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 usein a three-way merge. One common ancestor is better than another commonancestor if the latter is an ancestor of the former. A common ancestorthat does not have any better common ancestor is a best commonancestor, i.e. a merge base. Note that there can be more than onemerge base for a pair of commits.

OPERATION MODES

As the most common special case, specifying only two commits on thecommand 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 mergeacross all the remaining commits on the command line.

As a consequence, the merge base is not necessarily contained in each of thecommit arguments if more than two commits are specified. This is differentfrom 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 behaviorof git show-branch —merge-base.

• —independent

• Instead of printing merge bases, print a minimal subset ofthe supplied commits with the same ancestors. In other words,among the commits given, list those which cannot be reachedfrom any other. This mimics the behavior of git show-branch—independent.

• —is-ancestor

• Check if the first is an ancestor of the second ,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 leadsto ) forked from another branch (or any reference). This does not just look for the common ancestor ofthe two commits, but also takes into account the reflog of to see if the history leading to forked froman earlier incarnation of the branch (see discussionon 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 commitwhich 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 themerge base between A and a hypothetical commit M, which is a mergebetween 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 theequivalent 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 acommon 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 isthe best common ancestor of all commits.

When the history involves criss-cross merges, there can be more than onebest 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 thanthe 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 Aand B is (or at least used to be) to compute the merge base betweenA and B, and check if it is the same as A, in which case, A is anancestor 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 switch -ctopic origin/master, the history of remote-tracking branchorigin/master may have been rewound and rebuilt, leading to ahistory 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 itpoints at B, and your topic branch was started on top of it backwhen 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 workyou did on the topic on top of the updated origin/master.

In such a case, git merge-base origin/master topic would return theparent of B0 in the above picture, but B0^..D is not the range ofcommits you would want to replay on top of B (it includes B0, whichis not what you wrote; it is a commit the other side discarded whenit moved its tip from B0 to B1).

git merge-base —fork-point origin/master topic is designed tohelp 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’sreflog knows about) into account to see on which commit your topicbranch was built and finds B0, allowing you to replay only thecommits on your topic, excluding the commits the other side laterdiscarded.

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 thisshape:

         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 beexpired by git gc. If B0 no longer appears in the reflog of theremote-tracking branch origin/master, the —fork-point modeobviously cannot find it and fails, avoiding to give a random anduseless result (such as the parent of B0, like the same commandwithout the —fork-point option gives).

Also, the remote-tracking branch you use the —fork-point modewith must be the one your topic forked from its tip. If you forkedfrom an older commit than the tip, this mode would not find the forkpoint (imagine in the above sample history B0 did not exist,origin/master started at B1, moved to B2 and then B, and you forkedyour topic at origin/master^ when origin/master was B1; the shape ofthe history would be the same as above, without B0, and the parentof B1 is what git merge-base origin/master topic correctly finds,but the —fork-point mode will not, because it is not one of thecommits that used to be at the tip of origin/master).

See also

git-rev-list[1],git-show-branch[1],git-merge[1]

GIT

Part of the git[1] suite