- since the two lines are not equal.
- 
-    If `uniq' is not counting, and if the lines are equal, `count' is
- incremented.  Nothing is printed, since the point is to remove
- duplicates.  Otherwise, if `uniq' is counting repeated lines and more
- than one line is seen, or if `uniq' is counting nonrepeated lines and
- only one line is seen, then the line is printed, and `count' is reset.
- 
-    Finally, similar logic is used in the `END' rule to print the final
- line of input data:
- 
-      NR == 1 {
-          last = $0
-          next
-      }
- 
-      {
-          equal = are_equal()
- 
-          if (do_count) {    # overrides -d and -u
-              if (equal)
-                  count++
-              else {
-                  printf("%4d %s\n", count, last) > outputfile
-                  last = $0
-                  count = 1    # reset
-              }
-              next
-          }
- 
-          if (equal)
-              count++
-          else {
-              if ((repeated_only && count > 1) ||
-                  (non_repeated_only && count == 1))
-                      print last > outputfile
-              last = $0
-              count = 1
-          }
-      }
- 
-      END {
-          if (do_count)
-              printf("%4d %s\n", count, last) > outputfile
-          else if ((repeated_only && count > 1) ||
-                  (non_repeated_only && count == 1))
-              print last > outputfile
-          close(outputfile)
-      }
- 
- 
- File: gawk.info,  Node: Wc Program,  Prev: Uniq Program,  Up: Clones
- 
- 14.2.7 Counting Things
- ----------------------
- 
- The `wc' (word count) utility counts lines, words, and characters in
- one or more input files. Its usage is as follows:
- 
-      wc [-lwc] [ FILES ... ]
- 
-    If no files are specified on the command line, `wc' reads its
- standard input. If there are multiple files, it also prints total
- counts for all the files.  The options and their meanings are shown in
- the following list:
- 
- `-l'
-      Count only lines.
- 
- `-w'
-      Count only words.  A "word" is a contiguous sequence of
-      nonwhitespace characters, separated by spaces and/or TABs.
-      Luckily, this is the normal way `awk' separates fields in its
-      input data.
- 
- `-c'
-      Count only characters.
- 
-    Implementing `wc' in `awk' is particularly elegant, since `awk' does
- a lot of the work for us; it splits lines into words (i.e., fields) and
- counts them, it counts lines (i.e., records), and it can easily tell us
- how long a line is.
- 
-    This program uses the `getopt()' library function (*note Getopt
- Function::) and the file-transition functions (*note Filetrans
- Function::).
- 
-    This version has one notable difference from traditional versions of
- `wc': it always prints the counts in the order lines, words, and
- characters.  Traditional versions note the order of the `-l', `-w', and
- `-c' options on the command line, and print the counts in that order.
- 
-    The `BEGIN' rule does the argument processing.  The variable
- `print_total' is true if more than one file is named on the command
- line:
- 
-      # wc.awk --- count lines, words, characters
- 
-      # Options:
-      #    -l    only count lines
-      #    -w    only count words
-      #    -c    only count characters
-      #
-      # Default is to count lines, words, characters
-      #
-      # Requires getopt() and file transition library functions
- 
-      BEGIN {
-          # let getopt() print a message about
-          # invalid options. we ignore them
-          while ((c = getopt(ARGC, ARGV, "lwc")) != -1) {
-              if (c == "l")
-                  do_lines = 1
-              else if (c == "w")
-                  do_words = 1
-              else if (c == "c")
-                  do_chars = 1
-          }
-          for (i = 1; i < Optind; i++)
-              ARGV[i] = ""
- 
-          # if no options, do all
-          if (! do_lines && ! do_words && ! do_chars)
-              do_lines = do_words = do_chars = 1
- 
-          print_total = (ARGC - i > 2)
-      }
- 
-    The `beginfile()' function is simple; it just resets the counts of
- lines, words, and characters to zero, and saves the current file name in
- `fname':
- 
-      function beginfile(file)
-      {
-          lines = words = chars = 0
-          fname = FILENAME
-      }
- 
-    The `endfile()' function adds the current file's numbers to the
- running totals of lines, words, and characters.(1)  It then prints out
- those numbers for the file that was just read. It relies on
- `beginfile()' to reset the numbers for the following data file:
- 
-      function endfile(file)
-      {
-          tlines += lines
-          twords += words
-          tchars += chars
-          if (do_lines)
-              printf "\t%d", lines
-          if (do_words)
-              printf "\t%d", words
-          if (do_chars)
-              printf "\t%d", chars
-          printf "\t%s\n", fname
-      }
- 
-    There is one rule that is executed for each line. It adds the length
- of the record, plus one, to `chars'.(2) Adding one plus the record
- length is needed because the newline character separating records (the
- value of `RS') is not part of the record itself, and thus not included
- in its length.  Next, `lines' is incremented for each line read, and
- `words' is incremented by the value of `NF', which is the number of
- "words" on this line:
- 
-      # do per line
-      {
-          chars += length($0) + 1    # get newline
-          lines++
-          words += NF
-      }
- 
-    Finally, the `END' rule simply prints the totals for all the files:
- 
-      END {
-          if (print_total) {
-              if (do_lines)
-                  printf "\t%d", tlines
-              if (do_words)
-                  printf "\t%d", twords
-              if (do_chars)
-                  printf "\t%d", tchars
-              print "\ttotal"
-          }
-      }
- 
-    ---------- Footnotes ----------
- 
-    (1) `wc' can't just use the value of `FNR' in `endfile()'. If you
- examine the code in *note Filetrans Function::, you will see that `FNR'
- has already been reset by the time `endfile()' is called.
- 
-    (2) Since `gawk' understands multibyte locales, this code counts
- characters, not bytes.
- 
- 
- File: gawk.info,  Node: Miscellaneous Programs,  Prev: Clones,  Up: Sample 
Programs
- 
- 14.3 A Grab Bag of `awk' Programs
- =================================
- 
- This minor node is a large "grab bag" of miscellaneous programs.  We
- hope you find them both interesting and enjoyable.
- 
- * Menu:
- 
- * Dupword Program::             Finding duplicated words in a document.
- * Alarm Program::               An alarm clock.
- * Translate Program::           A program similar to the `tr' utility.
- * Labels Program::              Printing mailing labels.
- * Word Sorting::                A program to produce a word usage count.
- * History Sorting::             Eliminating duplicate entries from a history
-                                 file.
- * Extract Program::             Pulling out programs from Texinfo source
-                                 files.
- * Simple Sed::                  A Simple Stream Editor.
- * Igawk Program::               A wrapper for `awk' that includes
-                                 files.
- * Anagram Program::             Finding anagrams from a dictionary.
- * Signature Program::           People do amazing things with too much time on
-                                 their hands.
- 
- 
- File: gawk.info,  Node: Dupword Program,  Next: Alarm Program,  Up: 
Miscellaneous Programs
- 
- 14.3.1 Finding Duplicated Words in a Document
- ---------------------------------------------
- 
- A common error when writing large amounts of prose is to accidentally
- duplicate words.  Typically you will see this in text as something like
- "the the program does the following..."  When the text is online, often
- the duplicated words occur at the end of one line and the beginning of
- another, making them very difficult to spot.
- 
-    This program, `dupword.awk', scans through a file one line at a time
- and looks for adjacent occurrences of the same word.  It also saves the
- last word on a line (in the variable `prev') for comparison with the
- first word on the next line.
- 
-    The first two statements make sure that the line is all lowercase,
- so that, for example, "The" and "the" compare equal to each other.  The
- next statement replaces nonalphanumeric and nonwhitespace characters
- with spaces, so that punctuation does not affect the comparison either.
- The characters are replaced with spaces so that formatting controls
- don't create nonsense words (e.g., the Texinfo address@hidden' becomes
- `codeNF' if punctuation is simply deleted).  The record is then resplit
- into fields, yielding just the actual words on the line, and ensuring
- that there are no empty fields.
- 
-    If there are no fields left after removing all the punctuation, the
- current record is skipped.  Otherwise, the program loops through each
- word, comparing it to the previous one:
- 
-      # dupword.awk --- find duplicate words in text
-      {
-          $0 = tolower($0)
-          gsub(/[^[:alnum:][:blank:]]/, " ");
-          $0 = $0         # re-split
-          if (NF == 0)
-              next
-          if ($1 == prev)
-              printf("%s:%d: duplicate %s\n",
-                  FILENAME, FNR, $1)
-          for (i = 2; i <= NF; i++)
-              if ($i == $(i-1))
-                  printf("%s:%d: duplicate %s\n",
-                      FILENAME, FNR, $i)
-          prev = $NF
-      }
- 
- 
- File: gawk.info,  Node: Alarm Program,  Next: Translate Program,  Prev: 
Dupword Program,  Up: Miscellaneous Programs
- 
- 14.3.2 An Alarm Clock Program
- -----------------------------
- 
-      Nothing cures insomnia like a ringing alarm clock.
-      Arnold Robbins
- 
-    The following program is a simple "alarm clock" program.  You give
- it a time of day and an optional message.  At the specified time, it
- prints the message on the standard output. In addition, you can give it
- the number of times to repeat the message as well as a delay between
- repetitions.
- 
-    This program uses the `gettimeofday()' function from *note
- Gettimeofday Function::.
- 
-    All the work is done in the `BEGIN' rule.  The first part is argument
- checking and setting of defaults: the delay, the count, and the message
- to print.  If the user supplied a message without the ASCII BEL
- character (known as the "alert" character, `"\a"'), then it is added to
- the message.  (On many systems, printing the ASCII BEL generates an
- audible alert. Thus when the alarm goes off, the system calls attention
- to itself in case the user is not looking at the computer.)  Just for a
- change, this program uses a `switch' statement (*note Switch
- Statement::), but the processing could be done with a series of
- `if'-`else' statements instead.  Here is the program:
- 
-      # alarm.awk --- set an alarm
-      #
-      # Requires gettimeofday() library function
-      # usage: alarm time [ "message" [ count [ delay ] ] ]
- 
-      BEGIN    \
-      {
-          # Initial argument sanity checking
-          usage1 = "usage: alarm time ['message' [count [delay]]]"
-          usage2 = sprintf("\t(%s) time ::= hh:mm", ARGV[1])
- 
-          if (ARGC < 2) {
-              print usage1 > "/dev/stderr"
-              print usage2 > "/dev/stderr"
-              exit 1
-          }
-          switch (ARGC) {
-          case 5:
-              delay = ARGV[4] + 0
-              # fall through
-          case 4:
-              count = ARGV[3] + 0
-              # fall through
-          case 3:
-              message = ARGV[2]
-              break
-          default:
-              if (ARGV[1] !~ /[[:digit:]]?[[:digit:]]:[[:digit:]]{2}/) {
-                  print usage1 > "/dev/stderr"
-                  print usage2 > "/dev/stderr"
-                  exit 1
-              }
-              break
-          }
- 
-          # set defaults for once we reach the desired time
-          if (delay == 0)
-              delay = 180    # 3 minutes
-          if (count == 0)
-              count = 5
-          if (message == "")
-              message = sprintf("\aIt is now %s!\a", ARGV[1])
-          else if (index(message, "\a") == 0)
-              message = "\a" message "\a"
- 
-    The next minor node of code turns the alarm time into hours and
- minutes, converts it (if necessary) to a 24-hour clock, and then turns
- that time into a count of the seconds since midnight.  Next it turns
- the current time into a count of seconds since midnight.  The
- difference between the two is how long to wait before setting off the
- alarm:
- 
-          # split up alarm time
-          split(ARGV[1], atime, ":")
-          hour = atime[1] + 0    # force numeric
-          minute = atime[2] + 0  # force numeric
- 
-          # get current broken down time
-          gettimeofday(now)
- 
-          # if time given is 12-hour hours and it's after that
-          # hour, e.g., `alarm 5:30' at 9 a.m. means 5:30 p.m.,
-          # then add 12 to real hour
-          if (hour < 12 && now["hour"] > hour)
-              hour += 12
- 
-          # set target time in seconds since midnight
-          target = (hour * 60 * 60) + (minute * 60)
- 
-          # get current time in seconds since midnight
-          current = (now["hour"] * 60 * 60) + \
-                     (now["minute"] * 60) + now["second"]
- 
-          # how long to sleep for
-          naptime = target - current
-          if (naptime <= 0) {
-              print "time is in the past!" > "/dev/stderr"
-              exit 1
-          }
- 
-    Finally, the program uses the `system()' function (*note I/O
- Functions::) to call the `sleep' utility.  The `sleep' utility simply
- pauses for the given number of seconds.  If the exit status is not zero,
- the program assumes that `sleep' was interrupted and exits. If `sleep'
- exited with an OK status (zero), then the program prints the message in
- a loop, again using `sleep' to delay for however many seconds are
- necessary:
- 
-          # zzzzzz..... go away if interrupted
-          if (system(sprintf("sleep %d", naptime)) != 0)
-              exit 1
- 
-          # time to notify!
-          command = sprintf("sleep %d", delay)
-          for (i = 1; i <= count; i++) {
-              print message
-              # if sleep command interrupted, go away
-              if (system(command) != 0)
-                  break
-          }
- 
-          exit 0
-      }
- 
- 
- File: gawk.info,  Node: Translate Program,  Next: Labels Program,  Prev: 
Alarm Program,  Up: Miscellaneous Programs
- 
- 14.3.3 Transliterating Characters
- ---------------------------------
- 
- The system `tr' utility transliterates characters.  For example, it is
- often used to map uppercase letters into lowercase for further
- processing:
- 
-      GENERATE DATA | tr 'A-Z' 'a-z' | PROCESS DATA ...
- 
-    `tr' requires two lists of characters.(1)  When processing the
- input, the first character in the first list is replaced with the first
- character in the second list, the second character in the first list is
- replaced with the second character in the second list, and so on.  If
- there are more characters in the "from" list than in the "to" list, the
- last character of the "to" list is used for the remaining characters in
- the "from" list.
- 
-    Some time ago, a user proposed that a transliteration function should
- be added to `gawk'.  The following program was written to prove that
- character transliteration could be done with a user-level function.
- This program is not as complete as the system `tr' utility but it does
- most of the job.
- 
-    The `translate' program demonstrates one of the few weaknesses of
- standard `awk': dealing with individual characters is very painful,
- requiring repeated use of the `substr()', `index()', and `gsub()'
- built-in functions (*note String Functions::).(2) There are two
- functions.  The first, `stranslate()', takes three arguments:
- 
- `from'
-      A list of characters from which to translate.
- 
- `to'
-      A list of characters to which to translate.
- 
- `target'
-      The string on which to do the translation.
- 
-    Associative arrays make the translation part fairly easy. `t_ar'
- holds the "to" characters, indexed by the "from" characters.  Then a
- simple loop goes through `from', one character at a time.  For each
- character in `from', if the character appears in `target', it is
- replaced with the corresponding `to' character.
- 
-    The `translate()' function simply calls `stranslate()' using `$0' as
- the target.  The main program sets two global variables, `FROM' and
- `TO', from the command line, and then changes `ARGV' so that `awk'
- reads from the standard input.
- 
-    Finally, the processing rule simply calls `translate()' for each
- record:
- 
-      # translate.awk --- do tr-like stuff
-      # Bugs: does not handle things like: tr A-Z a-z, it has
-      # to be spelled out. However, if `to' is shorter than `from',
-      # the last character in `to' is used for the rest of `from'.
- 
-      function stranslate(from, to, target,     lf, lt, ltarget, t_ar, i, c,
-                                                                     result)
-      {
-          lf = length(from)
-          lt = length(to)
-          ltarget = length(target)
-          for (i = 1; i <= lt; i++)
-              t_ar[substr(from, i, 1)] = substr(to, i, 1)
-          if (lt < lf)
-              for (; i <= lf; i++)
-                  t_ar[substr(from, i, 1)] = substr(to, lt, 1)
-          for (i = 1; i <= ltarget; i++) {
-              c = substr(target, i, 1)
-              if (c in t_ar)
-                  c = t_ar[c]
-              result = result c
-          }
-          return result
-      }
- 
-      function translate(from, to)
-      {
-          return $0 = stranslate(from, to, $0)
-      }
- 
-      # main program
-      BEGIN {
-          if (ARGC < 3) {
-              print "usage: translate from to" > "/dev/stderr"
-              exit
-          }
-          FROM = ARGV[1]
-          TO = ARGV[2]
-          ARGC = 2
-          ARGV[1] = "-"
-      }
- 
-      {
-          translate(FROM, TO)
-          print
-      }
- 
-    While it is possible to do character transliteration in a user-level
- function, it is not necessarily efficient, and we (the `gawk' authors)
- started to consider adding a built-in function.  However, shortly after
- writing this program, we learned that the System V Release 4 `awk' had
- added the `toupper()' and `tolower()' functions (*note String
- Functions::).  These functions handle the vast majority of the cases
- where character transliteration is necessary, and so we chose to simply
- add those functions to `gawk' as well and then leave well enough alone.
- 
-    An obvious improvement to this program would be to set up the `t_ar'
- array only once, in a `BEGIN' rule. However, this assumes that the
- "from" and "to" lists will never change throughout the lifetime of the
- program.
- 
-    ---------- Footnotes ----------
- 
-    (1) On some older systems, `tr' may require that the lists be
- written as range expressions enclosed in square brackets (`[a-z]') and
- quoted, to prevent the shell from attempting a file name expansion.
- This is not a feature.
- 
-    (2) This program was written before `gawk' acquired the ability to
- split each character in a string into separate array elements.
- 
- 
- File: gawk.info,  Node: Labels Program,  Next: Word Sorting,  Prev: Translate 
Program,  Up: Miscellaneous Programs
- 
- 14.3.4 Printing Mailing Labels
- ------------------------------
- 
- Here is a "real world"(1) program.  This script reads lists of names and
- addresses and generates mailing labels.  Each page of labels has 20
- labels on it, two across and 10 down.  The addresses are guaranteed to
- be no more than five lines of data.  Each address is separated from the
- next by a blank line.
- 
-    The basic idea is to read 20 labels worth of data.  Each line of
- each label is stored in the `line' array.  The single rule takes care
- of filling the `line' array and printing the page when 20 labels have
- been read.
- 
-    The `BEGIN' rule simply sets `RS' to the empty string, so that `awk'
- splits records at blank lines (*note Records::).  It sets `MAXLINES' to
- 100, since 100 is the maximum number of lines on the page (20 * 5 =
- 100).
- 
-    Most of the work is done in the `printpage()' function.  The label
- lines are stored sequentially in the `line' array.  But they have to
- print horizontally; `line[1]' next to `line[6]', `line[2]' next to
- `line[7]', and so on.  Two loops are used to accomplish this.  The
- outer loop, controlled by `i', steps through every 10 lines of data;
- this is each row of labels.  The inner loop, controlled by `j', goes
- through the lines within the row.  As `j' goes from 0 to 4, `i+j' is
- the `j'-th line in the row, and `i+j+5' is the entry next to it.  The
- output ends up looking something like this:
- 
-      line 1          line 6
-      line 2          line 7
-      line 3          line 8
-      line 4          line 9
-      line 5          line 10
-      ...
- 
- The `printf' format string `%-41s' left-aligns the data and prints it
- within a fixed-width field.
- 
-    As a final note, an extra blank line is printed at lines 21 and 61,
- to keep the output lined up on the labels.  This is dependent on the
- particular brand of labels in use when the program was written.  You
- will also note that there are two blank lines at the top and two blank
- lines at the bottom.
- 
-    The `END' rule arranges to flush the final page of labels; there may
- not have been an even multiple of 20 labels in the data:
- 
-      # labels.awk --- print mailing labels
- 
-      # Each label is 5 lines of data that may have blank lines.
-      # The label sheets have 2 blank lines at the top and 2 at
-      # the bottom.
- 
-      BEGIN    { RS = "" ; MAXLINES = 100 }
- 
-      function printpage(    i, j)
-      {
-          if (Nlines <= 0)
-              return
- 
-          printf "\n\n"        # header
- 
-          for (i = 1; i <= Nlines; i += 10) {
-              if (i == 21 || i == 61)
-                  print ""
-              for (j = 0; j < 5; j++) {
-                  if (i + j > MAXLINES)
-                      break
-                  printf "   %-41s %s\n", line[i+j], line[i+j+5]
-              }
-              print ""
-          }
- 
-          printf "\n\n"        # footer
- 
-          delete line
-      }
- 
-      # main rule
-      {
-          if (Count >= 20) {
-              printpage()
-              Count = 0
-              Nlines = 0
-          }
-          n = split($0, a, "\n")
-          for (i = 1; i <= n; i++)
-              line[++Nlines] = a[i]
-          for (; i <= 5; i++)
-              line[++Nlines] = ""
-          Count++
-      }
- 
-      END    \
-      {
-          printpage()
-      }
- 
-    ---------- Footnotes ----------
- 
-    (1) "Real world" is defined as "a program actually used to get
- something done."
- 
- 
- File: gawk.info,  Node: Word Sorting,  Next: History Sorting,  Prev: Labels 
Program,  Up: Miscellaneous Programs
- 
- 14.3.5 Generating Word-Usage Counts
- -----------------------------------
- 
- When working with large amounts of text, it can be interesting to know
- how often different words appear.  For example, an author may overuse
- certain words, in which case she might wish to find synonyms to
- substitute for words that appear too often. This node develops a
- program for counting words and presenting the frequency information in
- a useful format.
- 
-    At first glance, a program like this would seem to do the job:
- 
-      # Print list of word frequencies
- 
-      {
-          for (i = 1; i <= NF; i++)
-              freq[$i]++
-      }
- 
-      END {
-          for (word in freq)
-              printf "%s\t%d\n", word, freq[word]
-      }
- 
-    The program relies on `awk''s default field splitting mechanism to
- break each line up into "words," and uses an associative array named
- `freq', indexed by each word, to count the number of times the word
- occurs. In the `END' rule, it prints the counts.
- 
-    This program has several problems that prevent it from being useful
- on real text files:
- 
-    * The `awk' language considers upper- and lowercase characters to be
-      distinct.  Therefore, "bartender" and "Bartender" are not treated
-      as the same word.  This is undesirable, since in normal text, words
-      are capitalized if they begin sentences, and a frequency analyzer
-      should not be sensitive to capitalization.
- 
-    * Words are detected using the `awk' convention that fields are
-      separated just by whitespace.  Other characters in the input
-      (except newlines) don't have any special meaning to `awk'.  This
-      means that punctuation characters count as part of words.
- 
-    * The output does not come out in any useful order.  You're more
-      likely to be interested in which words occur most frequently or in
-      having an alphabetized table of how frequently each word occurs.
- 
-    The first problem can be solved by using `tolower()' to remove case
- distinctions.  The second problem can be solved by using `gsub()' to
- remove punctuation characters.  Finally, we solve the third problem by
- using the system `sort' utility to process the output of the `awk'
- script.  Here is the new version of the program:
- 
-      # wordfreq.awk --- print list of word frequencies
- 
-      {
-          $0 = tolower($0)    # remove case distinctions
-          # remove punctuation
-          gsub(/[^[:alnum:]_[:blank:]]/, "", $0)
-          for (i = 1; i <= NF; i++)
-              freq[$i]++
-      }
- 
-      END {
-          for (word in freq)
-              printf "%s\t%d\n", word, freq[word]
-      }
- 
-    Assuming we have saved this program in a file named `wordfreq.awk',
- and that the data is in `file1', the following pipeline:
- 
-      awk -f wordfreq.awk file1 | sort -k 2nr
- 
- produces a table of the words appearing in `file1' in order of
- decreasing frequency.
- 
-    The `awk' program suitably massages the data and produces a word
- frequency table, which is not ordered.  The `awk' script's output is
- then sorted by the `sort' utility and printed on the screen.
- 
-    The options given to `sort' specify a sort that uses the second
- field of each input line (skipping one field), that the sort keys
- should be treated as numeric quantities (otherwise `15' would come
- before `5'), and that the sorting should be done in descending
- (reverse) order.
- 
-    The `sort' could even be done from within the program, by changing
- the `END' action to:
- 
-      END {
-          sort = "sort -k 2nr"
-          for (word in freq)
-              printf "%s\t%d\n", word, freq[word] | sort
-          close(sort)
-      }
- 
-    This way of sorting must be used on systems that do not have true
- pipes at the command-line (or batch-file) level.  See the general
- operating system documentation for more information on how to use the
- `sort' program.
- 
- 
- File: gawk.info,  Node: History Sorting,  Next: Extract Program,  Prev: Word 
Sorting,  Up: Miscellaneous Programs
- 
- 14.3.6 Removing Duplicates from Unsorted Text
- ---------------------------------------------
- 
- The `uniq' program (*note Uniq Program::), removes duplicate lines from
- _sorted_ data.
- 
-    Suppose, however, you need to remove duplicate lines from a data
- file but that you want to preserve the order the lines are in.  A good
- example of this might be a shell history file.  The history file keeps
- a copy of all the commands you have entered, and it is not unusual to
- repeat a command several times in a row.  Occasionally you might want
- to compact the history by removing duplicate entries.  Yet it is
- desirable to maintain the order of the original commands.
- 
-    This simple program does the job.  It uses two arrays.  The `data'
- array is indexed by the text of each line.  For each line, `data[$0]'
- is incremented.  If a particular line has not been seen before, then
- `data[$0]' is zero.  In this case, the text of the line is stored in
- `lines[count]'.  Each element of `lines' is a unique command, and the
- indices of `lines' indicate the order in which those lines are
- encountered.  The `END' rule simply prints out the lines, in order:
- 
-      # histsort.awk --- compact a shell history file
-      # Thanks to Byron Rakitzis for the general idea
- 
-      {
-          if (data[$0]++ == 0)
-              lines[++count] = $0
-      }
- 
-      END {
-          for (i = 1; i <= count; i++)
-              print lines[i]
-      }
- 
-    This program also provides a foundation for generating other useful
- information.  For example, using the following `print' statement in the
- `END' rule indicates how often a particular command is used:
- 
-      print data[lines[i]], lines[i]
- 
-    This works because `data[$0]' is incremented each time a line is
- seen.
- 
- 
- File: gawk.info,  Node: Extract Program,  Next: Simple Sed,  Prev: History 
Sorting,  Up: Miscellaneous Programs
- 
- 14.3.7 Extracting Programs from Texinfo Source Files
- ----------------------------------------------------
- 
- The nodes *note Library Functions::, and *note Sample Programs::, are
- the top level nodes for a large number of `awk' programs.  If you want
- to experiment with these programs, it is tedious to have to type them
- in by hand.  Here we present a program that can extract parts of a
- Texinfo input file into separate files.
- 
- This Info file is written in Texinfo (http://texinfo.org), the GNU
- project's document formatting language.  A single Texinfo source file
- can be used to produce both printed and online documentation.  The
- Texinfo language is described fully, starting with *note (Texinfo)Top::
- texinfo,Texinfo--The GNU Documentation Format.
- 
-    For our purposes, it is enough to know three things about Texinfo
- input files:
- 
-    * The "at" symbol (`@') is special in Texinfo, much as the backslash
-      (`\') is in C or `awk'.  Literal `@' symbols are represented in
-      Texinfo source files as `@@'.
- 
-    * Comments start with either address@hidden' or address@hidden'.  The
-      file-extraction program works by using special comments that start
-      at the beginning of a line.
- 
-    * Lines containing address@hidden' and address@hidden group' commands 
bracket
-      example text that should not be split across a page boundary.
-      (Unfortunately, TeX isn't always smart enough to do things exactly
-      right, so we have to give it some help.)
- 
-    The following program, `extract.awk', reads through a Texinfo source
- file and does two things, based on the special comments.  Upon seeing
- address@hidden system ...', it runs a command, by extracting the command text 
from
- the control line and passing it on to the `system()' function (*note
- I/O Functions::).  Upon seeing address@hidden file FILENAME', each subsequent 
line
- is sent to the file FILENAME, until address@hidden endfile' is encountered.  
The
- rules in `extract.awk' match either address@hidden' or address@hidden' by 
letting the
- `omment' part be optional.  Lines containing address@hidden' and 
address@hidden group'
- are simply removed.  `extract.awk' uses the `join()' library function
- (*note Join Function::).
- 
-    The example programs in the online Texinfo source for `GAWK:
- Effective AWK Programming' (`gawk.texi') have all been bracketed inside
- `file' and `endfile' lines.  The `gawk' distribution uses a copy of
- `extract.awk' to extract the sample programs and install many of them
- in a standard directory where `gawk' can find them.  The Texinfo file
- looks something like this:
- 
-      ...
-      This program has a @code{BEGIN} rule,
-      that prints a nice message:
- 
-      @example
-      @c file examples/messages.awk
-      BEGIN @{ print "Don't panic!" @}
-      @c end file
-      @end example
- 
-      It also prints some final advice:
- 
-      @example
-      @c file examples/messages.awk
-      END @{ print "Always avoid bored archeologists!" @}
-      @c end file
-      @end example
-      ...
- 
-    `extract.awk' begins by setting `IGNORECASE' to one, so that mixed
- upper- and lowercase letters in the directives won't matter.
- 
-    The first rule handles calling `system()', checking that a command is
- given (`NF' is at least three) and also checking that the command exits
- with a zero exit status, signifying OK:
- 
-      # extract.awk --- extract files and run programs
-      #                 from texinfo files
- 
-      BEGIN    { IGNORECASE = 1 }
- 
-      /address@hidden(omment)?[ \t]+system/    \
-      {
-          if (NF < 3) {
-              e = (FILENAME ":" FNR)
-              e = (e  ": badly formed `system' line")
-              print e > "/dev/stderr"
-              next
-          }
-          $1 = ""
-          $2 = ""
-          stat = system($0)
-          if (stat != 0) {
-              e = (FILENAME ":" FNR)
-              e = (e ": warning: system returned " stat)
-              print e > "/dev/stderr"
-          }
-      }
- 
- The variable `e' is used so that the rule fits nicely on the screen.
- 
-    The second rule handles moving data into files.  It verifies that a
- file name is given in the directive.  If the file named is not the
- current file, then the current file is closed.  Keeping the current file
- open until a new file is encountered allows the use of the `>'
- redirection for printing the contents, keeping open file management
- simple.
- 
-    The `for' loop does the work.  It reads lines using `getline' (*note
- Getline::).  For an unexpected end of file, it calls the
- `unexpected_eof()' function.  If the line is an "endfile" line, then it
- breaks out of the loop.  If the line is an address@hidden' or address@hidden 
group'
- line, then it ignores it and goes on to the next line.  Similarly,
- comments within examples are also ignored.
- 
-    Most of the work is in the following few lines.  If the line has no
- `@' symbols, the program can print it directly.  Otherwise, each
- leading `@' must be stripped off.  To remove the `@' symbols, the line
- is split into separate elements of the array `a', using the `split()'
- function (*note String Functions::).  The `@' symbol is used as the
- separator character.  Each element of `a' that is empty indicates two
- successive `@' symbols in the original line.  For each two empty
- elements (`@@' in the original file), we have to add a single `@'
- symbol back in.(1)
- 
-    When the processing of the array is finished, `join()' is called
- with the value of `SUBSEP', to rejoin the pieces back into a single
- line.  That line is then printed to the output file:
- 
-      /address@hidden(omment)?[ \t]+file/    \
-      {
-          if (NF != 3) {
-              e = (FILENAME ":" FNR ": badly formed `file' line")
-              print e > "/dev/stderr"
-              next
-          }
-          if ($3 != curfile) {
-              if (curfile != "")
-                  close(curfile)
-              curfile = $3
-          }
- 
-          for (;;) {
-              if ((getline line) <= 0)
-                  unexpected_eof()
-              if (line ~ /address@hidden(omment)?[ \t]+endfile/)
-                  break
-              else if (line ~ /^@(end[ \t]+)?group/)
-                  continue
-              else if (line ~ /address@hidden(omment+)?[ \t]+/)
-                  continue
-              if (index(line, "@") == 0) {
-                  print line > curfile
-                  continue
-              }
-              n = split(line, a, "@")
-              # if a[1] == "", means leading @,
-              # don't add one back in.
-              for (i = 2; i <= n; i++) {
-                  if (a[i] == "") { # was an @@
-                      a[i] = "@"
-                      if (a[i+1] == "")
-                          i++
-                  }
-              }
-              print join(a, 1, n, SUBSEP) > curfile
-          }
-      }
- 
-    An important thing to note is the use of the `>' redirection.
- Output done with `>' only opens the file once; it stays open and
- subsequent output is appended to the file (*note Redirection::).  This
- makes it easy to mix program text and explanatory prose for the same
- sample source file (as has been done here!) without any hassle.  The
- file is only closed when a new data file name is encountered or at the
- end of the input file.
- 
-    Finally, the function `unexpected_eof()' prints an appropriate error
- message and then exits.  The `END' rule handles the final cleanup,
- closing the open file:
- 
-      function unexpected_eof()
-      {
-          printf("%s:%d: unexpected EOF or error\n",
-              FILENAME, FNR) > "/dev/stderr"
-          exit 1
-      }
- 
-      END {
-          if (curfile)
-              close(curfile)
-      }
- 
-    ---------- Footnotes ----------
- 
-    (1) This program was written before `gawk' had the `gensub()'
- function. Consider how you might use it to simplify the code.
- 
- 
- File: gawk.info,  Node: Simple Sed,  Next: Igawk Program,  Prev: Extract 
Program,  Up: Miscellaneous Programs
- 
- 14.3.8 A Simple Stream Editor
- -----------------------------
- 
- The `sed' utility is a stream editor, a program that reads a stream of
- data, makes changes to it, and passes it on.  It is often used to make
- global changes to a large file or to a stream of data generated by a
- pipeline of commands.  While `sed' is a complicated program in its own
- right, its most common use is to perform global substitutions in the
- middle of a pipeline:
- 
-      command1 < orig.data | sed 's/old/new/g' | command2 > result
- 
-    Here, `s/old/new/g' tells `sed' to look for the regexp `old' on each
- input line and globally replace it with the text `new', i.e., all the
- occurrences on a line.  This is similar to `awk''s `gsub()' function
- (*note String Functions::).
- 
-    The following program, `awksed.awk', accepts at least two
- command-line arguments: the pattern to look for and the text to replace
- it with. Any additional arguments are treated as data file names to
- process. If none are provided, the standard input is used:
- 
-      # awksed.awk --- do s/foo/bar/g using just print
-      #    Thanks to Michael Brennan for the idea
- 
-      function usage()
-      {
-          print "usage: awksed pat repl [files...]" > "/dev/stderr"
-          exit 1
-      }
- 
-      BEGIN {
-          # validate arguments
-          if (ARGC < 3)
-              usage()
- 
-          RS = ARGV[1]
-          ORS = ARGV[2]
- 
-          # don't use arguments as files
-          ARGV[1] = ARGV[2] = ""
-      }
- 
-      # look ma, no hands!
-      {
-          if (RT == "")
-              printf "%s", $0
-          else
-              print
-      }
- 
-    The program relies on `gawk''s ability to have `RS' be a regexp, as
- well as on the setting of `RT' to the actual text that terminates the
- record (*note Records::).
- 
-    The idea is to have `RS' be the pattern to look for. `gawk'
- automatically sets `$0' to the text between matches of the pattern.
- This is text that we want to keep, unmodified.  Then, by setting `ORS'
- to the replacement text, a simple `print' statement outputs the text we
- want to keep, followed by the replacement text.
- 
-    There is one wrinkle to this scheme, which is what to do if the last
- record doesn't end with text that matches `RS'.  Using a `print'
- statement unconditionally prints the replacement text, which is not
- correct.  However, if the file did not end in text that matches `RS',
- `RT' is set to the null string.  In this case, we can print `$0' using
- `printf' (*note Printf::).
- 
-    The `BEGIN' rule handles the setup, checking for the right number of
- arguments and calling `usage()' if there is a problem. Then it sets
- `RS' and `ORS' from the command-line arguments and sets `ARGV[1]' and
- `ARGV[2]' to the null string, so that they are not treated as file names
- (*note ARGC and ARGV::).
- 
-    The `usage()' function prints an error message and exits.  Finally,
- the single rule handles the printing scheme outlined above, using
- `print' or `printf' as appropriate, depending upon the value of `RT'.
- 
- 
- File: gawk.info,  Node: Igawk Program,  Next: Anagram Program,  Prev: Simple 
Sed,  Up: Miscellaneous Programs
- 
- 14.3.9 An Easy Way to Use Library Functions
- -------------------------------------------
- 
- In *note Include Files::, we saw how `gawk' provides a built-in
- file-inclusion capability.  However, this is a `gawk' extension.  This
- minor node provides the motivation for making file inclusion available
- for standard `awk', and shows how to do it using a combination of shell
- and `awk' programming.
- 
-    Using library functions in `awk' can be very beneficial. It
- encourages code reuse and the writing of general functions. Programs are
- smaller and therefore clearer.  However, using library functions is
- only easy when writing `awk' programs; it is painful when running them,
- requiring multiple `-f' options.  If `gawk' is unavailable, then so too
- is the `AWKPATH' environment variable and the ability to put `awk'
- functions into a library directory (*note Options::).  It would be nice
- to be able to write programs in the following manner:
- 
-      # library functions
-      @include getopt.awk
-      @include join.awk
-      ...
- 
-      # main program
-      BEGIN {
-          while ((c = getopt(ARGC, ARGV, "a:b:cde")) != -1)
-              ...
-          ...
-      }
- 
-    The following program, `igawk.sh', provides this service.  It
- simulates `gawk''s searching of the `AWKPATH' variable and also allows
- "nested" includes; i.e., a file that is included with address@hidden' can
- contain further address@hidden' statements.  `igawk' makes an effort to only
- include files once, so that nested includes don't accidentally include
- a library function twice.
- 
-    `igawk' should behave just like `gawk' externally.  This means it
- should accept all of `gawk''s command-line arguments, including the
- ability to have multiple source files specified via `-f', and the
- ability to mix command-line and library source files.
- 
-    The program is written using the POSIX Shell (`sh') command
- language.(1) It works as follows:
- 
-   1. Loop through the arguments, saving anything that doesn't represent
-      `awk' source code for later, when the expanded program is run.
- 
-   2. For any arguments that do represent `awk' text, put the arguments
-      into a shell variable that will be expanded.  There are two cases:
- 
-        a. Literal text, provided with `--source' or `--source='.  This
-           text is just appended directly.
- 
-        b. Source file names, provided with `-f'.  We use a neat trick
-           and append address@hidden FILENAME' to the shell variable's
-           contents.  Since the file-inclusion program works the way
-           `gawk' does, this gets the text of the file included into the
-           program at the correct point.
- 
-   3. Run an `awk' program (naturally) over the shell variable's
-      contents to expand address@hidden' statements.  The expanded program is
-      placed in a second shell variable.
- 
-   4. Run the expanded program with `gawk' and any other original
-      command-line arguments that the user supplied (such as the data
-      file names).
- 
-    This program uses shell variables extensively: for storing
- command-line arguments, the text of the `awk' program that will expand
- the user's program, for the user's original program, and for the
- expanded program.  Doing so removes some potential problems that might
- arise were we to use temporary files instead, at the cost of making the
- script somewhat more complicated.
- 
-    The initial part of the program turns on shell tracing if the first
- argument is `debug'.
- 
-    The next part loops through all the command-line arguments.  There
- are several cases of interest:
- 
- `--'
-      This ends the arguments to `igawk'.  Anything else should be
-      passed on to the user's `awk' program without being evaluated.
- 
- `-W'
-      This indicates that the next option is specific to `gawk'.  To make
-      argument processing easier, the `-W' is appended to the front of
-      the remaining arguments and the loop continues.  (This is an `sh'
-      programming trick.  Don't worry about it if you are not familiar
-      with `sh'.)
- 
- `-v, -F'
-      These are saved and passed on to `gawk'.
- 
- `-f, --file, --file=, -Wfile='
-      The file name is appended to the shell variable `program' with an
-      address@hidden' statement.  The `expr' utility is used to remove the
-      leading option part of the argument (e.g., `--file=').  (Typical
-      `sh' usage would be to use the `echo' and `sed' utilities to do
-      this work.  Unfortunately, some versions of `echo' evaluate escape
-      sequences in their arguments, possibly mangling the program text.
-      Using `expr' avoids this problem.)
- 
- `--source, --source=, -Wsource='
-      The source text is appended to `program'.
- 
- `--version, -Wversion'
-      `igawk' prints its version number, runs `gawk --version' to get
-      the `gawk' version information, and then exits.
- 
-    If none of the `-f', `--file', `-Wfile', `--source', or `-Wsource'
- arguments are supplied, then the first nonoption argument should be the
- `awk' program.  If there are no command-line arguments left, `igawk'
- prints an error message and exits.  Otherwise, the first argument is
- appended to `program'.  In any case, after the arguments have been
- processed, `program' contains the complete text of the original `awk'
- program.
- 
-    The program is as follows:
- 
-      #! /bin/sh
-      # igawk --- like gawk but do @include processing
- 
-      if [ "$1" = debug ]
-      then
-          set -x
-          shift
-      fi
- 
-      # A literal newline, so that program text is formatted correctly
-      n='
-      '
- 
-      # Initialize variables to empty
-      program=
-      opts=
- 
-      while [ $# -ne 0 ] # loop over arguments
-      do
-          case $1 in
-          --)     shift
-                  break ;;
- 
-          -W)     shift
-                  # The ${x?'message here'} construct prints a
-                  # diagnostic if $x is the null string
-                  set -- -W"address@hidden'missing operand'}"
-                  continue ;;
- 
-          -[vF])  opts="$opts $1 '${2?'missing operand'}'"
-                  shift ;;
- 
-          -[vF]*) opts="$opts '$1'" ;;
- 
-          -f)     program="address@hidden ${2?'missing operand'}"
-                  shift ;;
- 
-          -f*)    f=$(expr "$1" : '-f\(.*\)')
-                  program="address@hidden $f" ;;
- 
-          -[W-]file=*)
-                  f=$(expr "$1" : '-.file=\(.*\)')
-                  program="address@hidden $f" ;;
- 
-          -[W-]file)
-                  program="address@hidden ${2?'missing operand'}"
-                  shift ;;
- 
-          -[W-]source=*)
-                  t=$(expr "$1" : '-.source=\(.*\)')
-                  program="$program$n$t" ;;
- 
-          -[W-]source)
-                  program="$program$n${2?'missing operand'}"
-                  shift ;;
- 
-          -[W-]version)
-                  echo igawk: version 3.0 1>&2
-                  gawk --version
-                  exit 0 ;;
- 
-          -[W-]*) opts="$opts '$1'" ;;
- 
-          *)      break ;;
-          esac
-          shift
-      done
- 
-      if [ -z "$program" ]
-      then
-           program=${1?'missing program'}
-           shift
-      fi
- 
-      # At this point, `program' has the program.
- 
-    The `awk' program to process address@hidden' directives is stored in the
- shell variable `expand_prog'.  Doing this keeps the shell script
- readable.  The `awk' program reads through the user's program, one line
- at a time, using `getline' (*note Getline::).  The input file names and
- address@hidden' statements are managed using a stack.  As each 
address@hidden' is
- encountered, the current file name is "pushed" onto the stack and the
- file named in the address@hidden' directive becomes the current file name.
- As each file is finished, the stack is "popped," and the previous input
- file becomes the current input file again.  The process is started by
- making the original file the first one on the stack.
- 
-    The `pathto()' function does the work of finding the full path to a
- file.  It simulates `gawk''s behavior when searching the `AWKPATH'
- environment variable (*note AWKPATH Variable::).  If a file name has a
- `/' in it, no path search is done.  Similarly, if the file name is
- `"-"', then that string is used as-is.  Otherwise, the file name is
- concatenated with the name of each directory in the path, and an
- attempt is made to open the generated file name.  The only way to test
- if a file can be read in `awk' is to go ahead and try to read it with
- `getline'; this is what `pathto()' does.(2) If the file can be read, it
- is closed and the file name is returned:
- 
-      expand_prog='
- 
-      function pathto(file,    i, t, junk)
-      {
-          if (index(file, "/") != 0)
-              return file
- 
-          if (file == "-")
-              return file
- 
-          for (i = 1; i <= ndirs; i++) {
-              t = (pathlist[i] "/" file)
-              if ((getline junk < t) > 0) {
-                  # found it
-                  close(t)
-                  return t
-              }
-          }
-          return ""
-      }
- 
-    The main program is contained inside one `BEGIN' rule.  The first
- thing it does is set up the `pathlist' array that `pathto()' uses.
- After splitting the path on `:', null elements are replaced with `"."',
- which represents the current directory:
- 
-      BEGIN {
-          path = ENVIRON["AWKPATH"]
-          ndirs = split(path, pathlist, ":")
-          for (i = 1; i <= ndirs; i++) {
-              if (pathlist[i] == "")
-                  pathlist[i] = "."
-          }
- 
-    The stack is initialized with `ARGV[1]', which will be `/dev/stdin'.
- The main loop comes next.  Input lines are read in succession. Lines
- that do not start with address@hidden' are printed verbatim.  If the line
- does start with address@hidden', the file name is in `$2'.  `pathto()' is
- called to generate the full path.  If it cannot, then the program
- prints an error message and continues.
- 
-    The next thing to check is if the file is included already.  The
- `processed' array is indexed by the full file name of each included
- file and it tracks this information for us.  If the file is seen again,
- a warning message is printed. Otherwise, the new file name is pushed
- onto the stack and processing continues.
- 
-    Finally, when `getline' encounters the end of the input file, the
- file is closed and the stack is popped.  When `stackptr' is less than
- zero, the program is done:
- 
-          stackptr = 0
-          input[stackptr] = ARGV[1] # ARGV[1] is first file
- 
-          for (; stackptr >= 0; stackptr--) {
-              while ((getline < input[stackptr]) > 0) {
-                  if (tolower($1) != "@include") {
-                      print
-                      continue
-                  }
-                  fpath = pathto($2)
-                  if (fpath == "") {
-                      printf("igawk:%s:%d: cannot find %s\n",
-                          input[stackptr], FNR, $2) > "/dev/stderr"
-                      continue
-                  }
-                  if (! (fpath in processed)) {
-                      processed[fpath] = input[stackptr]
-                      input[++stackptr] = fpath  # push onto stack
-                  } else
-                      print $2, "included in", input[stackptr],
-                          "already included in",
-                          processed[fpath] > "/dev/stderr"
-              }
-              close(input[stackptr])
-          }
-      }'  # close quote ends `expand_prog' variable
- 
-      processed_program=$(gawk -- "$expand_prog" /dev/stdin << EOF
-      $program
-      EOF
-      )
- 
-    The shell construct `COMMAND << MARKER' is called a "here document".
- Everything in the shell script up to the MARKER is fed to COMMAND as
- input.  The shell processes the contents of the here document for
- variable and command substitution (and possibly other things as well,
- depending upon the shell).
- 
-    The shell construct `$(...)' is called "command substitution".  The
- output of the command inside the parentheses is substituted into the
- command line.  Because the result is used in a variable assignment, it
- is saved as a single string, even if the results contain whitespace.
- 
-    The expanded program is saved in the variable `processed_program'.
- It's done in these steps:
- 
-   1. Run `gawk' with the address@hidden'-processing program (the value of
-      the `expand_prog' shell variable) on standard input.
- 
-   2. Standard input is the contents of the user's program, from the
-      shell variable `program'.  Its contents are fed to `gawk' via a
-      here document.
- 
-   3. The results of this processing are saved in the shell variable
-      `processed_program' by using command substitution.
- 
-    The last step is to call `gawk' with the expanded program, along
- with the original options and command-line arguments that the user
- supplied.
- 
-      eval gawk $opts -- '"$processed_program"' '"$@"'
- 
-    The `eval' command is a shell construct that reruns the shell's
- parsing process.  This keeps things properly quoted.
- 
-    This version of `igawk' represents my fifth version of this program.
- There are four key simplifications that make the program work better:
- 
-    * Using address@hidden' even for the files named with `-f' makes building
-      the initial collected `awk' program much simpler; all the
-      address@hidden' processing can be done once.
- 
-    * Not trying to save the line read with `getline' in the `pathto()'
-      function when testing for the file's accessibility for use with
-      the main program simplifies things considerably.
- 
-    * Using a `getline' loop in the `BEGIN' rule does it all in one
-      place.  It is not necessary to call out to a separate loop for
-      processing nested address@hidden' statements.
- 
-    * Instead of saving the expanded program in a temporary file,
-      putting it in a shell variable avoids some potential security
-      problems.  This has the disadvantage that the script relies upon
-      more features of the `sh' language, making it harder to follow for
-      those who aren't familiar with `sh'.
- 
-    Also, this program illustrates that it is often worthwhile to combine
- `sh' and `awk' programming together.  You can usually accomplish quite
- a lot, without having to resort to low-level programming in C or C++,
- and it is frequently easier to do certain kinds of string and argument
- manipulation using the shell than it is in `awk'.
- 
-    Finally, `igawk' shows that it is not always necessary to add new
- features to a program; they can often be layered on top.
- 
-    As an additional example of this, consider the idea of having two
- files in a directory in the search path:
- 
- `default.awk'
-      This file contains a set of default library functions, such as
-      `getopt()' and `assert()'.
- 
- `site.awk'
-      This file contains library functions that are specific to a site or
-      installation; i.e., locally developed functions.  Having a
-      separate file allows `default.awk' to change with new `gawk'
-      releases, without requiring the system administrator to update it
-      each time by adding the local functions.
- 
-    One user suggested that `gawk' be modified to automatically read
- these files upon startup.  Instead, it would be very simple to modify
- `igawk' to do this. Since `igawk' can process nested address@hidden'
- directives, `default.awk' could simply contain address@hidden' statements
- for the desired library functions.
- 
-    ---------- Footnotes ----------
- 
-    (1) Fully explaining the `sh' language is beyond the scope of this
- book. We provide some minimal explanations, but see a good shell
- programming book if you wish to understand things in more depth.
- 
-    (2) On some very old versions of `awk', the test `getline junk < t'
- can loop forever if the file exists but is empty.  Caveat emptor.
- 
- 
- File: gawk.info,  Node: Anagram Program,  Next: Signature Program,  Prev: 
Igawk Program,  Up: Miscellaneous Programs
- 
- 14.3.10 Finding Anagrams From A Dictionary
- ------------------------------------------
- 
- An interesting programming challenge is to search for "anagrams" in a
- word list (such as `/usr/share/dict/words' on many GNU/Linux systems).
- One word is an anagram of another if both words contain the same letters
- (for example, "babbling" and "blabbing").
- 
-    An elegant algorithm is presented in Column 2, Problem C of Jon
- Bentley's `Programming Pearls', second edition.  The idea is to give
- words that are anagrams a common signature, sort all the words together
- by their signature, and then print them.  Dr. Bentley observes that
- taking the letters in each word and sorting them produces that common
- signature.
- 
-    The following program uses arrays of arrays to bring together words
- with the same signature and array sorting to print the words in sorted
- order.
- 
-      # anagram.awk --- An implementation of the anagram finding algorithm
-      #                 from Jon Bentley's "Programming Pearls", 2nd edition.
-      #                 Addison Wesley, 2000, ISBN 0-201-65788-0.
-      #                 Column 2, Problem C, section 2.8, pp 18-20.
- 
-      /'s$/   { next }        # Skip possessives
- 
-    The program starts with a header, and then a rule to skip
- possessives in the dictionary file. The next rule builds up the data
- structure. The first dimension of the array is indexed by the
- signature; the second dimension is the word itself:
- 
-      {
-          key = word2key($1)  # Build signature
-          data[key][$1] = $1  # Store word with signature
-      }
- 
-    The `word2key()' function creates the signature.  It splits the word
- apart into individual letters, sorts the letters, and then joins them
- back together:
- 
-      # word2key --- split word apart into letters, sort, joining back together
- 
-      function word2key(word,     a, i, n, result)
-      {
-          n = split(word, a, "")
-          asort(a)
- 
-          for (i = 1; i <= n; i++)
-              result = result a[i]
- 
-          return result
-      }
- 
-    Finally, the `END' rule traverses the array and prints out the
- anagram lists.  It sends the output to the system `sort' command, since
- otherwise the anagrams would appear in arbitrary order:
- 
-      END {
-          sort = "sort"
-          for (key in data) {
-              # Sort words with same key
-              nwords = asorti(data[key], words)
-              if (nwords == 1)
-                  continue
- 
-              # And print. Minor glitch: trailing space at end of each line
-              for (j = 1; j <= nwords; j++)
-                  printf("%s ", words[j]) | sort
-              print "" | sort
-          }
-          close(sort)
-      }
- 
-    Here is some partial output when the program is run:
- 
-      $ gawk -f anagram.awk /usr/share/dict/words | grep '^b'
-      ...
-      babbled blabbed
-      babbler blabber brabble
-      babblers blabbers brabbles
-      babbling blabbing
-      babbly blabby
-      babel bable
-      babels beslab
-      babery yabber
-      ...
- 
- 
- File: gawk.info,  Node: Signature Program,  Prev: Anagram Program,  Up: 
Miscellaneous Programs
- 
- 14.3.11 And Now For Something Completely Different
- --------------------------------------------------
- 
- The following program was written by Davide Brini and is published on
- his website (http://backreference.org/2011/02/03/obfuscated-awk/).  It
- serves as his signature in the Usenet group `comp.lang.awk'.  He
- supplies the following copyright terms:
- 
-      Copyright (C) 2008 Davide Brini
- 
-      Copying and distribution of the code published in this page, with
-      or without modification, are permitted in any medium without
-      royalty provided the copyright notice and this notice are
-      preserved.
- 
-    Here is the program:
- 
-      awk 'BEGIN{O="~"~"~";o="=="=="==";o+=+o;x=O""O;while(X++<=x+o+o)c=c"%c";
-      printf c,(x-O)*(x-O),x*(x-o)-o,x*(x-O)+x-O-o,+x*(x-O)-x+o,X*(o*o+O)+x-O,
-      X*(X-x)-o*o,(x+X)*o*o+o,x*(X-x)-O-O,x-O+(O+o+X+x)*(o+O),X*X-X*(x-O)-x+O,
-      O+X*(o*(o+O)+O),+x+O+X*o,x*(x-o),(o+X+x)*o*o-(x-O-O),O+(X-x)*(X+O),x-O}'
- 
-    We leave it to you to determine what the program does.
- 
- 
- File: gawk.info,  Node: Debugger,  Next: Language History,  Prev: Sample 
Programs,  Up: Top
- 
- 15 Debugging `awk' Programs
- ***************************
- 
- It would be nice if computer programs worked perfectly the first time
- they were run, but in real life, this rarely happens for programs of
- any complexity.  Thus, most programming languages have facilities
- available for "debugging" programs, and now `awk' is no exception.
- 
-    The `gawk' debugger is purposely modeled after the GNU Debugger
- (GDB) (http://www.gnu.org/software/gdb/) command-line debugger.  If you
- are familiar with GDB, learning how to use `gawk' for debugging your
- program is easy.
- 
- * Menu:
- 
- * Debugging::                   Introduction to `gawk' debugger.
- * Sample Debugging Session::    Sample debugging session.
- * List of Debugger Commands::   Main debugger commands.
- * Readline Support::            Readline support.
- * Limitations::                 Limitations and future plans.
- 
- 
- File: gawk.info,  Node: Debugging,  Next: Sample Debugging Session,  Up: 
Debugger
- 
- 15.1 Introduction to `gawk' Debugger
- ====================================
- 
- This minor node introduces debugging in general and begins the
- discussion of debugging in `gawk'.
- 
- * Menu:
- 
- * Debugging Concepts::          Debugging in General.
- * Debugging Terms::             Additional Debugging Concepts.
- * Awk Debugging::               Awk Debugging.
- 
- 
- File: gawk.info,  Node: Debugging Concepts,  Next: Debugging Terms,  Up: 
Debugging
- 
- 15.1.1 Debugging in General
- ---------------------------
- 
- (If you have used debuggers in other languages, you may want to skip
- ahead to the next section on the specific features of the `awk'
- debugger.)
- 
-    Of course, a debugging program cannot remove bugs for you, since it
- has no way of knowing what you or your users consider a "bug" and what
- is a "feature."  (Sometimes, we humans have a hard time with this
- ourselves.)  In that case, what can you expect from such a tool?  The
- answer to that depends on the language being debugged, but in general,
- you can expect at least the following:
- 
-    * The ability to watch a program execute its instructions one by one,
-      giving you, the programmer, the opportunity to think about what is
-      happening on a time scale of seconds, minutes, or hours, rather
-      than the nanosecond time scale at which the code usually runs.
- 
-    * The opportunity to not only passively observe the operation of your
-      program, but to control it and try different paths of execution,
-      without having to change your source files.
- 
-    * The chance to see the values of data in the program at any point in
-      execution, and also to change that data on the fly, to see how that
-      affects what happens afterwards.  (This often includes the ability
-      to look at internal data structures besides the variables you
-      actually defined in your code.)
- 
-    * The ability to obtain additional information about your program's
-      state or even its internal structure.
- 
-    All of these tools provide a great amount of help in using your own
- skills and understanding of the goals of your program to find where it
- is going wrong (or, for that matter, to better comprehend a perfectly
- functional program that you or someone else wrote).
- 
- 
- File: gawk.info,  Node: Debugging Terms,  Next: Awk Debugging,  Prev: 
Debugging Concepts,  Up: Debugging
- 
- 15.1.2 Additional Debugging Concepts
- ------------------------------------
- 
- Before diving in to the details, we need to introduce several important
- concepts that apply to just about all debuggers.  The following list
- defines terms used throughout the rest of this major node.
- 
- "Stack Frame"
-      Programs generally call functions during the course of their
-      execution.  One function can call another, or a function can call
-      itself (recursion).  You can view the chain of called functions
-      (main program calls A, which calls B, which calls C), as a stack
-      of executing functions: the currently running function is the
-      topmost one on the stack, and when it finishes (returns), the next
-      one down then becomes the active function.  Such a stack is termed
-      a "call stack".
- 
-      For each function on the call stack, the system maintains a data
-      area that contains the function's parameters, local variables, and
-      return value, as well as any other "bookkeeping" information
-      needed to manage the call stack.  This data area is termed a
-      "stack frame".
- 
-      `gawk' also follows this model, and gives you access to the call
-      stack and to each stack frame. You can see the call stack, as well
-      as from where each function on the stack was invoked. Commands
-      that print the call stack print information about each stack frame
-      (as detailed later on).
- 
- "Breakpoint"
-      During debugging, you often wish to let the program run until it
-      reaches a certain point, and then continue execution from there one
-      statement (or instruction) at a time.  The way to do this is to set
-      a "breakpoint" within the program.  A breakpoint is where the
-      execution of the program should break off (stop), so that you can
-      take over control of the program's execution.  You can add and
-      remove as many breakpoints as you like.
- 
- "Watchpoint"
-      A watchpoint is similar to a breakpoint.  The difference is that
-      breakpoints are oriented around the code: stop when a certain
-      point in the code is reached.  A watchpoint, however, specifies
-      that program execution should stop when a _data value_ is changed.
-      This is useful, since sometimes it happens that a variable
-      receives an erroneous value, and it's hard to track down where
-      this happens just by looking at the code.  By using a watchpoint,
-      you can stop whenever a variable is assigned to, and usually find
-      the errant code quite quickly.
- 
- 
- File: gawk.info,  Node: Awk Debugging,  Prev: Debugging Terms,  Up: Debugging
- 
- 15.1.3 Awk Debugging
- --------------------
- 
- Debugging an `awk' program has some specific aspects that are not
- shared with other programming languages.
- 
-    First of all, the fact that `awk' programs usually take input
- line-by-line from a file or files and operate on those lines using
- specific rules makes it especially useful to organize viewing the
- execution of the program in terms of these rules.  As we will see, each
- `awk' rule is treated almost like a function call, with its own
- specific block of instructions.
- 
-    In addition, since `awk' is by design a very concise language, it is
- easy to lose sight of everything that is going on "inside" each line of
- `awk' code.  The debugger provides the opportunity to look at the
- individual primitive instructions carried out by the higher-level `awk'
- commands.
- 
- 
- File: gawk.info,  Node: Sample Debugging Session,  Next: List of Debugger 
Commands,  Prev: Debugging,  Up: Debugger
- 
- 15.2 Sample Debugging Session
- =============================
- 
- In order to illustrate the use of `gawk' as a debugger, let's look at a
- sample debugging session.  We will use the `awk' implementation of the
- POSIX `uniq' command described earlier (*note Uniq Program::) as our
- example.
- 
- * Menu:
- 
- * Debugger Invocation::         How to Start the Debugger.
- * Finding The Bug::             Finding the Bug.
- 
- 
- File: gawk.info,  Node: Debugger Invocation,  Next: Finding The Bug,  Up: 
Sample Debugging Session
- 
- 15.2.1 How to Start the Debugger
- --------------------------------
- 
- Starting the debugger is almost exactly like running `awk', except you
- have to pass an additional option `--debug' or the corresponding short
- option `-D'.  The file(s) containing the program and any supporting
- code are given on the command line as arguments to one or more `-f'
- options. (`gawk' is not designed to debug command-line programs, only
- programs contained in files.)  In our case, we invoke the debugger like
- this:
- 
-      $ gawk -D -f getopt.awk -f join.awk -f uniq.awk inputfile
- 
- where both `getopt.awk' and `uniq.awk' are in `$AWKPATH'.  (Experienced
- users of GDB or similar debuggers should note that this syntax is
- slightly different from what they are used to.  With `gawk' debugger,
- the arguments for running the program are given in the command line to
- the debugger rather than as part of the `run' command at the debugger
- prompt.)
- 
-    Instead of immediately running the program on `inputfile', as `gawk'
- would ordinarily do, the debugger merely loads all the program source
- files, compiles them internally, and then gives us a prompt:
- 
-      gawk>
- 
- from which we can issue commands to the debugger.  At this point, no
- code has been executed.
- 
- 
- File: gawk.info,  Node: Finding The Bug,  Prev: Debugger Invocation,  Up: 
Sample Debugging Session
- 
- 15.2.2 Finding the Bug
- ----------------------
- 
- Let's say that we are having a problem using (a faulty version of)
- `uniq.awk' in the "field-skipping" mode, and it doesn't seem to be
- catching lines which should be identical when skipping the first field,
- such as:
- 
-      awk is a wonderful program!
-      gawk is a wonderful program!
- 
-    This could happen if we were thinking (C-like) of the fields in a
- record as being numbered in a zero-based fashion, so instead of the
- lines:
- 
-      clast = join(alast, fcount+1, n)
-      cline = join(aline, fcount+1, m)
- 
- we wrote:
- 
-      clast = join(alast, fcount, n)
-      cline = join(aline, fcount, m)
- 
-    The first thing we usually want to do when trying to investigate a
- problem like this is to put a breakpoint in the program so that we can
- watch it at work and catch what it is doing wrong.  A reasonable spot
- for a breakpoint in `uniq.awk' is at the beginning of the function
- `are_equal()', which compares the current line with the previous one.
- To set the breakpoint, use the `b' (breakpoint) command:
- 
-      gawk> b are_equal
-      -| Breakpoint 1 set at file `awklib/eg/prog/uniq.awk', line 64
- 
-    The debugger tells us the file and line number where the breakpoint
- is.  Now type `r' or `run' and the program runs until it hits the
- breakpoint for the first time:
- 
-      gawk> r
-      -| Starting program:
-      -| Stopping in Rule ...
-      -| Breakpoint 1, are_equal(n, m, clast, cline, alast, aline)
-               at `awklib/eg/prog/uniq.awk':64
-      -| 64          if (fcount == 0 && charcount == 0)
-      gawk>
- 
-    Now we can look at what's going on inside our program.  First of all,
- let's see how we got to where we are.  At the prompt, we type `bt'
- (short for "backtrace"), and the debugger responds with a listing of
- the current stack frames:
- 
-      gawk> bt
-      -| #0  are_equal(n, m, clast, cline, alast, aline)
-               at `awklib/eg/prog/uniq.awk':69
-      -| #1  in main() at `awklib/eg/prog/uniq.awk':89
- 
-    This tells us that `are_equal()' was called by the main program at
- line 89 of `uniq.awk'.  (This is not a big surprise, since this is the
- only call to `are_equal()' in the program, but in more complex
- programs, knowing who called a function and with what parameters can be
- the key to finding the source of the problem.)
- 
-    Now that we're in `are_equal()', we can start looking at the values
- of some variables.  Let's say we type `p n' (`p' is short for "print").
- We would expect to see the value of `n', a parameter to `are_equal()'.
- Actually, the debugger gives us:
- 
-      gawk> p n
-      -| n = untyped variable
- 
- In this case, `n' is an uninitialized local variable, since the
- function was called without arguments (*note Function Calls::).
- 
-    A more useful variable to display might be the current record:
- 
-      gawk> p $0
-      -| $0 = string ("gawk is a wonderful program!")
- 
- This might be a bit puzzling at first since this is the second line of
- our test input above.  Let's look at `NR':
- 
-      gawk> p NR
-      -| NR = number (2)
- 
- So we can see that `are_equal()' was only called for the second record
- of the file.  Of course, this is because our program contained a rule
- for `NR == 1':
- 
-      NR == 1 {
-          last = $0
-          next
-      }
- 
-    OK, let's just check that that rule worked correctly:
- 
-      gawk> p last
-      -| last = string ("awk is a wonderful program!")
- 
-    Everything we have done so far has verified that the program has
- worked as planned, up to and including the call to `are_equal()', so
- the problem must be inside this function.  To investigate further, we
- must begin "stepping through" the lines of `are_equal()'.  We start by
- typing `n' (for "next"):
- 
-      gawk> n
-      -| 67          if (fcount > 0) {
- 
-    This tells us that `gawk' is now ready to execute line 67, which
- decides whether to give the lines the special "field skipping" treatment
- indicated by the `-f' command-line option.  (Notice that we skipped
- from where we were before at line 64 to here, since the condition in
- line 64
- 
-      if (fcount == 0 && charcount == 0)
- 
- was false.)
- 
-    Continuing to step, we now get to the splitting of the current and
- last records:
- 
-      gawk> n
-      -| 68              n = split(last, alast)
-      gawk> n
-      -| 69              m = split($0, aline)
- 
-    At this point, we should be curious to see what our records were
- split into, so we try to look:
- 
-      gawk> p n m alast aline
-      -| n = number (5)
-      -| m = number (5)
-      -| alast = array, 5 elements
-      -| aline = array, 5 elements
- 
- (The `p' command can take more than one argument, similar to `awk''s
- `print' statement.)
- 
-    This is kind of disappointing, though.  All we found out is that
- there are five elements in each of our arrays.  Useful enough (we now
- know that none of the words were accidentally left out), but what if we
- want to see inside the array?
- 
-    The first choice would be to use subscripts:
- 
-      gawk> p alast[0]
-      -| "0" not in array `alast'
- 
- Oops!
- 
-      gawk> p alast[1]
-      -| alast["1"] = string ("awk")
- 
-    This would be kind of slow for a 100-member array, though, so `gawk'
- provides a shortcut (reminiscent of another language not to be
- mentioned):
- 
-      gawk> p @alast
-      -| alast["1"] = string ("awk")
-      -| alast["2"] = string ("is")
-      -| alast["3"] = string ("a")
-      -| alast["4"] = string ("wonderful")
-      -| alast["5"] = string ("program!")
- 
-    It looks like we got this far OK.  Let's take another step or two:
- 
-      gawk> n
-      -| 70              clast = join(alast, fcount, n)
-      gawk> n
-      -| 71              cline = join(aline, fcount, m)
- 
-    Well, here we are at our error (sorry to spoil the suspense).  What
- we had in mind was to join the fields starting from the second one to
- make the virtual record to compare, and if the first field was numbered
- zero, this would work.  Let's look at what we've got:
- 
-      gawk> p cline clast
-      -| cline = string ("gawk is a wonderful program!")
-      -| clast = string ("awk is a wonderful program!")
- 
-    Hey, those look pretty familiar!  They're just our original,
- unaltered, input records.  A little thinking (the human brain is still
- the best debugging tool), and we realize that we were off by one!
- 
-    We get out of the debugger:
- 
-      gawk> q
-      -| The program is running. Exit anyway (y/n)? y
- 
- Then we get into an editor:
- 
-      clast = join(alast, fcount+1, n)
-      cline = join(aline, fcount+1, m)
- 
- and problem solved!
- 
- 
- File: gawk.info,  Node: List of Debugger Commands,  Next: Readline Support,  
Prev: Sample Debugging Session,  Up: Debugger
- 
- 15.3 Main Debugger Commands
- ===========================
- 
- The `gawk' debugger command set can be divided into the following
- categories:
- 
-    * Breakpoint control
- 
-    * Execution control
- 
-    * Viewing and changing data
- 
-    * Working with the stack
- 
-    * Getting information
- 
-    * Miscellaneous
- 
-    Each of these are discussed in the following subsections.  In the
- following descriptions, commands which may be abbreviated show the
- abbreviation on a second description line.  A debugger command name may
- also be truncated if that partial name is unambiguous. The debugger has
- the built-in capability to automatically repeat the previous command
- when just hitting <Enter>.  This works for the commands `list', `next',
- `nexti', `step', `stepi' and `continue' executed without any argument.
- 
- * Menu:
- 
- * Breakpoint Control::          Control of Breakpoints.
- * Debugger Execution Control::  Control of Execution.
- * Viewing And Changing Data::   Viewing and Changing Data.
- * Execution Stack::             Dealing with the Stack.
- * Debugger Info::               Obtaining Information about the Program and
-                                 the Debugger State.
- * Miscellaneous Debugger Commands:: Miscellaneous Commands.
- 
- 
- File: gawk.info,  Node: Breakpoint Control,  Next: Debugger Execution 
Control,  Up: List of Debugger Commands
- 
- 15.3.1 Control of Breakpoints
- -----------------------------
- 
- As we saw above, the first thing you probably want to do in a debugging
- session is to get your breakpoints set up, since otherwise your program
- will just run as if it was not under the debugger.  The commands for
- controlling breakpoints are:
- 
- `break' [[FILENAME`:']N | FUNCTION] [`"EXPRESSION"']
- `b' [[FILENAME`:']N | FUNCTION] [`"EXPRESSION"']
-      Without any argument, set a breakpoint at the next instruction to
-      be executed in the selected stack frame.  Arguments can be one of
-      the following:
- 
-     N
-           Set a breakpoint at line number N in the current source file.
- 
-     FILENAME`:'N
-           Set a breakpoint at line number N in source file FILENAME.
- 
-     FUNCTION
-           Set a breakpoint at entry to (the first instruction of)
-           function FUNCTION.
- 
-      Each breakpoint is assigned a number which can be used to delete
-      it from the breakpoint list using the `delete' command.
- 
-      With a breakpoint, you may also supply a condition.  This is an
-      `awk' expression (enclosed in double quotes) that the debugger
-      evaluates whenever the breakpoint is reached. If the condition is
-      true, then the debugger stops execution and prompts for a command.
-      Otherwise, it continues executing the program.
- 
- `clear' [[FILENAME`:']N | FUNCTION]
-      Without any argument, delete any breakpoint at the next instruction
-      to be executed in the selected stack frame. If the program stops at
-      a breakpoint, this deletes that breakpoint so that the program
-      does not stop at that location again.  Arguments can be one of the
-      following:
- 
-     N
-           Delete breakpoint(s) set at line number N in the current
-           source file.
- 
-     FILENAME`:'N
-           Delete breakpoint(s) set at line number N in source file
-           FILENAME.
- 
-     FUNCTION
-           Delete breakpoint(s) set at entry to function FUNCTION.
- 
- `condition' N `"EXPRESSION"'
-      Add a condition to existing breakpoint or watchpoint N. The
-      condition is an `awk' expression that the debugger evaluates
-      whenever the breakpoint or watchpoint is reached. If the condition
-      is true, then the debugger stops execution and prompts for a
-      command. Otherwise, the debugger continues executing the program.
-      If the condition expression is not specified, any existing
-      condition is removed; i.e., the breakpoint or watchpoint is made
-      unconditional.
- 
- `delete' [N1 N2 ...] [N-M]
- `d' [N1 N2 ...] [N-M]
-      Delete specified breakpoints or a range of breakpoints. Deletes
-      all defined breakpoints if no argument is supplied.
- 
- `disable' [N1 N2 ... | N-M]
-      Disable specified breakpoints or a range of breakpoints. Without
-      any argument, disables all breakpoints.
- 
- `enable' [`del' | `once'] [N1 N2 ...] [N-M]
- `e' [`del' | `once'] [N1 N2 ...] [N-M]
-      Enable specified breakpoints or a range of breakpoints. Without
-      any argument, enables all breakpoints.  Optionally, you can
-      specify how to enable the breakpoint:
- 
-     `del'
-           Enable the breakpoint(s) temporarily, then delete it when the
-           program stops at the breakpoint.
- 
-     `once'
-           Enable the breakpoint(s) temporarily, then disable it when
-           the program stops at the breakpoint.
- 
- `ignore' N COUNT
-      Ignore breakpoint number N the next COUNT times it is hit.
- 
- `tbreak' [[FILENAME`:']N | FUNCTION]
- `t' [[FILENAME`:']N | FUNCTION]
-      Set a temporary breakpoint (enabled for only one stop).  The
-      arguments are the same as for `break'.
- 
- 
- File: gawk.info,  Node: Debugger Execution Control,  Next: Viewing And 
Changing Data,  Prev: Breakpoint Control,  Up: List of Debugger Commands
- 
- 15.3.2 Control of Execution
- ---------------------------
- 
- Now that your breakpoints are ready, you can start running the program
- and observing its behavior.  There are more commands for controlling
- execution of the program than we saw in our earlier example:
- 
- `commands' [N]
- `silent'
- ...
- `end'
-      Set a list of commands to be executed upon stopping at a
-      breakpoint or watchpoint. N is the breakpoint or watchpoint number.
-      Without a number, the last one set is used. The actual commands
-      follow, starting on the next line, and terminated by the `end'
-      command.  If the command `silent' is in the list, the usual
-      messages about stopping at a breakpoint and the source line are
-      not printed. Any command in the list that resumes execution (e.g.,
-      `continue') terminates the list (an implicit `end'), and
-      subsequent commands are ignored.  For example:
- 
-           gawk> commands
-           > silent
-           > printf "A silent breakpoint; i = %d\n", i
-           > info locals
-           > set i = 10
-           > continue
-           > end
-           gawk>
- 
- `continue' [COUNT]
- `c' [COUNT]
-      Resume program execution. If continued from a breakpoint and COUNT
-      is specified, ignores the breakpoint at that location the next
-      COUNT times before stopping.
- 
- `finish'
-      Execute until the selected stack frame returns.  Print the
-      returned value.
- 
- `next' [COUNT]
- `n' [COUNT]
-      Continue execution to the next source line, stepping over function
-      calls.  The argument COUNT controls how many times to repeat the
-      action, as in `step'.
- 
- `nexti' [COUNT]
- `ni' [COUNT]
-      Execute one (or COUNT) instruction(s), stepping over function
-      calls.
- 
- `return' [VALUE]
-      Cancel execution of a function call. If VALUE (either a string or a
-      number) is specified, it is used as the function's return value.
-      If used in a frame other than the innermost one (the currently
-      executing function, i.e., frame number 0), discard all inner
-      frames in addition to the selected one, and the caller of that
-      frame becomes the innermost frame.
- 
- `run'
- `r'
-      Start/restart execution of the program. When restarting, the
-      debugger retains the current breakpoints, watchpoints, command
-      history, automatic display variables, and debugger options.
- 
- `step' [COUNT]
- `s' [COUNT]
-      Continue execution until control reaches a different source line
-      in the current stack frame. `step' steps inside any function
-      called within the line.  If the argument COUNT is supplied, steps
-      that many times before stopping, unless it encounters a breakpoint
-      or watchpoint.
- 
- `stepi' [COUNT]
- `si' [COUNT]
-      Execute one (or COUNT) instruction(s), stepping inside function
-      calls.  (For illustration of what is meant by an "instruction" in
-      `gawk', see the output shown under `dump' in *note Miscellaneous
-      Debugger Commands::.)
- 
- `until' [[FILENAME`:']N | FUNCTION]
- `u' [[FILENAME`:']N | FUNCTION]
-      Without any argument, continue execution until a line past the
-      current line in current stack frame is reached. With an argument,
-      continue execution until the specified location is reached, or the
-      current stack frame returns.
- 
- 
- File: gawk.info,  Node: Viewing And Changing Data,  Next: Execution Stack,  
Prev: Debugger Execution Control,  Up: List of Debugger Commands
- 
- 15.3.3 Viewing and Changing Data
- --------------------------------
- 
- The commands for viewing and changing variables inside of `gawk' are:
- 
- `display' [VAR | `$'N]
-      Add variable VAR (or field `$N') to the display list.  The value
-      of the variable or field is displayed each time the program stops.
-      Each variable added to the list is identified by a unique number:
- 
-           gawk> display x
-           -| 10: x = 1
- 
-      displays the assigned item number, the variable name and its
-      current value.  If the display variable refers to a function
-      parameter, it is silently deleted from the list as soon as the
-      execution reaches a context where no such variable of the given
-      name exists.  Without argument, `display' displays the current
-      values of items on the list.
- 
- `eval "AWK STATEMENTS"'
-      Evaluate AWK STATEMENTS in the context of the running program.
-      You can do anything that an `awk' program would do: assign values
-      to variables, call functions, and so on.
- 
- `eval' PARAM, ...
- AWK STATEMENTS
- `end'
-      This form of `eval' is similar, but it allows you to define "local
-      variables" that exist in the context of the AWK STATEMENTS,
-      instead of using variables or function parameters defined by the
-      program.
- 
- `print' VAR1[`,' VAR2 ...]
- `p' VAR1[`,' VAR2 ...]
-      Print the value of a `gawk' variable or field.  Fields must be
-      referenced by constants:
- 
-           gawk> print $3
- 
-      This prints the third field in the input record (if the specified
-      field does not exist, it prints `Null field'). A variable can be
-      an array element, with the subscripts being constant values. To
-      print the contents of an array, prefix the name of the array with
-      the `@' symbol:
- 
-           gawk> print @a
- 
-      This prints the indices and the corresponding values for all
-      elements in the array `a'.
- 
- `printf' FORMAT [`,' ARG ...]
-      Print formatted text. The FORMAT may include escape sequences,
-      such as `\n' (*note Escape Sequences::).  No newline is printed
-      unless one is specified.
- 
- `set' VAR`='VALUE
-      Assign a constant (number or string) value to an `awk' variable or
-      field.  String values must be enclosed between double quotes
-      (`"..."').
- 
-      You can also set special `awk' variables, such as `FS', `NF',
-      `NR', etc.
- 
- `watch' VAR | `$'N [`"EXPRESSION"']
- `w' VAR | `$'N [`"EXPRESSION"']
-      Add variable VAR (or field `$N') to the watch list.  The debugger
-      then stops whenever the value of the variable or field changes.
-      Each watched item is assigned a number which can be used to delete
-      it from the watch list using the `unwatch' command.
- 
-      With a watchpoint, you may also supply a condition.  This is an
-      `awk' expression (enclosed in double quotes) that the debugger
-      evaluates whenever the watchpoint is reached. If the condition is
-      true, then the debugger stops execution and prompts for a command.
-      Otherwise, `gawk' continues executing the program.
- 
- `undisplay' [N]
-      Remove item number N (or all items, if no argument) from the
-      automatic display list.
- 
- `unwatch' [N]
-      Remove item number N (or all items, if no argument) from the watch
-      list.
- 
- 
- 
- File: gawk.info,  Node: Execution Stack,  Next: Debugger Info,  Prev: Viewing 
And Changing Data,  Up: List of Debugger Commands
- 
- 15.3.4 Dealing with the Stack
- -----------------------------
- 
- Whenever you run a program which contains any function calls, `gawk'
- maintains a stack of all of the function calls leading up to where the
- program is right now.  You can see how you got to where you are, and
- also move around in the stack to see what the state of things was in the
- functions which called the one you are in.  The commands for doing this
- are:
- 
- `backtrace' [COUNT]
- `bt' [COUNT]
-      Print a backtrace of all function calls (stack frames), or
-      innermost COUNT frames if COUNT > 0. Print the outermost COUNT
-      frames if COUNT < 0.  The backtrace displays the name and
-      arguments to each function, the source file name, and the line
-      number.
- 
- `down' [COUNT]
-      Move COUNT (default 1) frames down the stack toward the innermost
-      frame.  Then select and print the frame.
- 
- `frame' [N]
- `f' [N]
-      Select and print (frame number, function and argument names,
-      source file, and the source line) stack frame N. Frame 0 is the
-      currently executing, or "innermost", frame (function call), frame
-      1 is the frame that called the innermost one. The highest numbered
-      frame is the one for the main program.
- 
- `up' [COUNT]
-      Move COUNT (default 1) frames up the stack toward the outermost
-      frame.  Then select and print the frame.
- 
- 
- File: gawk.info,  Node: Debugger Info,  Next: Miscellaneous Debugger 
Commands,  Prev: Execution Stack,  Up: List of Debugger Commands
- 
- 15.3.5 Obtaining Information about the Program and the Debugger State
- ---------------------------------------------------------------------
- 
- Besides looking at the values of variables, there is often a need to get
- other sorts of information about the state of your program and of the
- debugging environment itself.  The `gawk' debugger has one command which
- provides this information, appropriately called `info'.  `info' is used
- with one of a number of arguments that tell it exactly what you want to
- know:
- 
- `info' WHAT
- `i' WHAT
-      The value for WHAT should be one of the following:
- 
-     `args'
-           Arguments of the selected frame.
- 
-     `break'
-           List all currently set breakpoints.
- 
-     `display'
-           List all items in the automatic display list.
- 
-     `frame'
-           Description of the selected stack frame.
- 
-     `functions'
-           List all function definitions including source file names and
-           line numbers.
- 
-     `locals'
-           Local variables of the selected frame.
- 
-     `source'
-           The name of the current source file. Each time the program
-           stops, the current source file is the file containing the
-           current instruction.  When the debugger first starts, the
-           current source file is the first file included via the `-f'
-           option. The `list FILENAME:LINENO' command can be used at any
-           time to change the current source.
- 
-     `sources'
-           List all program sources.
- 
-     `variables'
-           List all global variables.
- 
-     `watch'
-           List all items in the watch list.
- 
-    Additional commands give you control over the debugger, the ability
- to save the debugger's state, and the ability to run debugger commands
- from a file.  The commands are:
- 
- `option' [NAME[`='VALUE]]
- `o' [NAME[`='VALUE]]
-      Without an argument, display the available debugger options and
-      their current values. `option NAME' shows the current value of the
-      named option. `option NAME=VALUE' assigns a new value to the named
-      option.  The available options are:
- 
-     `history_size'
-           The maximum number of lines to keep in the history file
-           `./.gawk_history'.  The default is 100.
- 
-     `listsize'
-           The number of lines that `list' prints. The default is 15.
- 
-     `outfile'
-           Send `gawk' output to a file; debugger output still goes to
-           standard output. An empty string (`""') resets output to
-           standard output.
- 
-     `prompt'
-           The debugger prompt. The default is `gawk> '.
- 
-     `save_history [on | off]'
-           Save command history to file `./.gawk_history'.  The default
-           is `on'.
- 
-     `save_options [on | off]'
-           Save current options to file `./.gawkrc' upon exit.  The
-           default is `on'.  Options are read back in to the next
-           session upon startup.
- 
-     `trace [on | off]'
-           Turn instruction tracing on or off. The default is `off'.
- 
- `save' FILENAME
-      Save the commands from the current session to the given file name,
-      so that they can be replayed using the `source' command.
- 
- `source' FILENAME
-      Run command(s) from a file; an error in any command does not
-      terminate execution of subsequent commands. Comments (lines
-      starting with `#') are allowed in a command file.  Empty lines are
-      ignored; they do _not_ repeat the last command.  You can't restart
-      the program by having more than one `run' command in the file.
-      Also, the list of commands may include additional `source'
-      commands; however, the `gawk' debugger will not source the same
-      file more than once in order to avoid infinite recursion.
- 
-      In addition to, or instead of the `source' command, you can use
-      the `-D FILE' or `--debug=FILE' command-line options to execute
-      commands from a file non-interactively (*note Options::.
- 
- 
- File: gawk.info,  Node: Miscellaneous Debugger Commands,  Prev: Debugger 
Info,  Up: List of Debugger Commands
- 
- 15.3.6 Miscellaneous Commands
- -----------------------------
- 
- There are a few more commands which do not fit into the previous
- categories, as follows:
- 
- `dump' [FILENAME]
-      Dump bytecode of the program to standard output or to the file
-      named in FILENAME.  This prints a representation of the internal
-      instructions which `gawk' executes to implement the `awk' commands
-      in a program.  This can be very enlightening, as the following
-      partial dump of Davide Brini's obfuscated code (*note Signature
-      Program::) demonstrates:
- 
-           gawk> dump
-           -|        # BEGIN
-           -|
-           -| [     2:0x89faef4] Op_rule             : [in_rule = BEGIN] 
[source_file = brini.awk]
-           -| [     3:0x89fa428] Op_push_i           : "~" [PERM|STRING|STRCUR]
-           -| [     3:0x89fa464] Op_push_i           : "~" [PERM|STRING|STRCUR]
-           -| [     3:0x89fa450] Op_match            :
-           -| [     3:0x89fa3ec] Op_store_var        : O [do_reference = FALSE]
-           -| [     4:0x89fa48c] Op_push_i           : "==" 
[PERM|STRING|STRCUR]
-           -| [     4:0x89fa4c8] Op_push_i           : "==" 
[PERM|STRING|STRCUR]
-           -| [     4:0x89fa4b4] Op_equal            :
-           -| [     4:0x89fa400] Op_store_var        : o [do_reference = FALSE]
-           -| [     5:0x89fa4f0] Op_push             : o
-           -| [     5:0x89fa4dc] Op_plus_i           : 0 [PERM|NUMCUR|NUMBER]
-           -| [     5:0x89fa414] Op_push_lhs         : o [do_reference = TRUE]
-           -| [     5:0x89fa4a0] Op_assign_plus      :
-           -| [      :0x89fa478] Op_pop              :
-           -| [     6:0x89fa540] Op_push             : O
-           -| [     6:0x89fa554] Op_push_i           : "" [PERM|STRING|STRCUR]
-           -| [      :0x89fa5a4] Op_no_op            :
-           -| [     6:0x89fa590] Op_push             : O
-           -| [      :0x89fa5b8] Op_concat           : [expr_count = 3] 
[concat_flag = 0]
-           -| [     6:0x89fa518] Op_store_var        : x [do_reference = FALSE]
-           -| [     7:0x89fa504] Op_push_loop        : [target_continue = 
0x89fa568] [target_break = 0x89fa680]
-           -| [     7:0x89fa568] Op_push_lhs         : X [do_reference = TRUE]
-           -| [     7:0x89fa52c] Op_postincrement    :
-           -| [     7:0x89fa5e0] Op_push             : x
-           -| [     7:0x89fa61c] Op_push             : o
-           -| [     7:0x89fa5f4] Op_plus             :
-           -| [     7:0x89fa644] Op_push             : o
-           -| [     7:0x89fa630] Op_plus             :
-           -| [     7:0x89fa5cc] Op_leq              :
-           -| [      :0x89fa57c] Op_jmp_false        : [target_jmp = 0x89fa680]
-           -| [     7:0x89fa694] Op_push_i           : "%c" 
[PERM|STRING|STRCUR]
-           -| [      :0x89fa6d0] Op_no_op            :
-           -| [     7:0x89fa608] Op_assign_concat    : c
-           -| [      :0x89fa6a8] Op_jmp              : [target_jmp = 0x89fa568]
-           -| [      :0x89fa680] Op_pop_loop         :
-           -|
-           ...
-           -|
-           -| [     8:0x89fa658] Op_K_printf         : [expr_count = 17] 
[redir_type = ""]
-           -| [      :0x89fa374] Op_no_op            :
-           -| [      :0x89fa3d8] Op_atexit           :
-           -| [      :0x89fa6bc] Op_stop             :
-           -| [      :0x89fa39c] Op_no_op            :
-           -| [      :0x89fa3b0] Op_after_beginfile  :
-           -| [      :0x89fa388] Op_no_op            :
-           -| [      :0x89fa3c4] Op_after_endfile    :
-           gawk>
- 
- `help'
- `h'
-      Print a list of all of the `gawk' debugger commands with a short
-      summary of their usage.  `help COMMAND' prints the information
-      about the command COMMAND.
- 
- `list' [`-' | `+' | N | FILENAME`:'N | N-M | FUNCTION]
- `l' [`-' | `+' | N | FILENAME`:'N | N-M | FUNCTION]
-      Print the specified lines (default 15) from the current source file
-      or the file named FILENAME. The possible arguments to `list' are
-      as follows:
- 
-     `-'
-           Print lines before the lines last printed.
- 
-     `+'
-           Print lines after the lines last printed.  `list' without any
-           argument does the same thing.
- 
-     N
-           Print lines centered around line number N.
- 
-     N-M
-           Print lines from N to M.
- 
-     FILENAME`:'N
-           Print lines centered around line number N in source file
-           FILENAME. This command may change the current source file.
- 
-     FUNCTION
-           Print lines centered around beginning of the function
-           FUNCTION. This command may change the current source file.
- 
- `quit'
- `q'
-      Exit the debugger.  Debugging is great fun, but sometimes we all
-      have to tend to other obligations in life, and sometimes we find
-      the bug, and are free to go on to the next one!  As we saw above,
-      if you are running a program, the debugger warns you if you
-      accidentally type `q' or `quit', to make sure you really want to
-      quit.
- 
- `trace' `on' | `off'
-      Turn on or off a continuous printing of instructions which are
-      about to be executed, along with printing the `awk' line which they
-      implement.  The default is `off'.
- 
-      It is to be hoped that most of the "opcodes" in these instructions
-      are fairly self-explanatory, and using `stepi' and `nexti' while
-      `trace' is on will make them into familiar friends.
- 
- 
- 
- File: gawk.info,  Node: Readline Support,  Next: Limitations,  Prev: List of 
Debugger Commands,  Up: Debugger
- 
- 15.4 Readline Support
- =====================
- 
- If `gawk' is compiled with the `readline' library, you can take
- advantage of that library's command completion and history expansion
- features. The following types of completion are available:
- 
- Command completion
-      Command names.
- 
- Source file name completion
-      Source file names. Relevant commands are `break', `clear', `list',
-      `tbreak', and `until'.
- 
- Argument completion
-      Non-numeric arguments to a command.  Relevant commands are
-      `enable' and `info'.
- 
- Variable name completion
-      Global variable names, and function arguments in the current
-      context if the program is running. Relevant commands are `display',
-      `print', `set', and `watch'.
- 
- 
- 
- File: gawk.info,  Node: Limitations,  Prev: Readline Support,  Up: Debugger
- 
- 15.5 Limitations and Future Plans
- =================================
- 
- We hope you find the `gawk' debugger useful and enjoyable to work with,
- but as with any program, especially in its early releases, it still has
- some limitations.  A few which are worth being aware of are:
- 
-    * At this point, the debugger does not give a detailed explanation of
-      what you did wrong when you type in something it doesn't like.
-      Rather, it just responds `syntax error'.  When you do figure out
-      what your mistake was, though, you'll feel like a real guru.
- 
-    * If you perused the dump of opcodes in *note Miscellaneous Debugger
-      Commands::, (or if you are already familiar with `gawk' internals),
-      you will realize that much of the internal manipulation of data in
-      `gawk', as in many interpreters, is done on a stack.  `Op_push',
-      `Op_pop', etc., are the "bread and butter" of most `gawk' code.
-      Unfortunately, as of now, the `gawk' debugger does not allow you
-      to examine the stack's contents.
- 
-      That is, the intermediate results of expression evaluation are on
-      the stack, but cannot be printed.  Rather, only variables which
-      are defined in the program can be printed.  Of course, a
-      workaround for this is to use more explicit variables at the
-      debugging stage and then change back to obscure, perhaps more
-      optimal code later.
- 
-    * There is no way to look "inside" the process of compiling regular
-      expressions to see if you got it right.  As an `awk' programmer,
-      you are expected to know what `/[^[:alnum:][:blank:]]/' means.
- 
-    * The `gawk' debugger is designed to be used by running a program
-      (with all its parameters) on the command line, as described in
-      *note Debugger Invocation::.  There is no way (as of now) to
-      attach or "break in" to a running program.  This seems reasonable
-      for a language which is used mainly for quickly executing, short
-      programs.
- 
-    * The `gawk' debugger only accepts source supplied with the `-f'
-      option.
- 
-    Look forward to a future release when these and other missing
- features may be added, and of course feel free to try to add them
- yourself!
- 
- 
- File: gawk.info,  Node: Language History,  Next: Installation,  Prev: 
Debugger,  Up: Top
- 
- Appendix A The Evolution of the `awk' Language
- **********************************************
- 
- This Info file describes the GNU implementation of `awk', which follows
- the POSIX specification.  Many long-time `awk' users learned `awk'
- programming with the original `awk' implementation in Version 7 Unix.
- (This implementation was the basis for `awk' in Berkeley Unix, through
- 4.3-Reno.  Subsequent versions of Berkeley Unix, and some systems
- derived from 4.4BSD-Lite, use various versions of `gawk' for their
- `awk'.)  This major node briefly describes the evolution of the `awk'
- language, with cross-references to other parts of the Info file where
- you can find more information.
- 
- * Menu:
- 
- * V7/SVR3.1::                   The major changes between V7 and System V
-                                 Release 3.1.
- * SVR4::                        Minor changes between System V Releases 3.1
-                                 and 4.
- * POSIX::                       New features from the POSIX standard.
- * BTL::                         New features from Brian Kernighan's version of
-                                 `awk'.
- * POSIX/GNU::                   The extensions in `gawk' not in POSIX
-                                 `awk'.
- * Common Extensions::           Common Extensions Summary.
- * Ranges and Locales::          How locales used to affect regexp ranges.
- * Contributors::                The major contributors to `gawk'.
- 
- 
- File: gawk.info,  Node: V7/SVR3.1,  Next: SVR4,  Up: Language History
- 
- A.1 Major Changes Between V7 and SVR3.1
- =======================================
- 
- The `awk' language evolved considerably between the release of Version
- 7 Unix (1978) and the new version that was first made generally
- available in System V Release 3.1 (1987).  This minor node summarizes
- the changes, with cross-references to further details:
- 
-    * The requirement for `;' to separate rules on a line (*note
-      Statements/Lines::).
- 
-    * User-defined functions and the `return' statement (*note
-      User-defined::).
- 
-    * The `delete' statement (*note Delete::).
- 
-    * The `do'-`while' statement (*note Do Statement::).
- 
-    * The built-in functions `atan2()', `cos()', `sin()', `rand()', and
-      `srand()' (*note Numeric Functions::).
- 
-    * The built-in functions `gsub()', `sub()', and `match()' (*note
-      String Functions::).
- 
-    * The built-in functions `close()' and `system()' (*note I/O
-      Functions::).
- 
-    * The `ARGC', `ARGV', `FNR', `RLENGTH', `RSTART', and `SUBSEP'
-      built-in variables (*note Built-in Variables::).
- 
-    * Assignable `$0' (*note Changing Fields::).
- 
-    * The conditional expression using the ternary operator `?:' (*note
-      Conditional Exp::).
- 
-    * The expression `INDEX-VARIABLE in ARRAY' outside of `for'
-      statements (*note Reference to Elements::).
- 
-    * The exponentiation operator `^' (*note Arithmetic Ops::) and its
-      assignment operator form `^=' (*note Assignment Ops::).
- 
-    * C-compatible operator precedence, which breaks some old `awk'
-      programs (*note Precedence::).
- 
-    * Regexps as the value of `FS' (*note Field Separators::) and as the
-      third argument to the `split()' function (*note String
-      Functions::), rather than using only the first character of `FS'.
- 
-    * Dynamic regexps as operands of the `~' and `!~' operators (*note
-      Regexp Usage::).
- 
-    * The escape sequences `\b', `\f', and `\r' (*note Escape
-      Sequences::).  (Some vendors have updated their old versions of
-      `awk' to recognize `\b', `\f', and `\r', but this is not something
-      you can rely on.)
- 
-    * Redirection of input for the `getline' function (*note Getline::).
- 
-    * Multiple `BEGIN' and `END' rules (*note BEGIN/END::).
- 
-    * Multidimensional arrays (*note Multi-dimensional::).
- 
- 
- File: gawk.info,  Node: SVR4,  Next: POSIX,  Prev: V7/SVR3.1,  Up: Language 
History
- 
- A.2 Changes Between SVR3.1 and SVR4
- ===================================
- 
- The System V Release 4 (1989) version of Unix `awk' added these features
- (some of which originated in `gawk'):
- 
-    * The `ENVIRON' array (*note Built-in Variables::).
- 
-    * Multiple `-f' options on the command line (*note Options::).
- 
-    * The `-v' option for assigning variables before program execution
-      begins (*note Options::).
- 
-    * The `--' option for terminating command-line options.
- 
-    * The `\a', `\v', and `\x' escape sequences (*note Escape
-      Sequences::).
- 
-    * A defined return value for the `srand()' built-in function (*note
-      Numeric Functions::).
- 
-    * The `toupper()' and `tolower()' built-in string functions for case
-      translation (*note String Functions::).
- 
-    * A cleaner specification for the `%c' format-control letter in the
-      `printf' function (*note Control Letters::).
- 
-    * The ability to dynamically pass the field width and precision
-      (`"%*.*d"') in the argument list of the `printf' function (*note
-      Control Letters::).
- 
-    * The use of regexp constants, such as `/foo/', as expressions, where
-      they are equivalent to using the matching operator, as in `$0 ~
-      /foo/' (*note Using Constant Regexps::).
- 
-    * Processing of escape sequences inside command-line variable
-      assignments (*note Assignment Options::).
- 
- 
- File: gawk.info,  Node: POSIX,  Next: BTL,  Prev: SVR4,  Up: Language History
- 
- A.3 Changes Between SVR4 and POSIX `awk'
- ========================================
- 
- The POSIX Command Language and Utilities standard for `awk' (1992)
- introduced the following changes into the language:
- 
-    * The use of `-W' for implementation-specific options (*note
-      Options::).
- 
-    * The use of `CONVFMT' for controlling the conversion of numbers to
-      strings (*note Conversion::).
- 
-    * The concept of a numeric string and tighter comparison rules to go
-      with it (*note Typing and Comparison::).
- 
-    * The use of built-in variables as function parameter names is
-      forbidden (*note Definition Syntax::.
- 
-    * More complete documentation of many of the previously undocumented
-      features of the language.
- 
-    *Note Common Extensions::, for a list of common extensions not
- permitted by the POSIX standard.
- 
-    The 2008 POSIX standard can be found online at
- `http://www.opengroup.org/onlinepubs/9699919799/'.
- 
- 
- File: gawk.info,  Node: BTL,  Next: POSIX/GNU,  Prev: POSIX,  Up: Language 
History
- 
- A.4 Extensions in Brian Kernighan's `awk'
- =========================================
- 
- Brian Kernighan has made his version available via his home page (*note
- Other Versions::).
- 
-    This minor node describes common extensions that originally appeared
- in his version of `awk'.
- 
-    * The `**' and `**=' operators (*note Arithmetic Ops:: and *note
-      Assignment Ops::).
- 
-    * The use of `func' as an abbreviation for `function' (*note
-      Definition Syntax::).
- 
-    * The `fflush()' built-in function for flushing buffered output
-      (*note I/O Functions::).
- 
- 
-    *Note Common Extensions::, for a full list of the extensions
- available in his `awk'.
- 
- 
- File: gawk.info,  Node: POSIX/GNU,  Next: Common Extensions,  Prev: BTL,  Up: 
Language History
- 
- A.5 Extensions in `gawk' Not in POSIX `awk'
- ===========================================
- 
- The GNU implementation, `gawk', adds a large number of features.  They
- can all be disabled with either the `--traditional' or `--posix' options
- (*note Options::).
- 
-    A number of features have come and gone over the years. This minor
- node summarizes the additional features over POSIX `awk' that are in
- the current version of `gawk'.
- 
-    * Additional built-in variables:
- 
-         - The `ARGIND' `BINMODE', `ERRNO', `FIELDWIDTHS', `FPAT',
-           `IGNORECASE', `LINT', `PROCINFO', `RT', and `TEXTDOMAIN'
-           variables (*note Built-in Variables::).
- 
-    * Special files in I/O redirections:
- 
-         - The `/dev/stdin', `/dev/stdout', `/dev/stderr' and
-           `/dev/fd/N' special file names (*note Special Files::).
- 
-         - The `/inet', `/inet4', and `/inet6' special files for TCP/IP
-           networking using `|&' to specify which version of the IP
-           protocol to use.  (*note TCP/IP Networking::).
- 
-    * Changes and/or additions to the language:
- 
-         - The `\x' escape sequence (*note Escape Sequences::).
- 
-         - Full support for both POSIX and GNU regexps (*note Regexp::).
- 
-         - The ability for `FS' and for the third argument to `split()'
-           to be null strings (*note Single Character Fields::).
- 
-         - The ability for `RS' to be a regexp (*note Records::).
- 
-         - The ability to use octal and hexadecimal constants in `awk'
-           program source code (*note Nondecimal-numbers::).
- 
-         - The `|&' operator for two-way I/O to a coprocess (*note
-           Two-way I/O::).
- 
-         - Indirect function calls (*note Indirect Calls::).
- 
-         - Directories on the command line produce a warning and are
-           skipped (*note Command line directories::).
- 
-    * New keywords:
- 
-         - The `BEGINFILE' and `ENDFILE' special patterns.  (*note
-           BEGINFILE/ENDFILE::).
- 
-         - The ability to delete all of an array at once with `delete
-           ARRAY' (*note Delete::).
- 
-         - The `nextfile' statement (*note Nextfile Statement::).
- 
-         - The `switch' statement (*note Switch Statement::).
- 
-    * Changes to standard `awk' functions:
- 
-         - The optional second argument to `close()' that allows closing
-           one end of a two-way pipe to a coprocess (*note Two-way
-           I/O::).
- 
-         - POSIX compliance for `gsub()' and `sub()'.
- 
-         - The `length()' function accepts an array argument and returns
-           the number of elements in the array (*note String
-           Functions::).
- 
-         - The optional third argument to the `match()' function for
-           capturing text-matching subexpressions within a regexp (*note
-           String Functions::).
- 
-         - Positional specifiers in `printf' formats for making
-           translations easier (*note Printf Ordering::).
- 
-         - The `split()' function's additional optional fourth argument
-           which is an array to hold the text of the field separators.
-           (*note String Functions::).
- 
-    * Additional functions only in `gawk':
- 
-         - The `and()', `compl()', `lshift()', `or()', `rshift()', and
-           `xor()' functions for bit manipulation (*note Bitwise
-           Functions::).
- 
-         - The `asort()' and `asorti()' functions for sorting arrays
-           (*note Array Sorting::).
- 
-         - The `bindtextdomain()', `dcgettext()' and `dcngettext()'
-           functions for internationalization (*note Programmer i18n::).
- 
-         - The `extension()' built-in function and the ability to add
-           new functions dynamically (*note Dynamic Extensions::).
- 
-         - The `fflush()' function from Brian Kernighan's version of
-           `awk' (*note I/O Functions::).
- 
-         - The `gensub()', `patsplit()', and `strtonum()' functions for
-           more powerful text manipulation (*note String Functions::).
- 
-         - The `mktime()', `systime()', and `strftime()' functions for
-           working with timestamps (*note Time Functions::).
- 
-    * Changes and/or additions in the command-line options:
- 
-         - The `AWKPATH' environment variable for specifying a path
-           search for the `-f' command-line option (*note Options::).
- 
-         - The `AWKLIBPATH' environment variable for specifying a path
-           search for the `-l' command-line option (*note Options::).
- 
-         - The ability to use GNU-style long-named options that start
-           with `--' and the `--characters-as-bytes', `--compat',
-           `--dump-variables', `--exec', `--gen-pot', `--lint',
-           `--lint-old', `--non-decimal-data', `--posix', `--profile',
-           `--re-interval', `--sandbox', `--source', `--traditional', and
-           `--use-lc-numeric' options (*note Options::).
- 
-    * Support for the following obsolete systems was removed from the
-      code and the documentation for `gawk' version 4.0:
- 
-         - Amiga
- 
-         - Atari
- 
-         - BeOS
- 
-         - Cray
- 
-         - MIPS RiscOS
- 
-         - MS-DOS with the Microsoft Compiler
- 
-         - MS-Windows with the Microsoft Compiler
- 
-         - NeXT
- 
-         - SunOS 3.x, Sun 386 (Road Runner)
- 
-         - Tandem (non-POSIX)
- 
-         - Prestandard VAX C compiler for VAX/VMS
- 
- 
- 
- 
- File: gawk.info,  Node: Common Extensions,  Next: Ranges and Locales,  Prev: 
POSIX/GNU,  Up: Language History
- 
- A.6 Common Extensions Summary
- =============================
- 
- This minor node summarizes the common extensions supported by `gawk',
- Brian Kernighan's `awk', and `mawk', the three most widely-used freely
- available versions of `awk' (*note Other Versions::).
- 
- Feature                      BWK Awk   Mawk   GNU Awk
- -------------------------------------------------------- 
- `\x' Escape sequence         X         X      X
- `RS' as regexp                         X      X
- `FS' as null string          X         X      X
- `/dev/stdin' special file    X                X
- `/dev/stdout' special file   X         X      X
- `/dev/stderr' special file   X         X      X
- `**' and `**=' operators     X                X
- `func' keyword               X                X
- `nextfile' statement         X         X      X
- `delete' without subscript   X         X      X
- `length()' of an array       X                X
- `fflush()' function          X         X      X
- `BINMODE' variable                     X      X
- 
- 
- File: gawk.info,  Node: Ranges and Locales,  Next: Contributors,  Prev: 
Common Extensions,  Up: Language History
- 
- A.7 Regexp Ranges and Locales: A Long Sad Story
- ===============================================
- 
- This minor node describes the confusing history of ranges within
- regular expressions and their interactions with locales, and how this
- affected different versions of `gawk'.
- 
-    The original Unix tools that worked with regular expressions defined
- character ranges (such as `[a-z]') to match any character between the
- first character in the range and the last character in the range,
- inclusive.  Ordering was based on the numeric value of each character
- in the machine's native character set.  Thus, on ASCII-based systems,
- `[a-z]' matched all the lowercase letters, and only the lowercase
- letters, since the numeric values for the letters from `a' through `z'
- were contiguous.  (On an EBCDIC system, the range `[a-z]' includes
- additional, non-alphabetic characters as well.)
- 
-    Almost all introductory Unix literature explained range expressions
- as working in this fashion, and in particular, would teach that the
- "correct" way to match lowercase letters was with `[a-z]', and that
- `[A-Z]' was the "correct" way to match uppercase letters.  And indeed,
- this was true.
- 
-    The 1993 POSIX standard introduced the idea of locales (*note
- Locales::).  Since many locales include other letters besides the plain
- twenty-six letters of the American English alphabet, the POSIX standard
- added character classes (*note Bracket Expressions::) as a way to match
- different kinds of characters besides the traditional ones in the ASCII
- character set.
- 
-    However, the standard _changed_ the interpretation of range
- expressions.  In the `"C"' and `"POSIX"' locales, a range expression
- like `[a-dx-z]' is still equivalent to `[abcdxyz]', as in ASCII.  But
- outside those locales, the ordering was defined to be based on
- "collation order".
- 
-    In many locales, `A' and `a' are both less than `B'.  In other
- words, these locales sort characters in dictionary order, and
- `[a-dx-z]' is typically not equivalent to `[abcdxyz]'; instead it might
- be equivalent to `[aBbCcdXxYyz]', for example.
- 
-    This point needs to be emphasized: Much literature teaches that you
- should use `[a-z]' to match a lowercase character.  But on systems with
- non-ASCII locales, this also matched all of the uppercase characters
- except `Z'!  This was a continuous cause of confusion, even well into
- the twenty-first century.
- 
-    To demonstrate these issues, the following example uses the `sub()'
- function, which does text replacement (*note String Functions::).  Here,
- the intent is to remove trailing uppercase characters:
- 
-      $ echo something1234abc | gawk-3.1.8 '{ sub("[A-Z]*$", ""); print }'
-      -| something1234a
- 
- This output is unexpected, since the `bc' at the end of
- `something1234abc' should not normally match `[A-Z]*'.  This result is
- due to the locale setting (and thus you may not see it on your system).
- 
-    Similar considerations apply to other ranges.  For example, `["-/]'
- is perfectly valid in ASCII, but is not valid in many Unicode locales,
- such as `en_US.UTF-8'.
- 
-    Early versions of `gawk' used regexp matching code that was not
- locale aware, so ranges had their traditional interpretation.
- 
-    When `gawk' switched to using locale-aware regexp matchers, the
- problems began; especially as both GNU/Linux and commercial Unix
- vendors started implementing non-ASCII locales, _and making them the
- default_.  Perhaps the most frequently asked question became something
- like "why does `[A-Z]' match lowercase letters?!?"
- 
-    This situation existed for close to 10 years, if not more, and the
- `gawk' maintainer grew weary of trying to explain that `gawk' was being
- nicely standards-compliant, and that the issue was in the user's
- locale.  During the development of version 4.0, he modified `gawk' to
- always treat ranges in the original, pre-POSIX fashion, unless
- `--posix' was used (*note Options::).
- 
-    Fortunately, shortly before the final release of `gawk' 4.0, the
- maintainer learned that the 2008 standard had changed the definition of
- ranges, such that outside the `"C"' and `"POSIX"' locales, the meaning
- of range expressions was _undefined_.(1)
- 
-    By using this lovely technical term, the standard gives license to
- implementors to implement ranges in whatever way they choose.  The
- `gawk' maintainer chose to apply the pre-POSIX meaning in all cases:
- the default regexp matching; with `--traditional', and with `--posix';
- in all cases, `gawk' remains POSIX compliant.
- 
-    ---------- Footnotes ----------
- 
-    (1) See the standard
- 
(http://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap09.html#tag_09_03_05)
- and its rationale
- 
(http://pubs.opengroup.org/onlinepubs/9699919799/xrat/V4_xbd_chap09.html#tag_21_09_03_05).
- 
- 
- File: gawk.info,  Node: Contributors,  Prev: Ranges and Locales,  Up: 
Language History
- 
- A.8 Major Contributors to `gawk'
- ================================
- 
-      Always give credit where credit is due.
-      Anonymous
- 
-    This minor node names the major contributors to `gawk' and/or this
- Info file, in approximate chronological order:
- 
-    * Dr. Alfred V. Aho, Dr. Peter J. Weinberger, and Dr. Brian W.
-      Kernighan, all of Bell Laboratories, designed and implemented Unix
-      `awk', from which `gawk' gets the majority of its feature set.
- 
-    * Paul Rubin did the initial design and implementation in 1986, and
-      wrote the first draft (around 40 pages) of this Info file.
- 
-    * Jay Fenlason finished the initial implementation.
- 
-    * Diane Close revised the first draft of this Info file, bringing it
-      to around 90 pages.
- 
-    * Richard Stallman helped finish the implementation and the initial
-      draft of this Info file.  He is also the founder of the FSF and
-      the GNU project.
- 
-    * John Woods contributed parts of the code (mostly fixes) in the
-      initial version of `gawk'.
- 
-    * In 1988, David Trueman took over primary maintenance of `gawk',
-      making it compatible with "new" `awk', and greatly improving its
-      performance.
- 
-    * Conrad Kwok, Scott Garfinkle, and Kent Williams did the initial
-      ports to MS-DOS with various versions of MSC.
- 
-    * Pat Rankin provided the VMS port and its documentation.
- 
-    * Hal Peterson provided help in porting `gawk' to Cray systems.
-      (This is no longer supported.)
- 
-    * Kai Uwe Rommel provided the initial port to OS/2 and its
-      documentation.
- 
-    * Michal Jaegermann provided the port to Atari systems and its
-      documentation.  (This port is no longer supported.)  He continues
-      to provide portability checking with DEC Alpha systems, and has
-      done a lot of work to make sure `gawk' works on non-32-bit systems.
- 
-    * Fred Fish provided the port to Amiga systems and its documentation.
-      (With Fred's sad passing, this is no longer supported.)
- 
-    * Scott Deifik currently maintains the MS-DOS port using DJGPP.
- 
-    * Eli Zaretskii currently maintains the MS-Windows port using MinGW.
- 
-    * Juan Grigera provided a port to Windows32 systems.  (This is no
-      longer supported.)
- 
-    * For many years, Dr. Darrel Hankerson acted as coordinator for the
-      various ports to different PC platforms and created binary
-      distributions for various PC operating systems.  He was also
-      instrumental in keeping the documentation up to date for the
-      various PC platforms.
- 
-    * Christos Zoulas provided the `extension()' built-in function for
-      dynamically adding new modules.
- 
-    * Ju"rgen Kahrs contributed the initial version of the TCP/IP
-      networking code and documentation, and motivated the inclusion of
-      the `|&' operator.
- 
-    * Stephen Davies provided the initial port to Tandem systems and its
-      documentation.  (However, this is no longer supported.)  He was
-      also instrumental in the initial work to integrate the byte-code
-      internals into the `gawk' code base.
- 
-    * Matthew Woehlke provided improvements for Tandem's POSIX-compliant
-      systems.
- 
-    * Martin Brown provided the port to BeOS and its documentation.
-      (This is no longer supported.)
- 
-    * Arno Peters did the initial work to convert `gawk' to use GNU
-      Automake and GNU `gettext'.
- 
-    * Alan J. Broder provided the initial version of the `asort()'
-      function as well as the code for the optional third argument to the
-      `match()' function.
- 
-    * Andreas Buening updated the `gawk' port for OS/2.
- 
-    * Isamu Hasegawa, of IBM in Japan, contributed support for multibyte
-      characters.
- 
-    * Michael Benzinger contributed the initial code for `switch'
-      statements.
- 
-    * Patrick T.J. McPhee contributed the code for dynamic loading in
-      Windows32 environments.  (This is no longer supported)
- 
-    * John Haque reworked the `gawk' internals to use a byte-code engine,
-      providing the `gawk' debugger for `awk' programs.
- 
-    * Efraim Yawitz contributed the original text for *note Debugger::.
- 
-    * Arnold Robbins has been working on `gawk' since 1988, at first
-      helping David Trueman, and as the primary maintainer since around
-      1994.
- 
- 
- File: gawk.info,  Node: Installation,  Next: Notes,  Prev: Language History,  
Up: Top
- 
- Appendix B Installing `gawk'
- ****************************
- 
- This appendix provides instructions for installing `gawk' on the
- various platforms that are supported by the developers.  The primary
- developer supports GNU/Linux (and Unix), whereas the other ports are
- contributed.  *Note Bugs::, for the electronic mail addresses of the
- people who did the respective ports.
- 
- * Menu:
- 
- * Gawk Distribution::           What is in the `gawk' distribution.
- * Unix Installation::           Installing `gawk' under various
-                                 versions of Unix.
- * Non-Unix Installation::       Installation on Other Operating Systems.
- * Bugs::                        Reporting Problems and Bugs.
- * Other Versions::              Other freely available `awk'
-                                 implementations.
- 
- 
- File: gawk.info,  Node: Gawk Distribution,  Next: Unix Installation,  Up: 
Installation
- 
- B.1 The `gawk' Distribution
- ===========================
- 
- This minor node describes how to get the `gawk' distribution, how to
- extract it, and then what is in the various files and subdirectories.
- 
- * Menu:
- 
- * Getting::                     How to get the distribution.
- * Extracting::                  How to extract the distribution.
- * Distribution contents::       What is in the distribution.
- 
- 
- File: gawk.info,  Node: Getting,  Next: Extracting,  Up: Gawk Distribution
- 
- B.1.1 Getting the `gawk' Distribution
- -------------------------------------
- 
- There are three ways to get GNU software:
- 
-    * Copy it from someone else who already has it.
- 
-    * Retrieve `gawk' from the Internet host `ftp.gnu.org', in the
-      directory `/gnu/gawk'.  Both anonymous `ftp' and `http' access are
-      supported.  If you have the `wget' program, you can use a command
-      like the following:
- 
-           wget http://ftp.gnu.org/gnu/gawk/gawk-4.0.1.tar.gz
- 
-    The GNU software archive is mirrored around the world.  The
- up-to-date list of mirror sites is available from the main FSF web site
- (http://www.gnu.org/order/ftp.html).  Try to use one of the mirrors;
- they will be less busy, and you can usually find one closer to your
- site.
- 
- 
- File: gawk.info,  Node: Extracting,  Next: Distribution contents,  Prev: 
Getting,  Up: Gawk Distribution
- 
- B.1.2 Extracting the Distribution
- ---------------------------------
- 
- `gawk' is distributed as several `tar' files compressed with different
- compression programs: `gzip', `bzip2', and `xz'. For simplicity, the
- rest of these instructions assume you are using the one compressed with
- the GNU Zip program, `gzip'.
- 
-    Once you have the distribution (for example, `gawk-4.0.1.tar.gz'),
- use `gzip' to expand the file and then use `tar' to extract it.  You
- can use the following pipeline to produce the `gawk' distribution:
- 
-      # Under System V, add 'o' to the tar options
-      gzip -d -c gawk-4.0.1.tar.gz | tar -xvpf -
- 
-    On a system with GNU `tar', you can let `tar' do the decompression
- for you:
- 
-      tar -xvpzf gawk-4.0.1.tar.gz
- 
- Extracting the archive creates a directory named `gawk-4.0.1' in the
- current directory.
- 
-    The distribution file name is of the form `gawk-V.R.P.tar.gz'.  The
- V represents the major version of `gawk', the R represents the current
- release of version V, and the P represents a "patch level", meaning
- that minor bugs have been fixed in the release.  The current patch
- level is 1, but when retrieving distributions, you should get the
- version with the highest version, release, and patch level.  (Note,
- however, that patch levels greater than or equal to 70 denote "beta" or
- nonproduction software; you might not want to retrieve such a version
- unless you don't mind experimenting.)  If you are not on a Unix or
- GNU/Linux system, you need to make other arrangements for getting and
- extracting the `gawk' distribution.  You should consult a local expert.
- 
- 
- File: gawk.info,  Node: Distribution contents,  Prev: Extracting,  Up: Gawk 
Distribution
- 
- B.1.3 Contents of the `gawk' Distribution
- -----------------------------------------
- 
- The `gawk' distribution has a number of C source files, documentation
- files, subdirectories, and files related to the configuration process
- (*note Unix Installation::), as well as several subdirectories related
- to different non-Unix operating systems:
- 
- Various `.c', `.y', and `.h' files
-      The actual `gawk' source code.
- 
- `README'
- `README_d/README.*'
-      Descriptive files: `README' for `gawk' under Unix and the rest for
-      the various hardware and software combinations.
- 
- `INSTALL'
-      A file providing an overview of the configuration and installation
-      process.
- 
- `ChangeLog'
-      A detailed list of source code changes as bugs are fixed or
-      improvements made.
- 
- `ChangeLog.0'
-      An older list of source code changes.
- 
- `NEWS'
-      A list of changes to `gawk' since the last release or patch.
- 
- `NEWS.0'
-      An older list of changes to `gawk'.
- 
- `COPYING'
-      The GNU General Public License.
- 
- `FUTURES'
-      A brief list of features and changes being contemplated for future
-      releases, with some indication of the time frame for the feature,
-      based on its difficulty.
- 
- `LIMITATIONS'
-      A list of those factors that limit `gawk''s performance.  Most of
-      these depend on the hardware or operating system software and are
-      not limits in `gawk' itself.
- 
- `POSIX.STD'
-      A description of behaviors in the POSIX standard for `awk' which
-      are left undefined, or where `gawk' may not comply fully, as well
-      as a list of things that the POSIX standard should describe but
-      does not.
- 
- `doc/awkforai.txt'
-      A short article describing why `gawk' is a good language for
-      Artificial Intelligence (AI) programming.
- 
- `doc/bc_notes'
-      A brief description of `gawk''s "byte code" internals.
- 
- `doc/README.card'
- `doc/ad.block'
- `doc/awkcard.in'
- `doc/cardfonts'
- `doc/colors'
- `doc/macros'
- `doc/no.colors'
- `doc/setter.outline'
-      The `troff' source for a five-color `awk' reference card.  A
-      modern version of `troff' such as GNU `troff' (`groff') is needed
-      to produce the color version. See the file `README.card' for
-      instructions if you have an older `troff'.
- 
- `doc/gawk.1'
-      The `troff' source for a manual page describing `gawk'.  This is
-      distributed for the convenience of Unix users.
- 
- `doc/gawk.texi'
-      The Texinfo source file for this Info file.  It should be
-      processed with TeX (via `texi2dvi' or `texi2pdf') to produce a
-      printed document, and with `makeinfo' to produce an Info or HTML
-      file.
- 
- `doc/gawk.info'
-      The generated Info file for this Info file.
- 
- `doc/gawkinet.texi'
-      The Texinfo source file for *note (General Introduction)Top::
-      gawkinet, TCP/IP Internetworking with `gawk'.  It should be
-      processed with TeX (via `texi2dvi' or `texi2pdf') to produce a
-      printed document and with `makeinfo' to produce an Info or HTML
-      file.
- 
- `doc/gawkinet.info'
-      The generated Info file for `TCP/IP Internetworking with `gawk''.
- 
- `doc/igawk.1'
-      The `troff' source for a manual page describing the `igawk'
-      program presented in *note Igawk Program::.
- 
- `doc/Makefile.in'
-      The input file used during the configuration process to generate
-      the actual `Makefile' for creating the documentation.
- 
- `Makefile.am'
- `*/Makefile.am'
-      Files used by the GNU `automake' software for generating the
-      `Makefile.in' files used by `autoconf' and `configure'.
- 
- `Makefile.in'
- `aclocal.m4'
- `configh.in'
- `configure.ac'
- `configure'
- `custom.h'
- `missing_d/*'
- `m4/*'
-      These files and subdirectories are used when configuring `gawk'
-      for various Unix systems.  They are explained in *note Unix
-      Installation::.
- 
- `po/*'
-      The `po' library contains message translations.
- 
- `awklib/extract.awk'
- `awklib/Makefile.am'
- `awklib/Makefile.in'
- `awklib/eg/*'
-      The `awklib' directory contains a copy of `extract.awk' (*note
-      Extract Program::), which can be used to extract the sample
-      programs from the Texinfo source file for this Info file. It also
-      contains a `Makefile.in' file, which `configure' uses to generate
-      a `Makefile'.  `Makefile.am' is used by GNU Automake to create
-      `Makefile.in'.  The library functions from *note Library
-      Functions::, and the `igawk' program from *note Igawk Program::,
-      are included as ready-to-use files in the `gawk' distribution.
-      They are installed as part of the installation process.  The rest
-      of the programs in this Info file are available in appropriate
-      subdirectories of `awklib/eg'.
- 
- `posix/*'
-      Files needed for building `gawk' on POSIX-compliant systems.
- 
- `pc/*'
-      Files needed for building `gawk' under MS-Windows and OS/2 (*note
-      PC Installation::, for details).
- 
- `vms/*'
-      Files needed for building `gawk' under VMS (*note VMS
-      Installation::, for details).
- 
- `test/*'
-      A test suite for `gawk'.  You can use `make check' from the
-      top-level `gawk' directory to run your version of `gawk' against
-      the test suite.  If `gawk' successfully passes `make check', then
-      you can be confident of a successful port.
- 
- 
- File: gawk.info,  Node: Unix Installation,  Next: Non-Unix Installation,  
Prev: Gawk Distribution,  Up: Installation
- 
- B.2 Compiling and Installing `gawk' on Unix-like Systems
- ========================================================
- 
- Usually, you can compile and install `gawk' by typing only two
- commands.  However, if you use an unusual system, you may need to
- configure `gawk' for your system yourself.
- 
- * Menu:
- 
- * Quick Installation::               Compiling `gawk' under Unix.
- * Additional Configuration Options:: Other compile-time options.
- * Configuration Philosophy::         How it's all supposed to work.
- 
- 
- File: gawk.info,  Node: Quick Installation,  Next: Additional Configuration 
Options,  Up: Unix Installation
- 
- B.2.1 Compiling `gawk' for Unix-like Systems
- --------------------------------------------
- 
- The normal installation steps should work on all modern commercial
- Unix-derived systems, GNU/Linux, BSD-based systems, and the Cygwin
- environment for MS-Windows.
- 
-    After you have extracted the `gawk' distribution, `cd' to
- `gawk-4.0.1'.  Like most GNU software, `gawk' is configured
- automatically for your system by running the `configure' program.  This
- program is a Bourne shell script that is generated automatically using
- GNU `autoconf'.  (The `autoconf' software is described fully starting
- with *note (Autoconf)Top:: autoconf,Autoconf--Generating Automatic
- Configuration Scripts.)
- 
-    To configure `gawk', simply run `configure':
- 
-      sh ./configure
- 
-    This produces a `Makefile' and `config.h' tailored to your system.
- The `config.h' file describes various facts about your system.  You
- might want to edit the `Makefile' to change the `CFLAGS' variable,
- which controls the command-line options that are passed to the C
- compiler (such as optimization levels or compiling for debugging).
- 
-    Alternatively, you can add your own values for most `make' variables
- on the command line, such as `CC' and `CFLAGS', when running
- `configure':
- 
-      CC=cc CFLAGS=-g sh ./configure
- 
- See the file `INSTALL' in the `gawk' distribution for all the details.
- 
-    After you have run `configure' and possibly edited the `Makefile',
- type:
- 
-      make
- 
- Shortly thereafter, you should have an executable version of `gawk'.
- That's all there is to it!  To verify that `gawk' is working properly,
- run `make check'.  All of the tests should succeed.  If these steps do
- not work, or if any of the tests fail, check the files in the
- `README_d' directory to see if you've found a known problem.  If the
- failure is not described there, please send in a bug report (*note
- Bugs::).
- 
- 
- File: gawk.info,  Node: Additional Configuration Options,  Next: 
Configuration Philosophy,  Prev: Quick Installation,  Up: Unix Installation
- 
- B.2.2 Additional Configuration Options
- --------------------------------------
- 
- There are several additional options you may use on the `configure'
- command line when compiling `gawk' from scratch, including:
- 
- `--disable-lint'
-      Disable all lint checking within `gawk'.  The `--lint' and
-      `--lint-old' options (*note Options::) are accepted, but silently
-      do nothing.  Similarly, setting the `LINT' variable (*note
-      User-modified::) has no effect on the running `awk' program.
- 
-      When used with GCC's automatic dead-code-elimination, this option
-      cuts almost 200K bytes off the size of the `gawk' executable on
-      GNU/Linux x86 systems.  Results on other systems and with other
-      compilers are likely to vary.  Using this option may bring you
-      some slight performance improvement.
- 
-      Using this option will cause some of the tests in the test suite
-      to fail.  This option may be removed at a later date.
- 
- `--disable-nls'
-      Disable all message-translation facilities.  This is usually not
-      desirable, but it may bring you some slight performance
-      improvement.
- 
- `--with-whiny-user-strftime'
-      Force use of the included version of the `strftime()' function for
-      deficient systems.
- 
-    Use the command `./configure --help' to see the full list of options
- that `configure' supplies.
- 
- 
- File: gawk.info,  Node: Configuration Philosophy,  Prev: Additional 
Configuration Options,  Up: Unix Installation
- 
- B.2.3 The Configuration Process
- -------------------------------
- 
- This minor node is of interest only if you know something about using
- the C language and Unix-like operating systems.
- 
-    The source code for `gawk' generally attempts to adhere to formal
- standards wherever possible.  This means that `gawk' uses library
- routines that are specified by the ISO C standard and by the POSIX
- operating system interface standard.  The `gawk' source code requires
- using an ISO C compiler (the 1990 standard).
- 
-    Many Unix systems do not support all of either the ISO or the POSIX
- standards.  The `missing_d' subdirectory in the `gawk' distribution
- contains replacement versions of those functions that are most likely
- to be missing.
- 
-    The `config.h' file that `configure' creates contains definitions
- that describe features of the particular operating system where you are
- attempting to compile `gawk'.  The three things described by this file
- are: what header files are available, so that they can be correctly
- included, what (supposedly) standard functions are actually available
- in your C libraries, and various miscellaneous facts about your
- operating system.  For example, there may not be an `st_blksize'
- element in the `stat' structure.  In this case, `HAVE_ST_BLKSIZE' is
- undefined.
- 
-    It is possible for your C compiler to lie to `configure'. It may do
- so by not exiting with an error when a library function is not
- available.  To get around this, edit the file `custom.h'.  Use an
- `#ifdef' that is appropriate for your system, and either `#define' any
- constants that `configure' should have defined but didn't, or `#undef'
- any constants that `configure' defined and should not have.  `custom.h'
- is automatically included by `config.h'.
- 
-    It is also possible that the `configure' program generated by
- `autoconf' will not work on your system in some other fashion.  If you
- do have a problem, the file `configure.ac' is the input for `autoconf'.
- You may be able to change this file and generate a new version of
- `configure' that works on your system (*note Bugs::, for information on
- how to report problems in configuring `gawk').  The same mechanism may
- be used to send in updates to `configure.ac' and/or `custom.h'.
- 
- 
- File: gawk.info,  Node: Non-Unix Installation,  Next: Bugs,  Prev: Unix 
Installation,  Up: Installation
- 
- B.3 Installation on Other Operating Systems
- ===========================================
- 
- This minor node describes how to install `gawk' on various non-Unix
- systems.
- 
- * Menu:
- 
- * PC Installation::             Installing and Compiling `gawk' on
-                                 MS-DOS and OS/2.
- * VMS Installation::            Installing `gawk' on VMS.
- 
- 
- File: gawk.info,  Node: PC Installation,  Next: VMS Installation,  Up: 
Non-Unix Installation
- 
- B.3.1 Installation on PC Operating Systems
- ------------------------------------------
- 
- This minor node covers installation and usage of `gawk' on x86 machines
- running MS-DOS, any version of MS-Windows, or OS/2.  In this minor
- node, the term "Windows32" refers to any of Microsoft
- Windows-95/98/ME/NT/2000/XP/Vista/7.
- 
-    The limitations of MS-DOS (and MS-DOS shells under Windows32 or
- OS/2) has meant that various "DOS extenders" are often used with
- programs such as `gawk'.  The varying capabilities of Microsoft Windows
- 3.1 and Windows32 can add to the confusion.  For an overview of the
- considerations, please refer to `README_d/README.pc' in the
- distribution.
- 
- * Menu:
- 
- * PC Binary Installation::      Installing a prepared distribution.
- * PC Compiling::                Compiling `gawk' for MS-DOS,
-                                 Windows32, and OS/2.
- * PC Testing::                  Testing `gawk' on PC systems.
- * PC Using::                    Running `gawk' on MS-DOS, Windows32
-                                 and OS/2.
- * Cygwin::                      Building and running `gawk' for
-                                 Cygwin.
- * MSYS::                        Using `gawk' In The MSYS Environment.
- 
- 
- File: gawk.info,  Node: PC Binary Installation,  Next: PC Compiling,  Up: PC 
Installation
- 
- B.3.1.1 Installing a Prepared Distribution for PC Systems
- .........................................................
- 
- If you have received a binary distribution prepared by the MS-DOS
- maintainers, then `gawk' and the necessary support files appear under
- the `gnu' directory, with executables in `gnu/bin', libraries in
- `gnu/lib/awk', and manual pages under `gnu/man'.  This is designed for
- easy installation to a `/gnu' directory on your drive--however, the
- files can be installed anywhere provided `AWKPATH' is set properly.
- Regardless of the installation directory, the first line of `igawk.cmd'
- and `igawk.bat' (in `gnu/bin') may need to be edited.
- 
-    The binary distribution contains a separate file describing the
- contents. In particular, it may include more than one version of the
- `gawk' executable.
- 
-    OS/2 (32 bit, EMX) binary distributions are prepared for the `/usr'
- directory of your preferred drive. Set `UNIXROOT' to your installation
- drive (e.g., `e:') if you want to install `gawk' onto another drive
- than the hardcoded default `c:'. Executables appear in `/usr/bin',
- libraries under `/usr/share/awk', manual pages under `/usr/man',
- Texinfo documentation under `/usr/info', and NLS files under
- `/usr/share/locale'.  Note that the files can be installed anywhere
- provided `AWKPATH' is set properly.
- 
-    If you already have a file `/usr/info/dir' from another package _do
- not overwrite it!_ Instead enter the following commands at your prompt
- (replace `x:' by your installation drive):
- 
-      install-info --info-dir=x:/usr/info x:/usr/info/gawk.info
-      install-info --info-dir=x:/usr/info x:/usr/info/gawkinet.info
- 
-    The binary distribution may contain a separate file containing
- additional or more detailed installation instructions.
- 
- 
- File: gawk.info,  Node: PC Compiling,  Next: PC Testing,  Prev: PC Binary 
Installation,  Up: PC Installation
- 
- B.3.1.2 Compiling `gawk' for PC Operating Systems
- .................................................
- 
- `gawk' can be compiled for MS-DOS, Windows32, and OS/2 using the GNU
- development tools from DJ Delorie (DJGPP: MS-DOS only) or Eberhard
- Mattes (EMX: MS-DOS, Windows32 and OS/2).  The file
- `README_d/README.pc' in the `gawk' distribution contains additional
- notes, and `pc/Makefile' contains important information on compilation
- options.
- 
-    To build `gawk' for MS-DOS and Windows32, copy the files in the `pc'
- directory (_except_ for `ChangeLog') to the directory with the rest of
- the `gawk' sources, then invoke `make' with the appropriate target name
- as an argument to build `gawk'.  The `Makefile' copied from the `pc'
- directory contains a configuration section with comments and may need
- to be edited in order to work with your `make' utility.
- 
-    The `Makefile' supports a number of targets for building various
- MS-DOS and Windows32 versions.  A list of targets is printed if the
- `make' command is given without a target.  As an example, to build
- `gawk' using the DJGPP tools, enter `make djgpp'.  (The DJGPP tools
- needed for the build may be found at
- `ftp://ftp.delorie.com/pub/djgpp/current/v2gnu/'.)  To build a native
- MS-Windows binary of `gawk', type `make mingw32'.
- 
-    The 32 bit EMX version of `gawk' works "out of the box" under OS/2.
- However, it is highly recommended to use GCC 2.95.3 for the compilation.
- In principle, it is possible to compile `gawk' the following way:
- 
-      $ ./configure
-      $ make
- 
-    This is not recommended, though.  To get an OMF executable you should
- use the following commands at your `sh' prompt:
- 
-      $ CFLAGS="-O2 -Zomf -Zmt"
-      $ export CFLAGS
-      $ LDFLAGS="-s -Zcrtdll -Zlinker /exepack:2 -Zlinker /pm:vio -Zstack 
0x6000"
-      $ export LDFLAGS
-      $ RANLIB="echo"
-      $ export RANLIB
-      $ ./configure --prefix=c:/usr
-      $ make AR=emxomfar
- 
-    These are just suggestions for use with GCC 2.x.  You may use any
- other set of (self-consistent) environment variables and compiler flags.
- 
-    If you use GCC 2.95 it is recommended to use also:
- 
-      $ LIBS="-lgcc"
-      $ export LIBS
- 
-    You can also get an `a.out' executable if you prefer:
- 
-      $ CFLAGS="-O2 -Zmt"
-      $ export CFLAGS
-      $ LDFLAGS="-s -Zstack 0x6000"
-      $ LIBS="-lgcc"
-      $ unset RANLIB
-      $ ./configure --prefix=c:/usr
-      $ make
- 
-      NOTE: Compilation of `a.out' executables also works with GCC 3.2.
-      Versions later than GCC 3.2 have not been tested successfully.
- 
-    `make install' works as expected with the EMX build.
- 
-      NOTE: Ancient OS/2 ports of GNU `make' are not able to handle the
-      Makefiles of this package.  If you encounter any problems with
-      `make', try GNU Make 3.79.1 or later versions.  You should find
-      the latest version on `ftp://hobbes.nmsu.edu/pub/os2/'.
- 
- 
- File: gawk.info,  Node: PC Testing,  Next: PC Using,  Prev: PC Compiling,  
Up: PC Installation
- 
- B.3.1.3 Testing `gawk' on PC Operating Systems
- ..............................................
- 
- Using `make' to run the standard tests and to install `gawk' requires
- additional Unix-like tools, including `sh', `sed', and `cp'. In order
- to run the tests, the `test/*.ok' files may need to be converted so
- that they have the usual MS-DOS-style end-of-line markers.
- Alternatively, run `make check CMP="diff -a"' to use GNU `diff' in text
- mode instead of `cmp' to compare the resulting files.
- 
-    Most of the tests work properly with Stewartson's shell along with
- the companion utilities or appropriate GNU utilities.  However, some
- editing of `test/Makefile' is required. It is recommended that you copy
- the file `pc/Makefile.tst' over the file `test/Makefile' as a
- replacement. Details can be found in `README_d/README.pc' and in the
- file `pc/Makefile.tst'.
- 
-    On OS/2 the `pid' test fails because `spawnl()' is used instead of
- `fork()'/`execl()' to start child processes.  Also the `mbfw1' and
- `mbprintf1' tests fail because the needed multibyte functionality is
- not available.
- 
- 
- File: gawk.info,  Node: PC Using,  Next: Cygwin,  Prev: PC Testing,  Up: PC 
Installation
- 
- B.3.1.4 Using `gawk' on PC Operating Systems
- ............................................
- 
- With the exception of the Cygwin environment, the `|&' operator and
- TCP/IP networking (*note TCP/IP Networking::) are not supported for
- MS-DOS or MS-Windows.  EMX (OS/2 only) does support at least the `|&'
- operator.
- 
-    The MS-DOS and MS-Windows versions of `gawk' search for program
- files as described in *note AWKPATH Variable::.  However, semicolons
- (rather than colons) separate elements in the `AWKPATH' variable.  If
- `AWKPATH' is not set or is empty, then the default search path for
- MS-Windows and MS-DOS versions is `".;c:/lib/awk;c:/gnu/lib/awk"'.
- 
-    The search path for OS/2 (32 bit, EMX) is determined by the prefix
- directory (most likely `/usr' or `c:/usr') that has been specified as
- an option of the `configure' script like it is the case for the Unix
- versions.  If `c:/usr' is the prefix directory then the default search
- path contains `.' and `c:/usr/share/awk'.  Additionally, to support
- binary distributions of `gawk' for OS/2 systems whose drive `c:' might
- not support long file names or might not exist at all, there is a
- special environment variable.  If `UNIXROOT' specifies a drive then
- this specific drive is also searched for program files.  E.g., if
- `UNIXROOT' is set to `e:' the complete default search path is
- `".;c:/usr/share/awk;e:/usr/share/awk"'.
- 
-    An `sh'-like shell (as opposed to `command.com' under MS-DOS or
- `cmd.exe' under MS-Windows or OS/2) may be useful for `awk' programming.
- The DJGPP collection of tools includes an MS-DOS port of Bash, and
- several shells are available for OS/2, including `ksh'.
- 
-    Under MS-Windows, OS/2 and MS-DOS, `gawk' (and many other text
- programs) silently translate end-of-line `"\r\n"' to `"\n"' on input
- and `"\n"' to `"\r\n"' on output.  A special `BINMODE' variable
- (c.e.)  allows control over these translations and is interpreted as
- follows:
- 
-    * If `BINMODE' is `"r"', or one, then binary mode is set on read
-      (i.e., no translations on reads).
- 
-    * If `BINMODE' is `"w"', or two, then binary mode is set on write
-      (i.e., no translations on writes).
- 
-    * If `BINMODE' is `"rw"' or `"wr"' or three, binary mode is set for
-      both read and write.
- 
-    * `BINMODE=NON-NULL-STRING' is the same as `BINMODE=3' (i.e., no
-      translations on reads or writes).  However, `gawk' issues a warning
-      message if the string is not one of `"rw"' or `"wr"'.
- 
- The modes for standard input and standard output are set one time only
- (after the command line is read, but before processing any of the `awk'
- program).  Setting `BINMODE' for standard input or standard output is
- accomplished by using an appropriate `-v BINMODE=N' option on the
- command line.  `BINMODE' is set at the time a file or pipe is opened
- and cannot be changed mid-stream.
- 
-    The name `BINMODE' was chosen to match `mawk' (*note Other
- Versions::).  `mawk' and `gawk' handle `BINMODE' similarly; however,
- `mawk' adds a `-W BINMODE=N' option and an environment variable that
- can set `BINMODE', `RS', and `ORS'.  The files `binmode[1-3].awk'
- (under `gnu/lib/awk' in some of the prepared distributions) have been
- chosen to match `mawk''s `-W BINMODE=N' option.  These can be changed
- or discarded; in particular, the setting of `RS' giving the fewest
- "surprises" is open to debate.  `mawk' uses `RS = "\r\n"' if binary
- mode is set on read, which is appropriate for files with the
- MS-DOS-style end-of-line.
- 
-    To illustrate, the following examples set binary mode on writes for
- standard output and other files, and set `ORS' as the "usual"
- MS-DOS-style end-of-line:
- 
-      gawk -v BINMODE=2 -v ORS="\r\n" ...
- 
- or:
- 
-      gawk -v BINMODE=w -f binmode2.awk ...
- 
- These give the same result as the `-W BINMODE=2' option in `mawk'.  The
- following changes the record separator to `"\r\n"' and sets binary mode
- on reads, but does not affect the mode on standard input:
- 
-      gawk -v RS="\r\n" --source "BEGIN { BINMODE = 1 }" ...
- 
- or:
- 
-      gawk -f binmode1.awk ...
- 
- With proper quoting, in the first example the setting of `RS' can be
- moved into the `BEGIN' rule.
- 
- 
- File: gawk.info,  Node: Cygwin,  Next: MSYS,  Prev: PC Using,  Up: PC 
Installation
- 
- B.3.1.5 Using `gawk' In The Cygwin Environment
- ..............................................
- 
- `gawk' can be built and used "out of the box" under MS-Windows if you
- are using the Cygwin environment (http://www.cygwin.com).  This
- environment provides an excellent simulation of Unix, using the GNU
- tools, such as Bash, the GNU Compiler Collection (GCC), GNU Make, and
- other GNU programs.  Compilation and installation for Cygwin is the
- same as for a Unix system:
- 
-      tar -xvpzf gawk-4.0.1.tar.gz
-      cd gawk-4.0.1
-      ./configure
-      make
- 
-    When compared to GNU/Linux on the same system, the `configure' step
- on Cygwin takes considerably longer.  However, it does finish, and then
- the `make' proceeds as usual.
- 
-      NOTE: The `|&' operator and TCP/IP networking (*note TCP/IP
-      Networking::) are fully supported in the Cygwin environment.  This
-      is not true for any other environment on MS-Windows.
- 
- 
- File: gawk.info,  Node: MSYS,  Prev: Cygwin,  Up: PC Installation
- 
- B.3.1.6 Using `gawk' In The MSYS Environment
- ............................................
- 
- In the MSYS environment under MS-Windows, `gawk' automatically uses
- binary mode for reading and writing files.  Thus there is no need to
- use the `BINMODE' variable.
- 
-    This can cause problems with other Unix-like components that have
- been ported to MS-Windows that expect `gawk' to do automatic
- translation of `"\r\n"', since it won't.  Caveat Emptor!
- 
- 
- File: gawk.info,  Node: VMS Installation,  Prev: PC Installation,  Up: 
Non-Unix Installation
- 
- B.3.2 How to Compile and Install `gawk' on VMS
- ----------------------------------------------
- 
- This node describes how to compile and install `gawk' under VMS.  The
- older designation "VMS" is used throughout to refer to OpenVMS.
- 
- * Menu:
- 
- * VMS Compilation::             How to compile `gawk' under VMS.
- * VMS Installation Details::    How to install `gawk' under VMS.
- * VMS Running::                 How to run `gawk' under VMS.
- * VMS Old Gawk::                An old version comes with some VMS systems.
- 
- 
- File: gawk.info,  Node: VMS Compilation,  Next: VMS Installation Details,  
Up: VMS Installation
- 
- B.3.2.1 Compiling `gawk' on VMS
- ...............................
- 
- To compile `gawk' under VMS, there is a `DCL' command procedure that
- issues all the necessary `CC' and `LINK' commands. There is also a
- `Makefile' for use with the `MMS' utility.  From the source directory,
- use either:
- 
-      $ @[.VMS]VMSBUILD.COM
- 
- or:
- 
-      $ MMS/DESCRIPTION=[.VMS]DESCRIP.MMS GAWK
- 
-    Older versions of `gawk' could be built with VAX C or GNU C on
- VAX/VMS, as well as with DEC C, but that is no longer supported.  DEC C
- (also briefly known as "Compaq C" and now known as "HP C," but referred
- to here as "DEC C") is required.  Both `VMSBUILD.COM' and `DESCRIP.MMS'
- contain some obsolete support for the older compilers but are set up to
- use DEC C by default.
- 
-    `gawk' has been tested under Alpha/VMS 7.3-1 using Compaq C V6.4,
- and on Alpha/VMS 7.3, Alpha/VMS 7.3-2, and IA64/VMS 8.3.(1)
- 
-    ---------- Footnotes ----------
- 
-    (1) The IA64 architecture is also known as "Itanium."
- 
- 
- File: gawk.info,  Node: VMS Installation Details,  Next: VMS Running,  Prev: 
VMS Compilation,  Up: VMS Installation
- 
- B.3.2.2 Installing `gawk' on VMS
- ................................
- 
- To install `gawk', all you need is a "foreign" command, which is a
- `DCL' symbol whose value begins with a dollar sign. For example:
- 
-      $ GAWK :== $disk1:[gnubin]GAWK
- 
- Substitute the actual location of `gawk.exe' for `$disk1:[gnubin]'. The
- symbol should be placed in the `login.com' of any user who wants to run
- `gawk', so that it is defined every time the user logs on.
- Alternatively, the symbol may be placed in the system-wide
- `sylogin.com' procedure, which allows all users to run `gawk'.
- 
-    Optionally, the help entry can be loaded into a VMS help library:
- 
-      $ LIBRARY/HELP SYS$HELP:HELPLIB [.VMS]GAWK.HLP
- 
- (You may want to substitute a site-specific help library rather than
- the standard VMS library `HELPLIB'.)  After loading the help text, the
- command:
- 
-      $ HELP GAWK
- 
- provides information about both the `gawk' implementation and the `awk'
- programming language.
- 
-    The logical name `AWK_LIBRARY' can designate a default location for
- `awk' program files.  For the `-f' option, if the specified file name
- has no device or directory path information in it, `gawk' looks in the
- current directory first, then in the directory specified by the
- translation of `AWK_LIBRARY' if the file is not found.  If, after
- searching in both directories, the file still is not found, `gawk'
- appends the suffix `.awk' to the filename and retries the file search.
- If `AWK_LIBRARY' has no definition, a default value of `SYS$LIBRARY:'
- is used for it.
- 
- 
- File: gawk.info,  Node: VMS Running,  Next: VMS Old Gawk,  Prev: VMS 
Installation Details,  Up: VMS Installation
- 
- B.3.2.3 Running `gawk' on VMS
- .............................
- 
- Command-line parsing and quoting conventions are significantly different
- on VMS, so examples in this Info file or from other sources often need
- minor changes.  They _are_ minor though, and all `awk' programs should
- run correctly.
- 
-    Here are a couple of trivial tests:
- 
-      $ gawk -- "BEGIN {print ""Hello, World!""}"
-      $ gawk -"W" version
-      ! could also be -"W version" or "-W version"
- 
- Note that uppercase and mixed-case text must be quoted.
- 
-    The VMS port of `gawk' includes a `DCL'-style interface in addition
- to the original shell-style interface (see the help entry for details).
- One side effect of dual command-line parsing is that if there is only a
- single parameter (as in the quoted string program above), the command
- becomes ambiguous.  To work around this, the normally optional `--'
- flag is required to force Unix-style parsing rather than `DCL' parsing.
- If any other dash-type options (or multiple parameters such as data
- files to process) are present, there is no ambiguity and `--' can be
- omitted.
- 
-    The default search path, when looking for `awk' program files
- specified by the `-f' option, is `"SYS$DISK:[],AWK_LIBRARY:"'.  The
- logical name `AWKPATH' can be used to override this default.  The format
- of `AWKPATH' is a comma-separated list of directory specifications.
- When defining it, the value should be quoted so that it retains a single
- translation and not a multitranslation `RMS' searchlist.
- 
- 
- File: gawk.info,  Node: VMS Old Gawk,  Prev: VMS Running,  Up: VMS 
Installation
- 
- B.3.2.4 Some VMS Systems Have An Old Version of `gawk'
- ......................................................
- 
- Some versions of VMS have an old version of `gawk'.  To access it,
- define a symbol, as follows:
- 
-      $ gawk :== $sys$common:[syshlp.examples.tcpip.snmp]gawk.exe
- 
-    This is apparently version 2.15.6, which is extremely old. We
- recommend compiling and using the current version.
- 
- 
- File: gawk.info,  Node: Bugs,  Next: Other Versions,  Prev: Non-Unix 
Installation,  Up: Installation
- 
- B.4 Reporting Problems and Bugs
- ===============================
- 
-      There is nothing more dangerous than a bored archeologist.
-      The Hitchhiker's Guide to the Galaxy
- 
-    If you have problems with `gawk' or think that you have found a bug,
- please report it to the developers; we cannot promise to do anything
- but we might well want to fix it.
- 
-    Before reporting a bug, make sure you have actually found a real bug.
- Carefully reread the documentation and see if it really says you can do
- what you're trying to do.  If it's not clear whether you should be able
- to do something or not, report that too; it's a bug in the
- documentation!
- 
-    Before reporting a bug or trying to fix it yourself, try to isolate
- it to the smallest possible `awk' program and input data file that
- reproduces the problem.  Then send us the program and data file, some
- idea of what kind of Unix system you're using, the compiler you used to
- compile `gawk', and the exact results `gawk' gave you.  Also say what
- you expected to occur; this helps us decide whether the problem is
- really in the documentation.
- 
-    Please include the version number of `gawk' you are using.  You can
- get this information with the command `gawk --version'.
- 
-    Once you have a precise problem, send email to <address@hidden>.
- 
-    Using this address automatically sends a copy of your mail to me.
- If necessary, I can be reached directly at <address@hidden>.  The
- bug reporting address is preferred since the email list is archived at
- the GNU Project.  _All email should be in English, since that is my
- native language._
- 
-      CAUTION: Do _not_ try to report bugs in `gawk' by posting to the
-      Usenet/Internet newsgroup `comp.lang.awk'.  While the `gawk'
-      developers do occasionally read this newsgroup, there is no
-      guarantee that we will see your posting.  The steps described
-      above are the official recognized ways for reporting bugs.  Really.
- 
-      NOTE: Many distributions of GNU/Linux and the various BSD-based
-      operating systems have their own bug reporting systems.  If you
-      report a bug using your distribution's bug reporting system,
-      _please_ also send a copy to <address@hidden>.
- 
-      This is for two reasons.  First, while some distributions forward
-      bug reports "upstream" to the GNU mailing list, many don't, so
-      there is a good chance that the `gawk'  maintainer won't even see
-      the bug report!  Second, mail to the GNU list is archived, and
-      having everything at the GNU project keeps things self-contained
-      and not dependant on other web sites.
- 
-    Non-bug suggestions are always welcome as well.  If you have
- questions about things that are unclear in the documentation or are
- just obscure features, ask me; I will try to help you out, although I
- may not have the time to fix the problem.  You can send me electronic
- mail at the Internet address noted previously.
- 
-    If you find bugs in one of the non-Unix ports of `gawk', please send
- an electronic mail message to the person who maintains that port.  They
- are named in the following list, as well as in the `README' file in the
- `gawk' distribution.  Information in the `README' file should be
- considered authoritative if it conflicts with this Info file.
- 
-    The people maintaining the non-Unix ports of `gawk' are as follows:
- 
- MS-DOS with DJGPP       Scott Deifik, <address@hidden>.
- MS-Windows with MINGW   Eli Zaretskii, <address@hidden>.
- OS/2                    Andreas Buening, <address@hidden>.
- VMS                     Pat Rankin, <address@hidden>
- z/OS (OS/390)           Dave Pitts, <address@hidden>.
- 
-    If your bug is also reproducible under Unix, please send a copy of
- your report to the <address@hidden> email list as well.
- 
- 
- File: gawk.info,  Node: Other Versions,  Prev: Bugs,  Up: Installation
- 
- B.5 Other Freely Available `awk' Implementations
- ================================================
- 
-      It's kind of fun to put comments like this in your awk code.
-      `// Do C++ comments work? answer: yes! of course'
-      Michael Brennan
- 
-    There are a number of other freely available `awk' implementations.
- This minor node briefly describes where to get them:
- 
- Unix `awk'
-      Brian Kernighan, one of the original designers of Unix `awk', has
-      made his implementation of `awk' freely available.  You can
-      retrieve this version via the World Wide Web from his home page
-      (http://www.cs.princeton.edu/~bwk).  It is available in several
-      archive formats:
- 
-     Shell archive
-           `http://www.cs.princeton.edu/~bwk/btl.mirror/awk.shar'
- 
-     Compressed `tar' file
-           `http://www.cs.princeton.edu/~bwk/btl.mirror/awk.tar.gz'
- 
-     Zip file
-           `http://www.cs.princeton.edu/~bwk/btl.mirror/awk.zip'
- 
-      This version requires an ISO C (1990 standard) compiler; the C
-      compiler from GCC (the GNU Compiler Collection) works quite nicely.
- 
-      *Note Common Extensions::, for a list of extensions in this `awk'
-      that are not in POSIX `awk'.
- 
- `mawk'
-      Michael Brennan wrote an independent implementation of `awk',
-      called `mawk'.  It is available under the GPL (*note Copying::),
-      just as `gawk' is.
- 
-      The original distribution site for the `mawk' source code no
-      longer has it.  A copy is available at
-      `http://www.skeeve.com/gawk/mawk1.3.3.tar.gz'.
- 
-      In 2009, Thomas Dickey took on `mawk' maintenance.  Basic
-      information is available on the project's web page
-      (http://www.invisible-island.net/mawk/mawk.html).  The download
-      URL is `http://invisible-island.net/datafiles/release/mawk.tar.gz'.
- 
-      Once you have it, `gunzip' may be used to decompress this file.
-      Installation is similar to `gawk''s (*note Unix Installation::).
- 
-      *Note Common Extensions::, for a list of extensions in `mawk' that
-      are not in POSIX `awk'.
- 
- `awka'
-      Written by Andrew Sumner, `awka' translates `awk' programs into C,
-      compiles them, and links them with a library of functions that
-      provides the core `awk' functionality.  It also has a number of
-      extensions.
- 
-      The `awk' translator is released under the GPL, and the library is
-      under the LGPL.
- 
-      To get `awka', go to `http://sourceforge.net/projects/awka'.
- 
-      The project seems to be frozen; no new code changes have been made
-      since approximately 2003.
- 
- `pawk'
-      Nelson H.F. Beebe at the University of Utah has modified Brian
-      Kernighan's `awk' to provide timing and profiling information.  It
-      is different from `gawk' with the `--profile' option.  (*note
-      Profiling::), in that it uses CPU-based profiling, not line-count
-      profiling.  You may find it at either
-      `ftp://ftp.math.utah.edu/pub/pawk/pawk-20030606.tar.gz' or
-      `http://www.math.utah.edu/pub/pawk/pawk-20030606.tar.gz'.
- 
- Busybox Awk
-      Busybox is a GPL-licensed program providing small versions of many
-      applications within a single executable. It is aimed at embedded
-      systems.  It includes a full implementation of POSIX `awk'.  When
-      building it, be careful not to do `make install' as it will
-      overwrite copies of other applications in your `/usr/local/bin'.
-      For more information, see the project's home page
-      (http://busybox.net).
- 
- The OpenSolaris POSIX `awk'
-      The version of `awk' in `/usr/xpg4/bin' on Solaris is more-or-less
-      POSIX-compliant. It is based on the `awk' from Mortice Kern
-      Systems for PCs.  The source code can be downloaded from the
-      OpenSolaris web site (http://www.opensolaris.org).  This author
-      was able to make it compile and work under GNU/Linux with 1-2
-      hours of work.  Making it more generally portable (using GNU
-      Autoconf and/or Automake) would take more work, and this has not
-      been done, at least to our knowledge.
- 
- `jawk'
-      This is an interpreter for `awk' written in Java. It claims to be
-      a full interpreter, although because it uses Java facilities for
-      I/O and for regexp matching, the language it supports is different
-      from POSIX `awk'.  More information is available on the project's
-      home page (http://jawk.sourceforge.net).
- 
- Libmawk
-      This is an embeddable `awk' interpreter derived from `mawk'. For
-      more information see `http://repo.hu/projects/libmawk/'.
- 
- QSE Awk
-      This is an embeddable `awk' interpreter. For more information see
-      `http://code.google.com/p/qse/' and `http://awk.info/?tools/qse'.
- 
- `QTawk'
-      This is an independent implementation of `awk' distributed under
-      the GPL. It has a large number of extensions over standard `awk'
-      and may not be 100% syntactically compatible with it.  See
-      `http://www.quiktrim.org/QTawk.html' for more information,
-      including the manual and a download link.
- 
- `xgawk'
-      XML `gawk'.  This is a fork of the `gawk' 3.1.6 source base to
-      support processing XML files. It has a number of interesting
-      extensions which should one day be integrated into the main `gawk'
-      code base.  For more information, see the XMLgawk project web site
-      (http://xmlgawk.sourceforge.net).
- 
- 
- 
- File: gawk.info,  Node: Notes,  Next: Basic Concepts,  Prev: Installation,  
Up: Top
- 
- Appendix C Implementation Notes
- *******************************
- 
- This appendix contains information mainly of interest to implementers
- and maintainers of `gawk'.  Everything in it applies specifically to
- `gawk' and not to other implementations.
- 
- * Menu:
- 
- * Compatibility Mode::          How to disable certain `gawk'
-                                 extensions.
- * Additions::                   Making Additions To `gawk'.
- * Dynamic Extensions::          Adding new built-in functions to
-                                 `gawk'.
- * Future Extensions::           New features that may be implemented one day.
- 
- 
- File: gawk.info,  Node: Compatibility Mode,  Next: Additions,  Up: Notes
- 
- C.1 Downward Compatibility and Debugging
- ========================================
- 
- *Note POSIX/GNU::, for a summary of the GNU extensions to the `awk'
- language and program.  All of these features can be turned off by
- invoking `gawk' with the `--traditional' option or with the `--posix'
- option.
- 
-    If `gawk' is compiled for debugging with `-DDEBUG', then there is
- one more option available on the command line:
- 
- `-Y'
- `--parsedebug'
-      Prints out the parse stack information as the program is being
-      parsed.
- 
-    This option is intended only for serious `gawk' developers and not
- for the casual user.  It probably has not even been compiled into your
- version of `gawk', since it slows down execution.
- 
- 
- File: gawk.info,  Node: Additions,  Next: Dynamic Extensions,  Prev: 
Compatibility Mode,  Up: Notes
- 
- C.2 Making Additions to `gawk'
- ==============================
- 
- If you find that you want to enhance `gawk' in a significant fashion,
- you are perfectly free to do so.  That is the point of having free
- software; the source code is available and you are free to change it as
- you want (*note Copying::).
- 
-    This minor node discusses the ways you might want to change `gawk'
- as well as any considerations you should bear in mind.
- 
- * Menu:
- 
- * Accessing The Source::        Accessing the Git repository.
- * Adding Code::                 Adding code to the main body of
-                                 `gawk'.
- * New Ports::                   Porting `gawk' to a new operating
-                                 system.
- 
- 
- File: gawk.info,  Node: Accessing The Source,  Next: Adding Code,  Up: 
Additions
- 
- C.2.1 Accessing The `gawk' Git Repository
- -----------------------------------------
- 
- As `gawk' is Free Software, the source code is always available.  *note
- Gawk Distribution::, describes how to get and build the formal,
- released versions of `gawk'.
- 
-    However, if you want to modify `gawk' and contribute back your
- changes, you will probably wish to work with the development version.
- To do so, you will need to access the `gawk' source code repository.
- The code is maintained using the Git distributed version control system
- (http://git-scm.com/).  You will need to install it if your system
- doesn't have it.  Once you have done so, use the command:
- 
-      git clone git://git.savannah.gnu.org/gawk.git
- 
- This will clone the `gawk' repository.  If you are behind a firewall
- that will not allow you to use the Git native protocol, you can still
- access the repository using:
- 
-      git clone http://git.savannah.gnu.org/r/gawk.git
- 
-    Once you have made changes, you can use `git diff' to produce a
- patch, and send that to the `gawk' maintainer; see *note Bugs:: for how
- to do that.
- 
-    Finally, if you cannot install Git (e.g., if it hasn't been ported
- yet to your operating system), you can use the Git-CVS gateway to check
- out a copy using CVS, as follows:
- 
-      cvs -d:pserver:address@hidden:/gawk.git co -d gawk master
- 
- 
- File: gawk.info,  Node: Adding Code,  Next: New Ports,  Prev: Accessing The 
Source,  Up: Additions
- 
- C.2.2 Adding New Features
- -------------------------
- 
- You are free to add any new features you like to `gawk'.  However, if
- you want your changes to be incorporated into the `gawk' distribution,
- there are several steps that you need to take in order to make it
- possible to include your changes:
- 
-   1. Before building the new feature into `gawk' itself, consider
-      writing it as an extension module (*note Dynamic Extensions::).
-      If that's not possible, continue with the rest of the steps in
-      this list.
- 
-   2. Be prepared to sign the appropriate paperwork.  In order for the
-      FSF to distribute your changes, you must either place those
-      changes in the public domain and submit a signed statement to that
-      effect, or assign the copyright in your changes to the FSF.  Both
-      of these actions are easy to do and _many_ people have done so
-      already. If you have questions, please contact me (*note Bugs::),
-      or <address@hidden>.
- 
-   3. Get the latest version.  It is much easier for me to integrate
-      changes if they are relative to the most recent distributed
-      version of `gawk'.  If your version of `gawk' is very old, I may
-      not be able to integrate them at all.  (*Note Getting::, for
-      information on getting the latest version of `gawk'.)
- 
-   4. See *note (Version)Top:: standards, GNU Coding Standards.  This
-      document describes how GNU software should be written. If you
-      haven't read it, please do so, preferably _before_ starting to
-      modify `gawk'.  (The `GNU Coding Standards' are available from the
-      GNU Project's web site
-      (http://www.gnu.org/prep/standards_toc.html).  Texinfo, Info, and
-      DVI versions are also available.)
- 
-   5. Use the `gawk' coding style.  The C code for `gawk' follows the
-      instructions in the `GNU Coding Standards', with minor exceptions.
-      The code is formatted using the traditional "K&R" style,
-      particularly as regards to the placement of braces and the use of
-      TABs.  In brief, the coding rules for `gawk' are as follows:
- 
-         * Use ANSI/ISO style (prototype) function headers when defining
-           functions.
- 
-         * Put the name of the function at the beginning of its own line.
- 
-         * Put the return type of the function, even if it is `int', on
-           the line above the line with the name and arguments of the
-           function.
- 
-         * Put spaces around parentheses used in control structures
-           (`if', `while', `for', `do', `switch', and `return').
- 
-         * Do not put spaces in front of parentheses used in function
-           calls.
- 
-         * Put spaces around all C operators and after commas in
-           function calls.
- 
-         * Do not use the comma operator to produce multiple side
-           effects, except in `for' loop initialization and increment
-           parts, and in macro bodies.
- 
-         * Use real TABs for indenting, not spaces.
- 
-         * Use the "K&R" brace layout style.
- 
-         * Use comparisons against `NULL' and `'\0'' in the conditions of
-           `if', `while', and `for' statements, as well as in the `case's
-           of `switch' statements, instead of just the plain pointer or
-           character value.
- 
-         * Use the `TRUE', `FALSE' and `NULL' symbolic constants and the
-           character constant `'\0'' where appropriate, instead of `1'
-           and `0'.
- 
-         * Provide one-line descriptive comments for each function.
- 
-         * Do not use the `alloca()' function for allocating memory off
-           the stack.  Its use causes more portability trouble than is
-           worth the minor benefit of not having to free the storage.
-           Instead, use `malloc()' and `free()'.
- 
-         * Do not use comparisons of the form `! strcmp(a, b)' or
-           similar.  As Henry Spencer once said, "`strcmp()' is not a
-           boolean!"  Instead, use `strcmp(a, b) == 0'.
- 
-         * If adding new bit flag values, use explicit hexadecimal
-           constants (`0x001', `0x002', `0x004', and son on) instead of
-           shifting one left by successive amounts (`(1<<0)', `(1<<1)',
-           and so on).
- 
-           NOTE: If I have to reformat your code to follow the coding
-           style used in `gawk', I may not bother to integrate your
-           changes at all.
- 
-   6. Update the documentation.  Along with your new code, please supply
-      new sections and/or chapters for this Info file.  If at all
-      possible, please use real Texinfo, instead of just supplying
-      unformatted ASCII text (although even that is better than no
-      documentation at all).  Conventions to be followed in `GAWK:
-      Effective AWK Programming' are provided after the address@hidden' at the
-      end of the Texinfo source file.  If possible, please update the
-      `man' page as well.
- 
-      You will also have to sign paperwork for your documentation
-      changes.
- 
-   7. Submit changes as unified diffs.  Use `diff -u -r -N' to compare
-      the original `gawk' source tree with your version.  I recommend
-      using the GNU version of `diff'.  Send the output produced by
-      either run of `diff' to me when you submit your changes.  (*Note
-      Bugs::, for the electronic mail information.)
- 
-      Using this format makes it easy for me to apply your changes to the
-      master version of the `gawk' source code (using `patch').  If I
-      have to apply the changes manually, using a text editor, I may not
-      do so, particularly if there are lots of changes.
- 
-   8. Include an entry for the `ChangeLog' file with your submission.
-      This helps further minimize the amount of work I have to do,
-      making it easier for me to accept patches.
- 
-    Although this sounds like a lot of work, please remember that while
- you may write the new code, I have to maintain it and support it. If it
- isn't possible for me to do that with a minimum of extra work, then I
- probably will not.
- 
- 
- File: gawk.info,  Node: New Ports,  Prev: Adding Code,  Up: Additions
- 
- C.2.3 Porting `gawk' to a New Operating System
- ----------------------------------------------
- 
- If you want to port `gawk' to a new operating system, there are several
- steps:
- 
-   1. Follow the guidelines in *note Adding Code::, concerning coding
-      style, submission of diffs, and so on.
- 
-   2. Be prepared to sign the appropriate paperwork.  In order for the
-      FSF to distribute your code, you must either place your code in
-      the public domain and submit a signed statement to that effect, or
-      assign the copyright in your code to the FSF.  Both of these
-      actions are easy to do and _many_ people have done so already. If
-      you have questions, please contact me, or <address@hidden>.
- 
-   3. When doing a port, bear in mind that your code must coexist
-      peacefully with the rest of `gawk' and the other ports. Avoid
-      gratuitous changes to the system-independent parts of the code. If
-      at all possible, avoid sprinkling `#ifdef's just for your port
-      throughout the code.
- 
-      If the changes needed for a particular system affect too much of
-      the code, I probably will not accept them.  In such a case, you
-      can, of course, distribute your changes on your own, as long as
-      you comply with the GPL (*note Copying::).
- 
-   4. A number of the files that come with `gawk' are maintained by other
-      people.  Thus, you should not change them unless it is for a very
-      good reason; i.e., changes are not out of the question, but
-      changes to these files are scrutinized extra carefully.  The files
-      are `dfa.c', `dfa.h', `getopt1.c', `getopt.c', `getopt.h',
-      `install-sh', `mkinstalldirs', `regcomp.c', `regex.c',
-      `regexec.c', `regexex.c', `regex.h', `regex_internal.c', and
-      `regex_internal.h'.
- 
-   5. Be willing to continue to maintain the port.  Non-Unix operating
-      systems are supported by volunteers who maintain the code needed
-      to compile and run `gawk' on their systems. If noone volunteers to
-      maintain a port, it becomes unsupported and it may be necessary to
-      remove it from the distribution.
- 
-   6. Supply an appropriate `gawkmisc.???' file.  Each port has its own
-      `gawkmisc.???' that implements certain operating system specific
-      functions. This is cleaner than a plethora of `#ifdef's scattered
-      throughout the code.  The `gawkmisc.c' in the main source
-      directory includes the appropriate `gawkmisc.???' file from each
-      subdirectory.  Be sure to update it as well.
- 
-      Each port's `gawkmisc.???' file has a suffix reminiscent of the
-      machine or operating system for the port--for example,
-      `pc/gawkmisc.pc' and `vms/gawkmisc.vms'. The use of separate
-      suffixes, instead of plain `gawkmisc.c', makes it possible to move
-      files from a port's subdirectory into the main subdirectory,
-      without accidentally destroying the real `gawkmisc.c' file.
-      (Currently, this is only an issue for the PC operating system
-      ports.)
- 
-   7. Supply a `Makefile' as well as any other C source and header files
-      that are necessary for your operating system.  All your code
-      should be in a separate subdirectory, with a name that is the same
-      as, or reminiscent of, either your operating system or the
-      computer system.  If possible, try to structure things so that it
-      is not necessary to move files out of the subdirectory into the
-      main source directory.  If that is not possible, then be sure to
-      avoid using names for your files that duplicate the names of files
-      in the main source directory.
- 
-   8. Update the documentation.  Please write a section (or sections)
-      for this Info file describing the installation and compilation
-      steps needed to compile and/or install `gawk' for your system.
- 
-    Following these steps makes it much easier to integrate your changes
- into `gawk' and have them coexist happily with other operating systems'
- code that is already there.
- 
-    In the code that you supply and maintain, feel free to use a coding
- style and brace layout that suits your taste.
- 
- 
- File: gawk.info,  Node: Dynamic Extensions,  Next: Future Extensions,  Prev: 
Additions,  Up: Notes
- 
- C.3 Adding New Built-in Functions to `gawk'
- ===========================================
- 
-      Danger Will Robinson!  Danger!!
-      Warning! Warning!
-      The Robot
- 
-    It is possible to add new built-in functions to `gawk' using
- dynamically loaded libraries. This facility is available on systems
- (such as GNU/Linux) that support the C `dlopen()' and `dlsym()'
- functions.  This minor node describes how to write and use dynamically
- loaded extensions for `gawk'.  Experience with programming in C or C++
- is necessary when reading this minor node.
- 
-      CAUTION: The facilities described in this minor node are very much
-      subject to change in a future `gawk' release.  Be aware that you
-      may have to re-do everything, at some future time.
- 
-      If you have written your own dynamic extensions, be sure to
-      recompile them for each new `gawk' release.  There is no guarantee
-      of binary compatibility between different releases, nor will there
-      ever be such a guarantee.
- 
-      NOTE: When `--sandbox' is specified, extensions are disabled
-      (*note Options::.
- 
- * Menu:
- 
- * Internals::                   A brief look at some `gawk' internals.
- * Plugin License::              A note about licensing.
- * Loading Extensions::          How to load dynamic extensions.
- * Sample Library::              A example of new functions.
- 
- 
- File: gawk.info,  Node: Internals,  Next: Plugin License,  Up: Dynamic 
Extensions
- 
- C.3.1 A Minimal Introduction to `gawk' Internals
- ------------------------------------------------
- 
- The truth is that `gawk' was not designed for simple extensibility.
- The facilities for adding functions using shared libraries work, but
- are something of a "bag on the side."  Thus, this tour is brief and
- simplistic; would-be `gawk' hackers are encouraged to spend some time
- reading the source code before trying to write extensions based on the
- material presented here.  Of particular note are the files `awk.h',
- `builtin.c', and `eval.c'.  Reading `awkgram.y' in order to see how the
- parse tree is built would also be of use.
- 
-    With the disclaimers out of the way, the following types, structure
- members, functions, and macros are declared in `awk.h' and are of use
- when writing extensions.  The next minor node shows how they are used:
- 
- `AWKNUM'
-      An `AWKNUM' is the internal type of `awk' floating-point numbers.
-      Typically, it is a C `double'.
- 
- `NODE'
-      Just about everything is done using objects of type `NODE'.  These
-      contain both strings and numbers, as well as variables and arrays.
- 
- `AWKNUM force_number(NODE *n)'
-      This macro forces a value to be numeric. It returns the actual
-      numeric value contained in the node.  It may end up calling an
-      internal `gawk' function.
- 
- `void force_string(NODE *n)'
-      This macro guarantees that a `NODE''s string value is current.  It
-      may end up calling an internal `gawk' function.  It also
-      guarantees that the string is zero-terminated.
- 
- `void force_wstring(NODE *n)'
-      Similarly, this macro guarantees that a `NODE''s wide-string value
-      is current.  It may end up calling an internal `gawk' function.
-      It also guarantees that the wide string is zero-terminated.
- 
- `nargs'
-      Inside an extension function, this is the actual number of
-      parameters passed to the current function.
- 
- `n->stptr'
- `n->stlen'
-      The data and length of a `NODE''s string value, respectively.  The
-      string is _not_ guaranteed to be zero-terminated.  If you need to
-      pass the string value to a C library function, save the value in
-      `n->stptr[n->stlen]', assign `'\0'' to it, call the routine, and
-      then restore the value.
- 
- `n->wstptr'
- `n->wstlen'
-      The data and length of a `NODE''s wide-string value, respectively.
-      Use `force_wstring()' to make sure these values are current.
- 
- `n->type'
-      The type of the `NODE'. This is a C `enum'. Values should be one
-      of `Node_var', `Node_var_new', or `Node_var_array' for function
-      parameters.
- 
- `n->vname'
-      The "variable name" of a node.  This is not of much use inside
-      externally written extensions.
- 
- `void assoc_clear(NODE *n)'
-      Clears the associative array pointed to by `n'.  Make sure that
-      `n->type == Node_var_array' first.
- 
- `NODE **assoc_lookup(NODE *symbol, NODE *subs)'
-      Finds, and installs if necessary, array elements.  `symbol' is the
-      array, `subs' is the subscript.  This is usually a value created
-      with `make_string()' (see below).
- 
- `NODE *make_string(char *s, size_t len)'
-      Take a C string and turn it into a pointer to a `NODE' that can be
-      stored appropriately.  This is permanent storage; understanding of
-      `gawk' memory management is helpful.
- 
- `NODE *make_number(AWKNUM val)'
-      Take an `AWKNUM' and turn it into a pointer to a `NODE' that can
-      be stored appropriately.  This is permanent storage; understanding
-      of `gawk' memory management is helpful.
- 
- `NODE *dupnode(NODE *n)'
-      Duplicate a node.  In most cases, this increments an internal
-      reference count instead of actually duplicating the entire `NODE';
-      understanding of `gawk' memory management is helpful.
- 
- `void unref(NODE *n)'
-      This macro releases the memory associated with a `NODE' allocated
-      with `make_string()' or `make_number()'.  Understanding of `gawk'
-      memory management is helpful.
- 
- `void make_builtin(const char *name, NODE *(*func)(NODE *), int count)'
-      Register a C function pointed to by `func' as new built-in
-      function `name'. `name' is a regular C string. `count' is the
-      maximum number of arguments that the function takes.  The function
-      should be written in the following manner:
- 
-           /* do_xxx --- do xxx function for gawk */
- 
-           NODE *
-           do_xxx(int nargs)
-           {
-               ...
-           }
- 
- `NODE *get_argument(int i)'
-      This function is called from within a C extension function to get
-      the `i'-th argument from the function call.  The first argument is
-      argument zero.
- 
- `NODE *get_actual_argument(int i,'
- `                          int optional, int wantarray);'
-      This function retrieves a particular argument `i'.  `wantarray' is
-      `TRUE' if the argument should be an array, `FALSE' otherwise. If
-      `optional' is `TRUE', the argument need not have been supplied.
-      If it wasn't, the return value is `NULL'.  It is a fatal error if
-      `optional' is `TRUE' but the argument was not provided.
- 
- `get_scalar_argument(i, opt)'
-      This is a convenience macro that calls `get_actual_argument()'.
- 
- `get_array_argument(i, opt)'
-      This is a convenience macro that calls `get_actual_argument()'.
- 
- `void update_ERRNO_int(int errno_saved)'
-      This function is called from within a C extension function to set
-      the value of `gawk''s `ERRNO' variable, based on the error value
-      provided as the argument.  It is provided as a convenience.
- 
- `void update_ERRNO_string(const char *string, enum errno_translate)'
-      This function is called from within a C extension function to set
-      the value of `gawk''s `ERRNO' variable to a given string.  The
-      second argument determines whether the string is translated before
-      being installed into `ERRNO'.  It is provided as a convenience.
- 
- `void unset_ERRNO(void)'
-      This function is called from within a C extension function to set
-      the value of `gawk''s `ERRNO' variable to a null string.  It is
-      provided as a convenience.
- 
- `void register_deferred_variable(const char *name, NODE *(*load_func)(void))'
-      This function is called to register a function to be called when a
-      reference to an undefined variable with the given name is
-      encountered.  The callback function will never be called if the
-      variable exists already, so, unless the calling code is running at
-      program startup, it should first check whether a variable of the
-      given name already exists.  The argument function must return a
-      pointer to a `NODE' containing the newly created variable.  This
-      function is used to implement the builtin `ENVIRON' and `PROCINFO'
-      arrays, so you can refer to them for examples.
- 
- `void register_open_hook(void *(*open_func)(IOBUF *))'
-      This function is called to register a function to be called
-      whenever a new data file is opened, leading to the creation of an
-      `IOBUF' structure in `iop_alloc()'.  After creating the new
-      `IOBUF', `iop_alloc()' will call (in reverse order of
-      registration, so the last function registered is called first)
-      each open hook until one returns non-`NULL'.  If any hook returns
-      a non-`NULL' value, that value is assigned to the `IOBUF''s
-      `opaque' field (which will presumably point to a structure
-      containing additional state associated with the input processing),
-      and no further open hooks are called.
- 
-      The function called will most likely want to set the `IOBUF''s
-      `get_record' method to indicate that future input records should
-      be retrieved by calling that method instead of using the standard
-      `gawk' input processing.
- 
-      And the function will also probably want to set the `IOBUF''s
-      `close_func' method to be called when the file is closed to clean
-      up any state associated with the input.
- 
-      Finally, hook functions should be prepared to receive an `IOBUF'
-      structure where the `fd' field is set to `INVALID_HANDLE', meaning
-      that `gawk' was not able to open the file itself. In this case,
-      the hook function must be able to successfully open the file and
-      place a valid file descriptor there.
- 
-      Currently, for example, the hook function facility is used to
-      implement the XML parser shared library extension.  For more info,
-      please look in `awk.h' and in `io.c'.
- 
-    An argument that is supposed to be an array needs to be handled with
- some extra code, in case the array being passed in is actually from a
- function parameter.
- 
-    The following boilerplate code shows how to do this:
- 
-      NODE *the_arg;
- 
-      /* assume need 3rd arg, 0-based */
-      the_arg = get_array_argument(2, FALSE);
- 
-    Again, you should spend time studying the `gawk' internals; don't
- just blindly copy this code.
- 
- 
- File: gawk.info,  Node: Plugin License,  Next: Loading Extensions,  Prev: 
Internals,  Up: Dynamic Extensions
- 
- C.3.2 Extension Licensing
- -------------------------
- 
- Every dynamic extension should define the global symbol
- `plugin_is_GPL_compatible' to assert that it has been licensed under a
- GPL-compatible license.  If this symbol does not exist, `gawk' will
- emit a fatal error and exit.
- 
-    The declared type of the symbol should be `int'.  It does not need
- to be in any allocated section, though.  The code merely asserts that
- the symbol exists in the global scope.  Something like this is enough:
- 
-      int plugin_is_GPL_compatible;
- 
- 
- File: gawk.info,  Node: Loading Extensions,  Next: Sample Library,  Prev: 
Plugin License,  Up: Dynamic Extensions
- 
- C.3.3 Loading a Dynamic Extension
- ---------------------------------
- 
- There are two ways to load a dynamically linked library. The first is
- to use the builtin `extension()':
- 
-      extension(libname, init_func)
- 
-    where `libname' is the library to load, and `init_func' is the name
- of the initialization or bootstrap routine to run once loaded.
- 
-    The second method for dynamic loading of a library is to use the
- command line option `-l':
- 
-      $ gawk -l libname -f myprog
- 
-    This will work only if the initialization routine is named
- `dlload()'.
- 
-    If you use `extension()', the library will be loaded at run time.
- This means that the functions are available only to the rest of your
- script. If you use the command line option `-l' instead, the library
- will be loaded before `gawk' starts comp