git-merge-base(1) — Linux manual page

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 switch -c
       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---B2
                           /
           ---o---o---B1--o---o---o---B (origin/master)
                   \
                    B0
                     \
                      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.

       Hence

           $ 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:

                            o---B2
                           /
           ---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
       ⟨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 2021-08-27.  (At that time,
       the date of the most recent commit that was found in the
       repository was 2021-08-24.)  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
       man-pages@man7.org

Git 2.33.0.69.gc420321         08/27/2021              GIT-MERGE-BASE(1)

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