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+git-merge-base(1)
+=================
+
+NAME
+----
+git-merge-base - Find as good common ancestors as possible for a merge
+
+
+SYNOPSIS
+--------
+[verse]
+'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 linkgit: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).
+
+
+See also
+--------
+linkgit:git-rev-list[1],
+linkgit:git-show-branch[1],
+linkgit:git-merge[1]
+
+GIT
+---
+Part of the linkgit:git[1] suite