NAME
git-merge-base - Find as good common ancestors as possible for a merge
SYNOPSIS
git merge-base [-a|--all] <commit> <commit>...
DESCRIPTION
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.
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 the most common special case,
specifying only two commits on the command line means computing the
merge base between the given two commits.
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 git-show-branch(1) when used with the --merge-base
option.
OPTIONS
-a, --all
Output all merge bases for the commits, instead of just one.
DISCUSSION
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:
o---o---o---B
/
---o---1---o---o---o---A
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---o---C
/
/ o---o---o---B
/ /
---2---1---o---o---o---A
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---o
/ \
/ o---o---o---o---M
/ /
---2---1---o---o---o---A
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.
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.
AUTHOR
Written by Linus Torvalds <torvalds@osdl.org[1]>
DOCUMENTATION
Documentation by David Greaves, Junio C Hamano and the git-list
<git@vger.kernel.org[2]>.
GIT
Part of the git(1) suite
NOTES
1. torvalds@osdl.org
mailto:torvalds@osdl.org
2. git@vger.kernel.org
mailto:git@vger.kernel.org