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>]
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
when used with the --merge-base
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 --independent.
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
, git merge-base A B
will output a
commit which is reachable from both A
through the parent
For example, with this topology:
the merge base between A
Given three commits A
, git merge-base A B C
will compute the merge base between A
and a hypothetical commit
, which is a merge between B
. For example, with
the result of git merge-base A B C
. This is because the
equivalent topology with a merge commit M
and the result of git merge-base A M
. Commit 2
a common ancestor between A
, 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 the best common ancestor of all commits.
When the history involves criss-cross merges, there can be more than one
common ancestor for two commits. For example, with this topology:
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 ...
After working on the topic
branch created with git checkout -b topic
, the history of remote-tracking branch origin/master
may have been rewound and rebuilt, leading to a history of this shape:
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
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.
$ 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:
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
. If B0 no longer appears in the reflog of the remote-tracking
, 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
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
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).
Part of the git(1)