Git: Getting the difference between two repositories

Question

How can we get the difference between two git repositories?

The scenario: We have a repo_a and repo_b. The latter was created as a copy of repo_a. There have been parallel development in both the repositories afterwards. Is there a way we can list the differences of the current versions of these two repositories?

Answer 1

See: http://pjps.tumblr.com/post/96756489/git-diff-with-a-remote-repository

In short:

• Create a local reference to the remote repository
• Then do the diff like you would do with any other local branch ...

Answer 2

Once you have both branches in one repository you can do a git diff. And getting them in one repository is as easy as

git fetch /the/other/repo/.git refs/heads/*:refs/remotes/other/*

Answer 3

In repo_a:

git remote add -f b path/to/repo_b.git
git diff master remotes/b
git remote rm b

Answer 4

See http://git.or.cz/gitwiki/GitTips, section "How to compare two local repositories" in "General".

In short you are using GIT_ALTERNATE_OBJECT_DIRECTORIES environment variable to have access to object database of the other repository, and using git rev-parse with --git-dir / GIT_DIR to convert symbolic name in other repository to SHA-1 identifier.

Modern version would look something like this (assuming that you are in 'repo_a'):

GIT_ALTERNATE_OBJECT_DIRECTORIES=../repo_b/.git/objects \
   git diff $(git --git-dir=../repo_b/.git rev-parse --verify HEAD) HEAD

where ../repo_b/.git is path to object database in repo_b (it would be repo_b.git if it were bare repository). Of course you can compare arbitrary versions, not only HEADs.

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Note that if repo_a and repo_b are the same repository, it might make more sense to put both of them in the same repository, either using "git remote add -f ..." to create nickname(s) for repository for repeated updates, or obe off "git fetch ..."; as described in other responses.