Re: confused git diff -G

[Taylor Blau <me@ttaylorr.com>]

On Thu, Aug 24, 2023 at 06:08:18PM -0400, Alexei Podtelezhnikov wrote:
> I find this sections of the docs confusing:
>
> git diff -G (https://git.kernel.org/pub/scm/git/git.git/tree/Documentation/diff-options.txt#n656)
>
>  I do not follow why the example talks about `git log -G` and commits.
> I see that thai file is included in git-log .txt but I do not
> understand how to use git  diff -G.

I agree that it can be somewhat confusing :-).

The linked section DIFFCORE-PICKAXE in gitdiffcore(7) may be helpful:

    DIFFCORE-PICKAXE: FOR DETECTING ADDITION/DELETION OF SPECIFIED STRING
           This transformation limits the set of filepairs to those that
           change specified strings between the preimage and the
           postimage in a certain way. -S<block of text> and -G<regular
           expression> options are used to specify different ways these
           strings are sought.

           [...]

           "-G<regular expression>" (mnemonic: grep) detects filepairs
           whose textual diff has an added or a deleted line that
           matches the given regular expression. This means that it will
           detect in-file (or what rename-detection considers the same
           file) moves, which is noise. The implementation runs diff
           twice and greps, and this can be quite expensive. To speed
           things up binary files without textconv filters will be
           ignored.

So if I have a setup like:

    $ git init repo
    $ git -C repo commit --allow-empty -m base
    $ for c in a b; do echo $c>repo/$c && git -C $repo add $c; done
    $ git -C repo commit -m changes

Then you can see `-G` has the effect of limiting the output of 'git
diff' to just those file(s) whose diff matches the regular expression
given to `-G`, like so:

    $ git -C repo diff --stat HEAD^
     a | 1 +
     b | 1 +
     2 files changed, 2 insertions(+)
    $ git -C repo diff --stat HEAD^ -G a
     a | 1 +
     1 file changed, 1 insertion(+)
    $ git -C repo diff --stat HEAD^ -G b
     b | 1 +
     1 file changed, 1 insertion(+)

(I'm using --stat here to keep the output brief, since the contents are
only important insomuch as files "a" and "b" match the regular
expressions "a" and "b", respectively).

Thanks,
Taylor