git-merge-base.txt   [plain text]


git-merge-base - Find as good common ancestors as possible for a merge

'git merge-base' [-a|--all] <commit> <commit>...
'git merge-base' [-a|--all] --octopus <commit>...
'git merge-base' --is-ancestor <commit> <commit>
'git merge-base' --independent <commit>...
'git merge-base' --fork-point <ref> [<commit>]


'git merge-base' finds best common ancestor(s) between two commits to use
in a three-way merge.  One common ancestor is 'better' than another common
ancestor if the latter is an ancestor of the former.  A common ancestor
that does not have any better common ancestor is a 'best common
ancestor', i.e. a 'merge base'.  Note that there can be more than one
merge base for a pair of commits.


As the most common special case, specifying only two commits on the
command line means computing the merge base between the given two commits.

More generally, among the two commits to compute the merge base from,
one is specified by the first commit argument on the command line;
the other commit is a (possibly hypothetical) commit that is a merge
across all the remaining commits on the command line.

As a consequence, the 'merge base' is not necessarily contained in each of the
commit arguments if more than two commits are specified. This is different
from linkgit:git-show-branch[1] when used with the `--merge-base` option.

	Compute the best common ancestors of all supplied commits,
	in preparation for an n-way merge.  This mimics the behavior
	of 'git show-branch --merge-base'.

	Instead of printing merge bases, print a minimal subset of
	the supplied commits with the same ancestors.  In other words,
	among the commits given, list those which cannot be reached
	from any other.  This mimics the behavior of 'git show-branch

	Check if the first <commit> is an ancestor of the second <commit>,
	and exit with status 0 if true, or with status 1 if not.
	Errors are signaled by a non-zero status that is not 1.

	Find the point at which a branch (or any history that leads
	to <commit>) forked from another branch (or any reference)
	<ref>. This does not just look for the common ancestor of
	the two commits, but also takes into account the reflog of
	<ref> to see if the history leading to <commit> forked from
	an earlier incarnation of the branch <ref> (see discussion
	on this mode below).

	Output all merge bases for the commits, instead of just one.


Given two commits 'A' and 'B', `git merge-base A B` will output a commit
which is reachable from both 'A' and 'B' through the parent relationship.

For example, with this topology:


the merge base between 'A' and 'B' is '1'.

Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the
merge base between 'A' and a hypothetical commit 'M', which is a merge
between 'B' and 'C'.  For example, with this topology:

	     /   o---o---o---B
	    /   /

the result of `git merge-base A B C` is '1'.  This is because the
equivalent topology with a merge commit 'M' between 'B' and 'C' is:

	      /                 \
	     /   o---o---o---o---M
	    /   /

and the result of `git merge-base A M` is '1'.  Commit '2' is also a
common ancestor between 'A' and 'M', but '1' is a better common ancestor,
because '2' is an ancestor of '1'.  Hence, '2' is not a merge base.

The result of `git merge-base --octopus A B C` is '2', because '2' is
the best common ancestor of all commits.

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

	   \ /
	   / \

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.

A common idiom to check "fast-forward-ness" between two commits A
and B is (or at least used to be) to compute the merge base between
A and B, and check if it is the same as A, in which case, A is an
ancestor of B.  You will see this idiom used often in older scripts.

	A=$(git rev-parse --verify A)
	if test "$A" = "$(git merge-base A B)"
		... A is an ancestor of B ...

In modern git, you can say this in a more direct way:

	if git merge-base --is-ancestor A B
		... A is an ancestor of B ...


Discussion on fork-point mode

After working on the `topic` branch created with `git checkout -b
topic origin/master`, the history of remote-tracking branch
`origin/master` may have been rewound and rebuilt, leading to a
history of this shape:

	---o---o---B2--o---o---o---B (origin/master)
		   Derived (topic)

where `origin/master` used to point at commits B3, B2, B1 and now it
points at B, and your `topic` branch was started on top of it back
when `origin/master` was at B3. This mode uses the reflog of
`origin/master` to find B3 as the fork point, so that the `topic`
can be rebased on top of the updated `origin/master` by:

    $ fork_point=$(git merge-base --fork-point origin/master topic)
    $ git rebase --onto origin/master $fork_point topic

See also

Part of the linkgit:git[1] suite