Right and fast way to test git directory and git branch?

Question

While trying to set up my new fish shell prompt, I ended up testing ways to get the branch name and also testing if I was on a git repository. I have found (1) git rev-parse --abbrev-ref HEAD, (2) git symbolic-ref HEAD | sed 's/refs\/heads\///' and (3) git describe --contains --all HEAD do the trick just fine.

I’m curious because I love the simplicity of (1), but on a new and untouched (just git inited) repository, it gives me a “fatal:” error, while (2) works as intended, that is, giving me a default master as output. The thing is, even with a “fatal:” error, the return code is 0. Is that the intended behavior?

To test if I was in a git repository, I ended up simply testing if the current directory had a “.git” folder: test -d ".git". Rudimentary solution, but it works, and it seems to be faster that using any git command.

So, my questions are:

1. Shouldn’t that “fatal:” with (1) return an exit code different from 0? If indeed it is a bug, since I don’t know how git people standardised their return codes, where should I report it?
2. I know if I redirect (1) with ^ /dev/null (for those who don’t know, ^ is the same as 2>), I won’t get the error, but it will say HEAD instead of master, but if I cat .git/HEAD, I get: ref: refs/heads/master. What should it really say? With master am I just being ignorant and picky, because it should instead say HEAD? I mean in the case of a just git inited repository.
3. Have you actually benchmarked any possible solutions during your quest to get a nice prompt?

This is the code I use to benchmark (for fish):

set -l before (date +%s%N)
git rev-parse --abbrev-ref HEAD
set -l after (date +%s%N)
set -l elapsed_time (expr $after - $before)
echo $elapsed_time

Thanks in advance!

PS: The tools are the latest GNU versions, I mean, sed, date and expr. I’m sorry if there is too much information or if something doesn’t make any sense. Bear with me. Thanks!

EDIT: As someone correctly guessed in an answer, the whole error is:

fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree.
Use '--' to separate paths from revisions

Answer 1

Doesn't fully answer your questions but probably a bit more than just a comment:

1. Based on other comments in your answer, I think the error you saw is fatal: ambiguous argument 'HEAD': unknown revision or path not in the working tree. That occurs because you're conducting these tests on a brand new git repository, one that has no committed work. Not only is this not really a fair test (your performance will by definition be worse on a larger repository), it is in nearly every way an edge case. For example, .git/HEAD shows ref: refs/heads/master by convention. The error is thrown because HEAD is ambiguous. You have no history, thus any value passed to rev-parse would be treated the same. Additionally, this doesn't exit 0 for me:

   $ git init /tmp/test && cd /tmp/test
   $ git rev-parse --abbrev-ref HEAD
   $ echo $?
   128
   

   In sum, don't sweat this error. Grab yourself one of the huge android repos, or perhaps the linux kernel, and test your prompt there.

2. HEAD is not a branch. It's a pointer to the checked-out state. When you've got a branch checked out, it's a symbolic reference to that branch. When you're in 'detached HEAD' state, the HEAD pointer is pointing at a commit. You get HEAD as the return using method (1) because rev-parse is just echoing the passed refspec value in the error. Try it with blah; you'll see the same thing.

3. I haven't actually benchmarked solutions, but a few things you might try (both are used in my zsh prompt, which I didn't write, and both have performed quite well):

   git rev-parse --is-inside-work-tree ;# avoids your 'test -d .git' call
   ref=$(git symbolic-ref HEAD 2> /dev/null) ;# grab the full refspec
   branch=$(echo ${ref#refs/heads/}) ;# extract the branch name