NAME | SYNOPSIS | DESCRIPTION | OPERATION MODES | OPTIONS | DISCUSSION | DISCUSSION ON FORK-POINT MODE | SEE ALSO | GIT | COLOPHON

GIT-MERGE-BASE(1)                Git Manual                GIT-MERGE-BASE(1)

NAME         top

       git-merge-base - Find as good common ancestors as possible for a
       merge

SYNOPSIS         top

       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         top

       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         top

       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         top

       -a, --all
           Output all merge bases for the commits, instead of just one.

DISCUSSION         top

       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         top

       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         top

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

GIT         top

       Part of the git(1) suite

COLOPHON         top

       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
       2017-03-13.  If you discover any rendering problems in this HTML ver‐
       sion 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 man‐
       ual page), send a mail to man-pages@man7.org

Git 2.11.0.22.g8d7a455           12/10/2016                GIT-MERGE-BASE(1)