git: get effective GIT_DIR, if current dir is a working tree subdir and $GIT_DIR is unset

Question

From a script, how to determine the current GIT_DIR? If not overriden, it's just the top level dir or the top level dir of a bare repo.

I want something like:

[/git/repo1/subdir]$ git show-git-dir
/git/repo1/.git

Answer 1

This can be done with rev-parse:

git rev-parse --git-dir