How do I get the interdiff between these two git commits?

Question

I'm trying to view the difference between two revisions of the same commit in git. A diff of the diffs, basically. From what I've read so far, this is known as the "interdiff". I've read several tutorials on how to create interdiffs of git patches, but I haven't been able to get these methods to work in my particular case.

So here's the setup. I have two diverging branches, each with slightly different commits:

* 29e734f - (origin/feature_branch, new_commits) New commit 3 (69 minutes ago) <Ajedi32>
* b22ebea - New commit 2 (89 minutes ago) <Ajedi32>
* 09d42c2 - New commit 1 (2 hours ago) <Ajedi32>
| * 467e08f - (old_commits) Old commit 3 (4 weeks ago) <Ajedi32>
| * f2bf1cb - Old commit 2 (4 weeks ago) <Ajedi32>
| * 34a2187 - Old commit 1 (4 weeks ago) <Ajedi32>
|/  
*   1b05a4a - (origin/base, base) Base commit (5 weeks ago) <Ajedi32>

In this example, I want to find the interdiff between "Old commit 3" and "New commit 3". I've tried creating a patch file from these two commits and running them through the interdiff utility, but all I got was this:

1 out of 2 hunks FAILED -- saving rejects to file /tmp/interdiff-1.Kgwx8u.rej
interdiff: Error applying patch1 to reconstructed file

I'm not really sure what that means, so I'm kind of stuck now. Where do I go from here?

Note: I'm am not looking for git diff old_commits new_commits here. I don't want the revisions to commits 1 and 2 included in the output.

Answer 1

Git 2.19 introduces a new command, git range-diff which does this:

git-range-diff - Compare two commit ranges (e.g. two versions of a branch)

git range-diff [--color=[<when>]] [--no-color] [<diff-options>] [--no-dual-color]
               [--creation-factor=<factor>]\
               ( <range1> <range2> | <rev1>...<rev2> | <base> <rev1> <rev2> )

Description

This command shows the differences between two versions of a patch series, or more generally, two commit ranges (ignoring merge commits).

To that end, it first finds pairs of commits from both commit ranges that correspond with each other. Two commits are said to correspond when the diff between their patches (i.e. the author information, the commit message and the commit diff) is reasonably small compared to the patches' size. See Algorithm below for details.

Finally, the list of matching commits is shown in the order of the second commit range, with unmatched commits being inserted just after all of their ancestors have been shown.

So, in your case:

git range-diff base old_commits new_commits

Would automatically match up the commits made in the old_commits branch with the commits in the new_commits branch, and display a summary of the differences between each commit.

Or, if you just want the changes from the last commit in each of those branches, you'd run:

git range-diff old_commits~..old_commits new_commits~..new_commits

For more on range-diff, see the official documentation.

Answer 2

Perhaps something like this:

git log -p -1 new_commits > patch.new
git log -p -1 old_commits > patch.old
diff patch.old patch.new

Or for a terse one-liner (in bash):

diff <(git log -p -1 old_commits) <(git log -p -1 new_commits)

Answer 3

The GNU interdiff on your 2 patches fails because they have no direct common base / parent.

While a git range-diff produces an outer diff of diffs (which is often hard to read; unaware / inconsistent regarding already consumed / merged changes in previous commits; not an applicable patch format), you can get an effective well readable interdiff by merge-rebasing a copy of the one commit onto the parent of the other (preferably newer) commit. There are several ways.

Trivial would be to compare <New commit 1> with <Old commit 1> in your example because they already have an immediate common parent:

diff NC1 OC1

Further away from the common base, according to your actual question, we get it for example like this:

# move to the parent of NC3 - entering detached HEAD state
git checkout NC3~1
# merge-rebase the single patch <Old-commit-3> onto here
git cherry-pick OC3
# show the diff or reverse diff (or diff in gitk GUI)
git diff -R NC3
# back to feature_branch
git checkout feature_branch

Of course merge conflicts may appear here - which then (necessarily) expose changes that are not compatible to a sort of interdiff with 'virtual common base' due to the lack of a real direct common parent.

A (temporary) tag or branch could be created to conserve this rebased comparison commit (or range of commits) - e.g. for discussion and work with other users. Otherwise the temporary unreferenced dangling commit(s) would be auto-recycled soon.