git-merge-base.txt

git-merge-base(1)
=================

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


SYNOPSIS
--------
[verse]
'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>...

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.

OPERATION MODE
--------------

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.

--octopus::
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'.

--independent::
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
--independent'.

--is-ancestor::
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.


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.

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:

 ---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.

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)"
then
... A is an ancestor of B ...
fi

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

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

instead.


See also
--------
linkgit:git-rev-list[1],
linkgit:git-show-branch[1],
linkgit:git-merge[1]

GIT
---
Part of the linkgit:git[1] suite