CODEWITHSUNDEEP
git
champion
champion

Reputation: 349

Difference between commits exists but git log doesn't show changes

I have some curious situation and not undestand it.

When I use git log -p -<n> I don't see changes what exists in file, but If I try git diff <prev>..<next> <file> difference exists and shown. Why can it be?

Git diff returns blob numbers index cd785c4..967453b 100644, what I can do with that?

Can that information be helpful for me?

Upvotes: 1

Views: 1091

Answers (1)

torek
torek

Reputation: 488053

When I use git log -p -<n> I don't see changes what exists in file, but If I try git diff <prev>..<next> <file> difference exists and shown. Why can it be?

This means the commit in question is a merge commit, i.e., a commit with at least two parent IDs. When git log -p or git show is showing a merge, their default is to use git's combined diff format.

The precise details of combined diffs are in the documentation under the combined diff format section, but I'll quote from the "diff format for merges" section:

Note that combined diff lists only files which were modified from all parents.

(boldface mine). Suppose, for instance, that you merge branch feature into branch develop, and that in develop, file README says, in part:

There will be a feature X soon.

Suppose that in feature, README says:

Feature X is now supported.

When you merge feature into develop, the file README picks up the change—but the resulting README exactly matches the version in develop.

If you now git show the merge commit (or it comes up in git log -p), the combined diff compares README in the merge against README in the first parent (it's different, it has the "now supported" text when develop didn't before), then compares README in the merge against README in the second parent. The second comparison shows no change since the merge picked up the feature version, so the combined diff omits file README.

When you find the first parent ID of the merge, i.e., the commit that was the tip of develop before the merge, and git diff that commit against the merge commit, you get a regular (non-combined) diff, and since README picked up the change, you see the change.

Note that you can use the -m option to tell things that show merges (git log and git show, for instance) to display one diff against each parent, rather than a single combined diff. (For typical two-parent merges, this shows two diffs against the one commit. Internally, these commands generate a fake extra commit to do this. You should not have to know about the internal-only fake commit, but it leaks out in some cases.)

Git diff returns blob numbers index cd785c4..967453b 100644, what I can do with that?

See this answer.

Upvotes: 2

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