Re: git-diff: --ignore-matching-lines has no effect on the output when --name-only is used

From: Lidong Yan <yldhome2d2@gmail.com>
Date: Jul 23, 2025, 08:00 UTC
Message-ID: <12095AC3-BC38-48DB-95B4-394F9F7DE054@gmail.com>
In-Reply-To: <87a54v8nmz.fsf@arnes.space>

hi@arnes.space writes:

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> 
> I discovered that diff output is not filtered when the `--name-only`
> flag is given, it always lists all affected files. Is this a known bug?
> I could not find anything in the documentation.
> 
> Perhaps related, `--ignore-matching-lines` is ignored by my external
> diff helper difftastic[1]. I'm not sure if this is an upstream issue, or
> if the way external diff helpers are implemented is related to this at all.

The `—ignore-matching-lines` options is introduced in 296d4a94e72 (Michał Kępień 2020-10-20 08:48:09 +0200 5803) Since the Git documentation doesn't specify how git diff --ignore-matching-lines=<regex> is supposed to behave when used with --name-only, I'm not sure whether this is a bug. However, I'm confident that this is not an issue with your diff tool. The reason is that git diff uses the built-in diffcore_std() to filter out diff file pairs that shouldn't be output. However, in the latest Git source code, diffcore_std() does not filter file pairs based on ignore_regex.

It appears that ignore_regex only takes effect when using the built-in diff (builtin_diff). I'm not entirely certain about this, but if that's the case, then regardless of whether your external diff tool supports --ignore-matching-lines or not, this option is likely to have no effect when using an external diff.

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> 
> The behavior can be observed in git 2.49.0 using `git diff --no-ext-diff
> --name-only --ignore-matching-lines`.
> 
> Best,
> Arne
> 
> [1]: https://difftastic.wilfred.me.uk/

Cool diff tool

Thanks, Lidong