gitgit-status

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

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?
Solution

  • update

    As pointed out by amalloy, recent versions of git support finding the matching tracking branch for a given branch by giving "branchname@{upstream}" (or "branchname@{u}", or "@{u}" for the tracking branch of HEAD). This effectively supercedes the script below. You can do:

    git rev-list @{u}..
git rev-list --left-right --boundary @{u}...
gitk @{u}...

    etc. For example, I have git q aliased to git log --pretty='...' @{u}.. to show me "queued" commits ready for pushing.

    original answer

    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.