git-merge-base - Online in the Cloud

PROGRAM:

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

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