How do I find if a git SHA points to a branch HEAD?

Question

To avoid unnecessarily detached HEADs when checking out a certain git SHA with a (python) script, I'd like to check out a branch instead, if that SHA happens to be the current HEAD of a branch.

Ideally, I want to feed git an SHA, and it returns a branch name if the SHA is on a branch's current tip, or error out if not.

git describe --all --exact-match <SHA> very nearly is what I need, only that it is primarily aimed at tags, and so if a branch and a tag point at my SHA (which often happens in our release branches, for example), only the tag is given. This is not useful, because checking out the tag leads to a detached HEAD (even if a branch points at the same SHA).

Note, I don't want to do git branch --contains - I don't need to know which branches contain my commit.

If there's no command like git describe, just for branches, I know I can cross-check my SHA against branch SHAs via git show-ref. That's not the most elegant solution, though.

I could also do git name-rev --name-only <hash>, but I'd have to manually check the output for ~ characters, which feels unelegant if there's a git command somewhere to do the same thing.

Answer 1

With the upcoming git 2.7 (Q4 2015) you will get a more complete version of git for-each-ref, which now support the --points-at

git for-each-ref --points-at <branch_name>

With the doc:

--points-at <object>:

Only list refs which points at the given object.

That will allow to check if a commit is part of that list or not.

---

See commit 4a71109, commit ee2bd06, commit f266c91, commit 9d306b5, commit 7c32834, commit 35257aa, commit 5afcb90, ..., commit b2172fd (07 Jul 2015), and commit af83baf (09 Jul 2015) by Karthik Nayak (KarthikNayak).
^{(Merged by Junio C Hamano -- gitster -- in commit 9958dd8, 05 Oct 2015)}

Some features from "git tag -l" and "git branch -l" have been made available to "git for-each-ref" so that eventually the unified implementation can be shared across all three, in a follow-up series or two.

* kn/for-each-tag-branch:
  for-each-ref: add '--contains' option
  ref-filter: implement '--contains' option
  parse-options.h: add macros for '--contains' option
  parse-option: rename parse_opt_with_commit()
  for-each-ref: add '--merged' and '--no-merged' options
  ref-filter: implement '--merged' and '--no-merged' options
  ref-filter: add parse_opt_merge_filter()
  for-each-ref: add '--points-at' option
  ref-filter: implement '--points-at' option