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git-merge-base(1)                  Git Manual                  git-merge-base(1)


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


       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>]


       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 option.

           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 below).


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


       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 topology:

               \ /
               / \

       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 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---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


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


       Part of the git(1) suite

Git 2.39.0                         12/12/2022                  git-merge-base(1)

git 2.39.0 - Generated Tue Dec 13 18:29:32 CST 2022
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