GIT-MERGE-BASE(1) Git Manual GIT-MERGE-BASE(1)
NAME
git-merge-base - Find as good common ancestors as possible for a merge
SYNOPSIS
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>]
DESCRIPTION
git merge-base finds the 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 MODES
In 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 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.
--fork-point
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 of this mode below).
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.
DISCUSSION ON FORK-POINT MODE
After working on the topic branch created with git switch -c 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---B2
/
---o---o---B1--o---o---o---B (origin/master)
\
B0
\
D0---D1---D (topic)
where origin/master used to point at commits B0, B1, B2 and now it
points at B, and your topic branch was started on top of it back when
origin/master was at B0, and you built three commits, D0, D1, and D, on
top of it. Imagine that you now want to rebase the work you did on the
topic on top of the updated origin/master.
In such a case, git merge-base origin/master topic would return the
parent of B0 in the above picture, but B0^..D is not the range of
commits you would want to replay on top of B (it includes B0, which is
not what you wrote; it is a commit the other side discarded when it
moved its tip from B0 to B1).
git merge-base --fork-point origin/master topic is designed to help in
such a case. It takes not only B but also B0, B1, and B2 (i.e. old tips
of the remote-tracking branches your repository's reflog knows about)
into account to see on which commit your topic branch was built and
finds B0, allowing you to replay only the commits on your topic,
excluding the commits the other side later discarded.
Hence
$ fork_point=$(git merge-base --fork-point origin/master topic)
will find B0, and
$ git rebase --onto origin/master $fork_point topic
will replay D0, D1, and D on top of B to create a new history of this
shape:
o---B2
/
---o---o---B1--o---o---o---B (origin/master)
\ \
B0 D0'--D1'--D' (topic - updated)
\
D0---D1---D (topic - old)
A caveat is that older reflog entries in your repository may be expired
by git gc. If B0 no longer appears in the reflog of the remote-tracking
branch origin/master, the --fork-point mode obviously cannot find it
and fails, avoiding to give a random and useless result (such as the
parent of B0, like the same command without the --fork-point option
gives).
Also, the remote-tracking branch you use the --fork-point mode with
must be the one your topic forked from its tip. If you forked from an
older commit than the tip, this mode would not find the fork point
(imagine in the above sample history B0 did not exist, origin/master
started at B1, moved to B2 and then B, and you forked your topic at
origin/master^ when origin/master was B1; the shape of the history
would be the same as above, without B0, and the parent of B1 is what
git merge-base origin/master topic correctly finds, but the
--fork-point mode will not, because it is not one of the commits that
used to be at the tip of origin/master).
SEE ALSO
git-rev-list(1), git-show-branch(1), git-merge(1)
GIT
Part of the git(1) suite
Git 2.43.5 05/31/2024 GIT-MERGE-BASE(1)
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