GNU bug report logs - #28056
Substitution doing global when it should not.

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From: Techwolf Lupindo <techwolf.lupindo <at> gmail.com>
To: Eric Blake <eblake <at> redhat.com>
Cc: 28056-done <at> debbugs.gnu.org
Subject: bug#28056: Substitution doing global when it should not.
Date: Fri, 11 Aug 2017 22:19:46 -0400

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The docs say:
g
Apply the replacement to all matches to the regexp, not just the first.

Nothing in the docs say it will matches all lines in the input, wither
piped or file inplace. Doc bug?

On Fri, Aug 11, 2017 at 6:39 PM, Eric Blake <eblake <at> redhat.com> wrote:

> tag 28056 notabug
> thanks
>
> On 08/11/2017 03:04 PM, Techwolf Lupindo wrote:
>
> > techwolf <at> laptop ~/test $ sed -e 's:COMMAND ${MERCURIAL}:COMMAND
> > ${MERCURIAL} --cwd ${CMAKE_SOURCE_DIR}:' BuildVersion.cmake >
> > BuildVersion1.cmake
> > techwolf <at> laptop ~/test $ sed -e 's:COMMAND ${MERCURIAL}:COMMAND
> > ${MERCURIAL} --cwd ${CMAKE_SOURCE_DIR}:g' BuildVersion.cmake >
> > BuildVersion2.cmake
>
>
> >
> > The only difference in the sed commands was s/// and s///g.
>
> Which tells sed whether to make one substitution on every affected line,
> or as many substitutions as possible on every affected line.  But the
> number of affected lines remains unchanged for the two variants.
>
> > The diff
> > between BuildVersion1.cmake and BuildVersion2.cmake should had one
> change.
>
> Rather, one change per affected line.
>
> > But sed is matching two lines without the s///g. All docs I have read
> says
> > that without 'g', only the first match is changed.
>
> Only the first match per line - but when both lines affected only had
> one match per line, there's no difference in using the 'g' flag.
>
> If you want sed to stop processing as soon as it has changed one line,
> rather than going on to look for additional affected lines, you can do so:
>
> sed -e '/COMMAND ${MERCURIAL}/ { s::COMMAND ${MERCURIAL} -- cwd
> ${CMAKE_SOURCE_DIR}: ; q }'
>
> which says find the first line that matches the pattern, and on that
> line make the substitution and immediately quit.
>
> Since sed is behaving as documented, I'm closing this as not a bug;
> however, feel free to add further comments to this thread if you need
> followup clarification.
>
> --
> Eric Blake, Principal Software Engineer
> Red Hat, Inc.           +1-919-301-3266
> Virtualization:  qemu.org | libvirt.org
>
>

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