Is git merge-base commutative?

Question

Consider

$ git merge-base <original-branch> <new-branch>

Question: is git merge-base commutative? That is, does the above command yield the same result as

$ git merge-base <new-branch> <original-branch>

I'm not sure, but since there can be multiple merge bases, I would guess that the answer is **no**. — jub0bs, Aug 18 '15 at 14:49
See also the great answer from Christopher here: http://stackoverflow.com/a/12198219/5414160 — Thomas Vuillaume, Aug 01 '16 at 10:09

score 4 · Accepted Answer · answered Aug 18 '15 at 14:56

If there is only one merge-base, then yes.
Meaning: git merge-base --all <original-branch> <new-branch> returns only one commit.

But, as mentioned in the git merge-base man page:

When the history involves criss-cross merges, there can be more than one best common ancestor for two commits.
For example, with this topology:

---1---o---A
\ /
 X
/ \
---2---o---o---B

both 1 and 2 are merge-bases of A and B. Neither one is better than the other (both are best merge bases).
When the --all option is not given, it is unspecified which best one is output.

In short, `git merge-base` may happen to be commutative, but the spec of that command offers no such guarantee. — jub0bs, Aug 18 '15 at 16:39

Is git merge-base commutative?

1 Answers1