ignoring changes matching a string in git diff

No more grep needed!

With Git 2.30 (Q1 2021), "git diff"^(man) family of commands learned the "-I<regex>" option to ignore hunks whose changed lines all match the given pattern.

(Note: regex means here, as seen in git diff man page, POSIX Extended Regular Expressions (ERE) via the C library's regex functions (like regcomp(), used in diff.c with the REG_EXTENDED flag).
It is not based on Perl-compatible regexes. That means that Perl-specific features (such as lookahead or non-capturing groups) are not supported.
Instead, you need to use the standard syntax available in POSIX ERE or findutils)

See commit 296d4a9, commit ec7967c (20 Oct 2020) by Michał Kępień (kempniu).
^{(Merged by Junio C Hamano -- gitster -- in commit 1ae0949, 02 Nov 2020)}

diff: add -I<regex> that ignores matching changes

^{Signed-off-by: Michał Kępień}

Add a new diff option that enables ignoring changes whose all lines (changed, removed, and added) match a given regular expression.
This is similar to the -I/--ignore-matching-lines option in standalone diff utilities and can be used e.g. to ignore changes which only affect code comments or to look for unrelated changes in commits containing a large number of automatically applied modifications (e.g. a tree-wide string replacement).

The difference between -G/-S and the new -I option is that the latter filters output on a per-change basis.

Use the 'ignore' field of xdchange_t for marking a change as ignored or not.
Since the same field is used by --ignore-blank-lines, identical hunk emitting rules apply for --ignore-blank-lines and -I.
These two options can also be used together in the same git invocation (they are complementary to each other).

Rename xdl_mark_ignorable() to xdl_mark_ignorable_lines(), to indicate that it is logically a "sibling" of xdl_mark_ignorable_regex() rather than its "parent".

diff-options now includes in its man page:

-I<regex>

--ignore-matching-lines=<regex>

Ignore changes whose all lines match <regex>.
This option may be specified more than once.

Examples:

git diff --ignore-blank-lines -I"ten.*e" -I"^[124-9]" 

A small memleak in "diff -I<regexp>" has been corrected with Git 2.31 (Q1 2021).

See commit c45dc9c, commit e900d49 (11 Feb 2021) by Ævar Arnfjörð Bjarmason (avar).
^{(Merged by Junio C Hamano -- gitster -- in commit 45df6c4, 22 Feb 2021)}

diff: plug memory leak from regcomp() on {log,diff} -I

^{Signed-off-by: Ævar Arnfjörð Bjarmason}

Fix a memory leak in 296d4a9 ("diff: add -I that ignores matching changes", 2020-10-20, Git v2.30.0-rc0 -- merge listed in batch #3) by freeing the memory it allocates in the newly introduced diff_free().

This memory leak was intentionally introduced in 296d4a9, see the discussion on a previous iteration of it.

At that time freeing the memory was somewhat tedious, but since it isn't anymore with the newly introduced diff_free() let's use it.

Let's retain the pattern for diff_free_file() and add a diff_free_ignore_regex(), even though (unlike "diff_free_file") we don't need to call it elsewhere.
I think this will make for more readable code than gradually accumulating a giant diff_free() function, sharing "int i" across unrelated code etc.

Use git difftool to run a real diff.

Example: https://github.com/cben/kubernetes-discovery-samples/commit/b1e946434e73d8d1650c887f7d49b46dcbd835a6
I've created a script running diff the way I want to (here I'm keeping curl --verbose outputs in the repo, resulting in boring changes each time I rerun the curl):

#!/bin/bash diff --recursive --unified=1 --color \ --ignore-matching-lines=serverAddress \ --ignore-matching-lines='^\* subject:' \ --ignore-matching-lines='^\* start date:' \ --ignore-matching-lines='^\* expire date:' \ --ignore-matching-lines='^\* issuer:' \ --ignore-matching-lines='^< Date:' \ --ignore-matching-lines='^< Content-Length:' \ --ignore-matching-lines='--:--:--' \ --ignore-matching-lines='{ \[[0-9]* bytes data\]' \ "$@" 

And now I can run git difftool --dir-diff --extcmd=path/to/above/script.sh and see only interesting changes.

An important caveat about GNU diff -I aka --ignore-matching-lines: this merely prevents such lines from making a chunk "intersting" but when these changes appear in same chunk with other non-ignored changes, it will still show them. I used --unified=1 above to reduce this effect by making chunks smaller (only 1 context line above and below each change).

I think that I have a different solution using pipes and grep. I had two files that needed to be checked for differences that didn't include @@ and g:, so I did this (borrowing from here and here and here:

$ git diff -U0 --color-words --no-index file1.tex file2.tex | grep -v -e "@@" -e "g:" 

and that seemed to do the trick. Colors still were there.

So I assume you could take a simpler git diff command/output and do the same thing. What I like about this is that it doesn't require making new files or redirection (other than a pipe).