git: programmatically know by how much the branch is ahead/behind a remote branch

Question

I would like to extract the information that is printed after a git status, which looks like:

# On branch master
# Your branch is ahead of 'origin/master' by 2 commits.

Of course I can parse the output of git status but this is not recommended since this human readable output is liable to change.

There are two problems:

1. How to know the remote tracked branch? It is often origin/branch but need not be.
2. How to get the numbers? How to know whether it is ahead/behind? By how many commits? And what about the diverged branch case?

Answer 1

git rev-list origin..HEAD will show the commits that are in your current branch, but not origin -- i.e., whether you're ahead of origin and by which commits.

git rev-list HEAD..origin will show the opposite.

If both commands show commits, then you have diverged branches.

Answer 2

There doesn't seem to be an easy way to find the tracking branch in general, without parsing lots more git config than is practical in a few shell commands. But for many cases this will go a long way:

# work out the current branch name
currentbranch=$(expr $(git symbolic-ref HEAD) : 'refs/heads/\(.*\)')
[ -n "$currentbranch" ] || die "You don't seem to be on a branch"
# look up this branch in the configuration
remote=$(git config branch.$currentbranch.remote)
remote_ref=$(git config branch.$currentbranch.merge)
# convert the remote ref into the tracking ref... this is a hack
remote_branch=$(expr $remote_ref : 'refs/heads/\(.*\)')
tracking_branch=refs/remotes/$remote/$remote_branch
# now $tracking_branch should be the local ref tracking HEAD
git rev-list $tracking_branch..HEAD

Another, more brute-force, approach:

git rev-list HEAD --not --remotes

jamessan's answer explains how to find the relative differences between $tracking_branch and HEAD using git rev-list. One fun thing you can do:

git rev-list --left-right $tracking_branch...HEAD

(note three dots between $tracking_branch and HEAD). This will show commits on both "arms" with a distinguishing mark at the front: "<" for commits on $tracking_branch, and ">" for commits on HEAD.

Answer 3

You can try git branch -v -v. With -v flag given twice, it outputs names of upstream branches. Sample output:

* devel  7a5ff2c [origin/devel: ahead 1] smaller file status overlay icons
  master 37ca389 [origin/master] initial project check-in.

I think this format is more stable than git status output.

Answer 4

git log origin..HEAD with more info