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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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NAME | SYNOPSIS | DESCRIPTION | OPERATION MODES | OPTIONS | DISCUSSION | DISCUSSION ON FORK-POINT MODE | SEE ALSO | GIT | COLOPHON |
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GIT-MERGE-BASE(1) Git Manual GIT-MERGE-BASE(1)
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 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.
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).
-a, --all
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:
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.
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).
git-rev-list(1), git-show-branch(1), git-merge(1)
Part of the git(1) suite
This page is part of the git (Git distributed version control
system) project. Information about the project can be found at
⟨http://git-scm.com/⟩. If you have a bug report for this manual
page, see ⟨http://git-scm.com/community⟩. This page was obtained
from the project's upstream Git repository
⟨https://github.com/git/git.git⟩ on 2025-08-11. (At that time,
the date of the most recent commit that was found in the
repository was 2025-08-07.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
Git 2.51.0.rc1 2025-08-07 GIT-MERGE-BASE(1)
Pages that refer to this page: git(1), git-rebase(1), git-show-branch(1)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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