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 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

   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 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 on 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 checkout -b 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---B1
                       /
       ---o---o---B2--o---o---o---B (origin/master)
               \
                B3
                 \
                  Derived (topic)

   where origin/master 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
   origin/master was at B3. This mode uses the reflog of origin/master to
   find B3 as the fork point, so that the topic can be rebased on top of
   the updated origin/master by:

       $ fork_point=$(git merge-base --fork-point origin/master topic)
       $ git rebase --onto origin/master $fork_point topic

SEE ALSO

   git-rev-list(1), git-show-branch(1), git-merge(1)

GIT

   Part of the git(1) suite

---

Free and Open Source Software