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 B3, B2, B1 and now it points
at B, and your topic
branch was started on top of it back when
was at B3. This mode uses the reflog of
to find B3 as the fork point, so that the topic
can be rebased on top of the updated origin/master
$ fork_point=$(git merge-base --fork-point origin/master topic)
$ git rebase --onto origin/master $fork_point topic
Part of the git(1)