grep problems

[Michal Zalewski]

grep (GNU grep) 2.4.2... Not necessarily bugs, but I think it would be a
good idea to implement it:

1) -A, -B and -num options have undesired effects in -v mode:

  $ echo -e 'one\ntwo\nthree\nfour\nfive' | grep -1 three
  two
  three
  four

  This is correct. "Three" and one line before and after is picked.
  Now, -v option is supposed to reverse 'grep' behavior. Thus,
  we should get everything except lines that normal 'grep' returned:

  $ echo -e 'one\ntwo\nthree\nfour\nfive' | grep -v -1 three
  one
  two
  three
  four
  five

  Wouldn't it be more reasonable, suitable and useful for the last command
  to return "one, five"? In my opinion, as -v mode should be an opposite
  of normal mode - so grep -v something always return everything that
  'grep something' didn't return and nothing more... -A, -B and -nnn options 
  should refer to number of lines skipped along with the matching line.

  Of course, you might disagree, but then, is there any chance to have
  appropriate option to modify default behavior and make -A and -B
  work the way I described?

2) Buffering. Very often, grep is used this way:
 
   ./somecommand | grep something | othercommands

  ...or this way:

   ./somecommand | grep something | grep -v something

  In both cases, the second command (grep something) is having stdout
  redirected to something that is not a terminal. As a result, libc
  deploys rather heavy buffering, and output is not continous. For
  example, if somecommand generates one line of output every second,
  this sequence will cause 'othercommands' or 'grep -v something'
  to get 10 lines of output every 10 seconds, and nothing in between.
  This is very often something not desirable, especially if time
  intervals for 'somecommand' output are irregular and longer than one
  second. The most obvious example would be:

  tail -f /var/log/messages | grep -i pop3 | grep -vF '123.123.123.123'

  You won't see continous output in real-time.

  Some commands that are supposed to be used in such sequences (e.g.
  tcpdump), implement an option to perform fflush(stdout) after
  every line (for tcpdump, this would be -l option). This enables
  the user to arrange constant flow of text information in such
  command sequence. Many others simply fflush(stdout) after 
  every line, no matter what (e.g. tail -f).

  Do you believe it makes sense to implement similar option for grep?
  It certainly wouldn't hurt, and might be very useful for many
  people.

Thanks,
-- 
_____________________________________________________
Michal Zalewski address@hidden [security]
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