Re: git grep bug with --column and --only-matching

[Phillip Wood <phillip.wood123@gmail.com>]

On 21/04/2026 21:33, René Scharfe wrote:
> On 4/21/26 7:03 AM, Brandon Chinn wrote:
>> I'm encountering a bug when using `git grep` with both `--column` and
>> `--only-matching`, is this a known limitation?
>>
>> Repro:
>>
>> ```
>> $ echo 'x   x   x' > repro.txt
>>
>> $ grep -bo x repro.txt
>> 0:x
>> 4:x
>> 8:x
>>
>> $ git grep --no-index -o -n --column x repro.txt
>> repro.txt:1:  1:x
>> repro.txt:1:  2:x
>> repro.txt:1:  6:x
> 
> The first column value matches (1-based for git grep, 0-based for
> grep(1)), the remaining ones are different.  grep(1) shows the offset
> like for the first match, but git grep adds the relative position of the
> end of the previous match.

I'm having a hard time understanding what git is doing. Looking at your 
example with the ruler

>    (`man gitcvs-migration` or `git help cvs-migration` if git is
>          ^^^                   ^^^                        ^^^
>             1111111111222222222233333333334444444444555555555566
>    123456789 123456789 123456789 123456789 123456789 123456789 1
>             
>    7:git
>    16:git
>    38:git

The first match ends at column 9 and the second match starts at column 
29 so how do we end up with 16 as the "column" of the second match when 
the difference between them is 20?

> I don't know how to use the resulting numbers and I agree that it seems
> like a bug -- showing the column of each match within its line makes
> more sense for an option named --column.

Indeed

> However, the original submission of this feature in
> https://lore.kernel.org/git/cover.1529961706.git.me@ttaylorr.com/
> gave similar examples in its commit message and its tests, called
> them "as one would expect" and was not challenged on that, so I may be
> missing something.

I wonder how hard anyone looked at the examples and tests, the 
discussion seemed to have focused on other things.

Thanks

Phillip

> Here's its first example with underlined matches, a ruler and the
> intended output (without unnecessary headers):
> 
>    (`man gitcvs-migration` or `git help cvs-migration` if git is
>          ^^^                   ^^^                        ^^^
>             1111111111222222222233333333334444444444555555555566
>    123456789 123456789 123456789 123456789 123456789 123456789 1
>             
>    7:git
>    16:git
>    38:git
> 
> And here's the last line of the test file from t7810 with underlined
> matches (the other four lines are very similar), a ruler and the
> expected output (unncessary headers removed):
> 
>    foo_mmap bar mmap baz
>        ^^^^     ^^^^
>             111111111122
>    123456789 123456789 1
> 
>    5:mmap
>    13:mmap
> 
> If we wanted to show the column of matches 2 and beyond then we could do
> something like this:
> 
> 
> diff --git a/grep.c b/grep.c
> index c7e1dc1e0e..a54e5d86a9 100644
> --- a/grep.c
> +++ b/grep.c
> @@ -1267,6 +1267,7 @@ static void show_line(struct grep_opt *opt,
>   		regmatch_t match;
>   		enum grep_context ctx = GREP_CONTEXT_BODY;
>   		int eflags = 0;
> +		const char *start = bol;
>   
>   		if (want_color(opt->color)) {
>   			if (sign == ':')
> @@ -1285,6 +1286,7 @@ static void show_line(struct grep_opt *opt,
>   			if (match.rm_so == match.rm_eo)
>   				break;
>   
> +			cno = bol - start + match.rm_so + 1;
>   			if (opt->only_matching)
>   				show_line_header(opt, name, lno, cno, sign);
>   			else
> @@ -1294,7 +1296,6 @@ static void show_line(struct grep_opt *opt,
>   			if (opt->only_matching)
>   				opt->output(opt, "\n", 1);
>   			bol += match.rm_eo;
> -			cno += match.rm_eo;
>   			rest -= match.rm_eo;
>   			eflags = REG_NOTBOL;
>   		}
> diff --git a/t/t7810-grep.sh b/t/t7810-grep.sh
> index 64ac4f04ee..bd439563d6 100755
> --- a/t/t7810-grep.sh
> +++ b/t/t7810-grep.sh
> @@ -322,11 +322,11 @@ do
>   		${HC}file:1:5:mmap
>   		${HC}file:2:5:mmap
>   		${HC}file:3:5:mmap
> -		${HC}file:3:13:mmap
> +		${HC}file:3:14:mmap
>   		${HC}file:4:5:mmap
> -		${HC}file:4:13:mmap
> +		${HC}file:4:14:mmap
>   		${HC}file:5:5:mmap
> -		${HC}file:5:13:mmap
> +		${HC}file:5:14:mmap
>   		EOF
>   		git grep --column -n -o -e mmap $H >actual &&
>   		test_cmp expected actual
> 
>