Re: diffutils file mode (was Re: [PATCH 5.15 00/37] 5.15.96-rc2 review)

[Linus Torvalds]

On Fri, Feb 24, 2023 at 11:32 AM Andrew Morton
<akpm@xxxxxxxxxxxxxxxxxxxx> wrote:
>
> Can we use git instead of diff?  I tried once, but it didn't work -
> perhaps because it didn't like doing stuff outside a git repo.

Just using "git diff" does work *but* I would strongly suggest using
"--no-index" as in

    git diff --no-index -- path1 path2

because without the "--no-index" you will find that "git diff" will
use heuristics to decide what it is you want to do.

So if you do just

    git diff path1 path2

and both of those paths are *inside* a git directory, then git thinks
that "oh, you want to see the diff of those two paths against the
current git index", and does something *very* different from showing
the diff between those two paths.

And I suspect that is the exact reason you *thought* it didn't work,
but now that you tried it in a new test-directory, it did work for
you.

With the "--no-index", the ambiguity of "do you want a diff against
git state, or the files against each other" goes away

Just to give another example of this:

 (a) when I'm in my kernel tree, I can do

      $ git diff .config /etc/kernel-config

and it will show me the diff between the two files, because while my
".config" file is inside the repository, "/etc/kernel-config" is
clearly not, so I get that "diff between two files" behavior.

 (b) but then if I do a conceptually similar

    $ git diff .config arch/x86/configs/x86_64_defconfig

then git will see that both paths *are* inside the repository, and
think I'm doing a diff vs the git index state, and since I have no
changes wrt any checked in state in any paths that match, it will show
no diff at all.

So if I actually want to see the file diff between those two paths, I have to do

    $ git diff --no-index .config arch/x86/configs/x86_64_defconfig

to clarify what it is that I want.

Also note that "git diff" is *not* a replacement for the 'diff' binary
from diffutils in general.

Doing a 'git diff' will *only* generate the extended git unified
diffs. There's no support for any other diff format, and while there
is overlap in the command line switches, there's a lot of differences
too.

So "git diff" is very much a "you can use it as a replacement for
plain 'diff', but only in very specific circumstances" thing.

                 Linus