Re: command for relative path

[Peng Yu]

On Sun, Nov 13, 2011 at 12:45 PM, Dmitry V. Levin <address@hidden> wrote:
> Hi,
>
> On Sat, Nov 12, 2011 at 11:45:56AM -0700, Eric Blake wrote:
> [...]
>> http://docs.python.org/library/os.path.html
>> >> os.path.relpath(path[, start])
>> >>
>> >>     Return a relative filepath to path either from the current directory 
>> >> or from an optional start point.
>>
>> Ah, so the idea is that python has a function that computes a relative
>> pathname to one path given a starting point:
>>
>> $ relpath /usr/bin /tmp
>> ../usr/bin
>> $ relpath /usr/bin /usr/share
>> ../bin
>
> Btw, there is an utility in freebsd ports called relpath:

This is good to know.

Just to double check. Due to the license for the FreeBSD's relpath, it
can not be copied to coreutils. Right?

> http://www.freebsd.org/cgi/cvsweb.cgi/ports/sysutils/relpath/
> ftp://ftp.freebsd.org/pub/FreeBSD/ports/local-distfiles/beech/relpath-0.1.0.tar.gz
>
> $ lftp -c 'cat 
> http://www.freebsd.org/cgi/cvsweb.cgi/~checkout~/ports/sysutils/relpath/pkg-descr'
> Usage: relpath [-d DIR] START_DIR END_PATH
>
> Find a relative path from START_DIR to END_PATH.
> Prints the relative path on standard out.
>
> If -d DIR, then only emit a relative path if both
> START_DIR and END_PATH are sub-directories of DIR;
> otherwise, emit an absolute path to END_PATH.
>
>> This seems like you could do it in shell without resorting to python, by
>> computing a canonical name for both the destination and the starting
>> point, then comparing common prefixes, and for every directory component
>> that differs after the common prefix, replacing the directory with
>> '../'.  This has been done before; for example, this is from gnulib-tool:
>>
>> # func_relativize DIR1 DIR2
>> # computes a relative pathname RELDIR such that DIR1/RELDIR = DIR2.
>> # Input:
>> # - DIR1            relative pathname, relative to the current directory
>> # - DIR2            relative pathname, relative to the current directory
>> # Output:
>> # - reldir          relative pathname of DIR2, relative to DIR1
>> func_relativize ()
>> {
>>   dir0=`pwd`
>>   dir1="$1"
>>   dir2="$2"
>>   sed_first='s,^\([^/]*\)/.*$,\1,'
>>   sed_rest='s,^[^/]*/*,,'
>>   sed_last='s,^.*/\([^/]*\)$,\1,'
>>   sed_butlast='s,/*[^/]*$,,'
>>   while test -n "$dir1"; do
>>     first=`echo "$dir1" | sed -e "$sed_first"`
>>     if test "$first" != "."; then
>>       if test "$first" = ".."; then
>>         dir2=`echo "$dir0" | sed -e "$sed_last"`/"$dir2"
>>         dir0=`echo "$dir0" | sed -e "$sed_butlast"`
>>       else
>>         first2=`echo "$dir2" | sed -e "$sed_first"`
>>         if test "$first2" = "$first"; then
>>           dir2=`echo "$dir2" | sed -e "$sed_rest"`
>>         else
>>           dir2="../$dir2"
>>         fi
>>         dir0="$dir0"/"$first"
>>       fi
>>     fi
>>     dir1=`echo "$dir1" | sed -e "$sed_rest"`
>>   done
>>   reldir="$dir2"
>> }
>>
>> Doing it in fewer processes seems possible with something like a single
>> awk script, although I haven't tried writing one; at any rate, awk would
>> be more portable than python for the same task.
>>
>> But yes, providing this as an alternative mode of coreutils' realpath
>> instead of scripting it in shell or awk seems like a useful addition -
>> would you care to submit a patch?
>
> AFAIR the coreutils' realpath is called readlink. ;)
>
> Would "readlink --relative" or something like this be a good choice?
> Or should it rather be a new utility?

I think a new utility relpath will be more descriptive.


-- 
Regards,
Peng