Subject: Re: bug#28056: Substitution doing global when it should not.

From: Eric Blake <[email-masked]>
Date: Fri, 11 Aug 2017 17:39:27 -0500

tag 28056 notabug
thanks

On 08/11/2017 03:04 PM, Techwolf Lupindo wrote:

> techwolf@laptop ~/test $ sed -e 's:COMMAND ${MERCURIAL}:COMMAND
> ${MERCURIAL} --cwd ${CMAKE_SOURCE_DIR}:' BuildVersion.cmake >
> BuildVersion1.cmake
> techwolf@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