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

NAME         top

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

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.


       As the most common special case, specifying only two commits on the
       command line means computing the merge base between the given two

       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

           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

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:


       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---B
               /   /

       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---M
               /   /

       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

               \ /
               / \

       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)"
                   ... 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
       origin/master, the history of remote-tracking branch origin/master
       may have been rewound and rebuilt, leading to a history of this

           ---o---o---B1--o---o---o---B (origin/master)
                      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.


           $ 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

           ---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).

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 
       ⟨⟩.  If you have a bug report for this manual page,
       see ⟨⟩.  This page was obtained from the
       project's upstream Git repository ⟨⟩ on
       2018-10-29.  (At that time, the date of the most recent commit that
       was found in the repository was 2018-10-26.)  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

Git           01/23/2018                GIT-MERGE-BASE(1)

Pages that refer to this page: git(1)git-rebase(1)git-show-branch(1)