# git-merge-base

## 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 the 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



           In 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 of 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 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).
    


## GIT



           Part of the git(1) suite
    


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