Finding the current Git branch in CodeBuild

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I want to use the name of the current git branch in nearly every automated build I run on CodeBuild, usually for tagging docker images. CodeBuild doesn’t make this value available, just the commit hash it fetched.

No problem! There’s a command that’s pretty commonly seen for grabbing the branch name yourself:

git rev-parse --abbrev-ref HEAD

The command rev-parse fetches information about a revision, the parameter HEAD means where we are now, and the option --abbrev-ref asks for a non-ambiguous name for the revision in question. This typically results in the name of the current branch.

There actually is a problem, though. This command doesn’t work as expected on CodeBuild. It will always return HEAD instead of the name.

No worries! We’ll just take a look in the git directory and see if we can sort this out ourselves.

ls .git
.git

Whoops, it turns out they’re using a detached worktree which makes .git a file that contains the name of the real data directory. Well, we can still use the worktree porcelain to take a look at what we’ve got:

git worktree list --porcelain
worktree /codebuild/local-cache/workspace/037017878334821233e13c831b4e3454b9a39f7d3678db91b07531daf15e12be/.git
HEAD 6a2dbaadee105594f481e3dd524ad4f2a59922ca
detached

Shoot. That explains why the rev-parse command fails. The last line of this output should include a branch, but it’s omitted when in a detached state.

However, it turns out we can still look in the git data directory and see some information about available refs. They had to pull something, right?

If you look, you’ll see refs/heads/ with at least one head pointing to a ref. It stands to reason that one of these will be the current commit. Of course we also have that hash inside the HEAD file. Using this info, we can find the branch name we’re looking for with the following bash script:

branch_name() {
  # Get the name the standard way
  name="$(git rev-parse --abbrev-ref HEAD)"

  # If we get the value "HEAD" and .git is a file,
  # we're probably in CodeBuild.
  if [ "$name" == "HEAD" ] && [ -f .git ]; then
    # Read in the real git directory location
    dir="$(awk '{print $2}' < .git)"

    # Get the current commit hash
    commit="$(cat "$dir/HEAD")"

    # Look through the heads we know about
    for head in "$dir/refs/heads"/*; do

      # Find the named branch with the same commit we're on
      if [ "$(cat "$head")" = "$commit" ]; then
        name="$(basename "$head")"
        break
      fi
    done
  fi

  # Report what we found
  echo "$name"
  return 0
}

If you know of a better way to get this information (particularly with a git builtin), please let me know! This was the best solution I could figure out in a CodeBuild build.