git rev-parse --show-toplevel
could be enough if executed within a git repo.
From git rev-parse man page:
--show-toplevel
Show the absolute path of the top-level directory.
For older versions (before 1.7.x), the other options are listed in "Is there a way to get the git root directory in one command?":
git rev-parse --git-dir
That would give the path of the .git directory.
The OP mentions:
git rev-parse --show-prefix
which returns the local path under the git repo root. (empty if you are at the git repo root)
Note: for simply checking if one is in a git repo, I find the following command quite expressive:
git rev-parse --is-inside-work-tree
And yes, if you need to check if you are in a .git git-dir folder:
git rev-parse --is-inside-git-dir