Howto get the merge-base ?

Hi,

I'm trying to know which commit it the parent of a merge. For exemple if I do that :

   o Merge
  / \
 /   \
 |   |
 |   o Commit D
 |   |
 |   o Commit C
 |   |
 o   | Commit B
 \  /
  \/
  o Commit A
  |
  o Init

How could I know that ``Commit A'' is the merge-base of ``Merge'' ?

I try to get this git-merge-base but result is strange and quiet mysterious as he return me always second args I passed to. I'm using git 1.3.2

"Bertrand Jacquin" <beber.mailing@gmail.com> writes:

It is mysterious to me because you did not say what you gave as arguments ;-).

If I am reading you correctly, you already have a "Merge" commit, made by you or somebody else, and are trying to figure out where the merge base was. If that is the case:

	git-merge-base Merge^1 Merge^2

in other words

	git-merge-base CommitB CommitD        

is what you are looking for?

But what do you need that information for? To reproduce somebody else's merge?

On 5/14/06, Junio C Hamano <junkio@cox.net> wrote:

I was using git-merge-base ``merge-ish'' ``comit-ish''. Docs about it is atrocious so was trying many but not the one good. Blam me.

That's ok :)

Yes ! Thanks :)

No, that's just on the following of git-send-mail-commit.sh thread (or something near). To make a readable merge mail with diffstat and summury.

"Bertrand Jacquin" <beber.mailing@gmail.com> writes:

For that you do not want merge-base. If your mainline was A-B and you merged a side branch B-C-D with the merge, then to people who tracked your head (that's the audience of "merge mail", I presume) they just need to see:

	git diff --stat M^1..M
	git rev-list --no-merges --pretty M^1..M | git shortlog

The diffstat is "what damage was inflicted on the branch you have been following with this merge", so the diff between the trees before and after the merge is what you want. And the rev-list piped to shortlog is to show "what commits were _not_ present on this branch before the merge, but are present after the merge" (it could be spelled M^1..M^2 but the above would handle octopus as well).

On Sun, 14 May 2006, Bertrand Jacquin wrote:

Well, Junio already answered, but I'd like to comment a bit further.

There may not be "one" merge-base. There can be several:

		Merge
		  |
		  A
		 / \
		B   C
		|\ /|
		| X |
		|/ \|
		D   E
		 \ /
		  F
		  |

Here the merge (A) has two equally good merge bases: D and E.

If you want all of these merge-bases, you need to add the "--all" flag to git-merge-base.

On the git archive, try this trivia shell pipeline:

	git-rev-list --parents HEAD |
		sed -n '/^[0-9a-f]* [0-9a-f]* [0-9a-f]*$/ p' |
		while read a b c
		do
			echo $a: $(git merge-base --all $b $c)
		done | less -S

and you'll be able to easily pick up examples of where there are real multiple merge bases. For example, commit 4da8cbc2.. has that.

If you want to examine _why_ it has multiple merge-bases, do:

	gitk 4da8cbc2 --not 5910e997 52b70d56

(where the two "not" commits are the merge bases for it) which shows that merge and the criss-crossing nature of the history leading up to it.

NOTE! Most of the time, multiple merge bases do not really matter all that much, and you'd get the same merge regardless of which one you would choose as the base. Still, they _can_ matter.

			Linus