git-merge-base(1)



GIT-MERGE-BASE(1)                 Git Manual                 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

Git 2.14.2                        09/26/2017                 GIT-MERGE-BASE(1)

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