In a Unix or GNU scripting environment (e.g. a Linux distro, Cygwin, OSX), what is the best way to determine which Git branch is currently checked out in a working directory?

One use of this technique would be automatically labeling a release (like svnversion would do with Subversion).

Please also see my related question: How to programmatically determine whether a Git checkout is a tag, and if so what is the tag name?

The correct solution is to take a peek at contrib/completions/git-completion.bash does that for bash prompt in __git_ps1. Removing all extras like selecting how to describe detached HEAD situation, i.e. when we are on unnamed branch, it is:

branch_name="$(git symbolic-ref HEAD 2>/dev/null)" ||
branch_name="(unnamed branch)"     # detached HEAD
branch_name=${branch_name##refs/heads/}

git symbolic-ref is used to extract fully qualified branch name from symbolic reference; we use it for HEAD, which is currently checked out branch.

Alternate solution could be:

branch_name=$(git symbolic-ref -q HEAD)
branch_name=${branch_name##refs/heads/}
branch_name=${branch_name:-HEAD}

where in last line we deal with the detached HEAD situation, using simply "HEAD" to denote such situation.

Added 11-06-2013

Junio C. Hamano (git maintainer) blog post, Checking the current branch programatically, from June 10, 2013 explains whys (and hows) in more detail.