How to tell git branch name from commit hash?

Question

I have a bash script which accepts a string of either a branch name (e.g., \"master\", or \"feature/foo\") or a commit hash (e.g. \"1234abcd\").

I have the repository c

Answer 1

You can use git show-ref:

git show-ref --head | grep refs

If it is empty, it is a SHA1 (or an invalid object, which isn't good).
If not, it is a branch name.

---

A better technique comes from "Validate if commit exists", using git merge-base:

A branch name will result in a different string (the SHA1)

C:\Users\vonc\prog\b2d>git merge-base master master
de4accfd28c5f25fcc057d56996b83450be5dc60

a SHA1 will result in the same result (or at least starts with the same result):

C:\Users\vonc\prog\b2d>git merge-base 03949c3d3f88a378c6a08e57daa97059b52813f1 03949c3d3f88a378c6a08e57daa97059b52813f1
03949c3d3f88a378c6a08e57daa97059b52813f1

foobar will fail:

C:\Users\vonc\prog\b2d>git merge-base xxx xxx
fatal: Not a valid object name xxx

That means something like:

if [[ git merge-base $string $string ]]; then
  if [[ $(git merge-base $string $string) == $string* ]]; then
    echo "SHA1"
  else
    echo "branch"
  fi
else
  echo "Not a valid object name '$string'"
fi