Show history of commits for a line or section in a file

Question

I'm looking to step through the history of commits for a section in a file. Particularly, there are two lines I'm interested in. I want to see what changes were made to these two lines going back to a few months ago, or even the origin of the file. Is there a quick way to get the changes using the command line? I know there are GUI's for git that allow you to do this, but I'd prefer not to. I'd rather use vim or sublime if I'm going to do that.

Ideally I want something like the commit hash, date, name, and change.

34hi5u3k 4/13/2013 Someone Name (Line 408)  $text = 'Something';
72wbedfj 4/05/2013 Someone Else (Line 408)  $text = 'Something else';
827y3hrj 3/29/2013 Someone Nice (Line 408)  $text = 'This one time...';

Answer 1

Is there a quick way to get the changes using the command line?

With Git 2.28 (Q3 2020), it will be even quicker, considering "git log -L..." now takes advantage of the "which paths are touched by this commit?" info stored in the commit-graph system.

See commit f32dde8 (11 May 2020) by Derrick Stolee (derrickstolee).
See commit 002933f, commit 3cb9d2b, commit 48da94b, commit d554672 (11 May 2020) by SZEDER Gábor (szeder).
^{(Merged by Junio C Hamano -- gitster -- in commit c3a0282, 09 Jun 2020)}

line-log: more responsive, incremental 'git log -L'

^{Signed-off-by: SZEDER Gábor
Signed-off-by: Derrick Stolee}

The current line-level log implementation performs a preprocessing step in prepare_revision_walk(), during which the line_log_filter() function filters and rewrites history to keep only commits modifying the given line range.

This preprocessing affects both responsiveness and correctness:

• Git doesn't produce any output during this preprocessing step.
  Checking whether a commit modified the given line range is somewhat expensive, so depending on the size of the given revision range this preprocessing can result in a significant delay before the first commit is shown.

• Limiting the number of displayed commits (e.g. 'git log -3 -L...') doesn't limit the amount of work during preprocessing, because that limit is applied during history traversal.
  Alas, by that point this expensive preprocessing step has already churned through the whole revision range to find all commits modifying the revision range, even though only a few of them need to be shown.

• It rewrites parents, with no way to turn it off.
  Without the user explicitly requesting parent rewriting any parent object ID shown should be that of the immediate parent, just like in case of a pathspec-limited history traversal without parent rewriting.

However, after that preprocessing step rewrote history, the subsequent "regular" history traversal (i.e. get_revision() in a loop) only sees commits modifying the given line range.
Consequently, it can only show the object ID of the last ancestor that modified the given line range (which might happen to be the immediate parent, but many-many times it isn't).

This patch addresses both the correctness and, at least for the common case, the responsiveness issues by integrating line-level log filtering into the regular revision walking machinery:

• Make process_ranges_arbitrary_commit(), the static function in 'line-log.c' deciding whether a commit modifies the given line range, public by removing the static keyword and adding the 'line_log_' prefix, so it can be called from other parts of the revision walking machinery.

• If the user didn't explicitly ask for parent rewriting (which, I believe, is the most common case):

  • Call this now-public function during regular history traversal, namely from get_commit_action() to ignore any commits not modifying the given line range.
    Note that while this check is relatively expensive, it must be performed before other, much cheaper conditions, because the tracked line range must be adjusted even when the commit will end up being ignored by other conditions.

  • Skip the line_log_filter() call, i.e. the expensive preprocessing step, in prepare_revision_walk(), because, thanks to the above points, the revision walking machinery is now able to filter out commits not modifying the given line range while traversing history.
    This way the regular history traversal sees the unmodified history, and is therefore able to print the object ids of the immediate parents of the listed commits.
    The eliminated preprocessing step can greatly reduce the delay before the first commit is shown, see the numbers below.

• However, if the user did explicitly ask for parent rewriting via '--parents' or a similar option, then stick with the current implementation for now, i.e. perform that expensive filtering and history rewriting in the preprocessing step just like we di before, leaving the initial delay as long as it was.

I tried to integrate line-level log filtering with parent rewriting into the regular history traversal, but, unfortunately, several subtleties resisted... :) Maybe someday we'll figure out how to do that, but until then at least the simple and common (i.e. without parent rewriting) 'git log -L:func:file' commands can benefit from the reduced delay.

The reduced delay is most noticable when there's a commit modifying the line range near the tip of a large-ish revision range:

# no parent rewriting requested, no commit-graph present
$ time git --no-pager log -L:read_alternate_refs:sha1-file.c -1 v2.23.0

Before:

real    0m9.570s
user    0m9.494s
sys     0m0.076s

After:

real    0m0.718s
user    0m0.674s
sys     0m0.044s

A significant part of the remaining delay is spent reading and parsing commit objects in limit_list().

With the help of the commit-graph we can eliminate most of that reading and parsing overhead, so here are the timing results of the same command as above, but this time using the commit-graph:

Before:

real    0m8.874s
user    0m8.816s
sys     0m0.057s

After:

real    0m0.107s
user    0m0.091s
sys     0m0.013s

The next patch will further reduce the remaining delay.

To be clear: this patch doesn't actually optimize the line-level log, but merely moves most of the work from the preprocessing step to the history traversal, so the commits modifying the line range can be shown as soon as they are processed, and the traversal can be terminated as soon as the given number of commits are shown.
Consequently, listing the full history of a line range, potentially all the way to the root commit, will take the same time as before (but at least the user might start reading the output earlier).
Furthermore, if the most recent commit modifying the line range is far away from the starting revision, then that initial delay will still be significant.

Additional testing by Derrick Stolee: In the Linux kernel repository, the MAINTAINERS file was changed ~3,500 times across the ~915,000 commits.
In addition to that edit frequency, the file itself is quite large (~18,700 lines).
This means that a significant portion of the computation is taken up by computing the patch-diff of the file. This patch improves the real time it takes to output the first result quite a bit:

Command: git log -L 100,200:MAINTAINERS -n 1 >/dev/null
Before: 3.88 s 
After: 0.71 s

If we drop the "-n 1" in the command, then there is no change in end-to-end process time. This is because the command still needs to walk the entire commit history, which negates the point of this patch. This is expected.

As a note for future reference, the ~4.3 seconds in the old code spends ~2.6 seconds computing the patch-diffs, and the rest of the time is spent walking commits and computing diffs for which paths changed at each commit
The changed-path Bloom filters could improve the end-to-end computation time (i.e. no "-n 1" in the command).

Answer 2

Show all changes to lines 95-105 in $filename:

git log -L 95,105:$filename

Show all changes to 10 lines starting at regex in $filename:

git log -L /<regex>/,+10:$filename

The output produced is not in the ideal format you mentioned, but it does show all diffs for the specified section for the entire history.