SYNOPSIS
--------
-'git merge-base' [--all] <commit> <commit>...
+[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>...
DESCRIPTION
-----------
-'git-merge-base' finds best common ancestor(s) between two commits to use
+'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.
-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 the most common special case, specifying only
-two commits on the command line means computing the merge base between
-the given two commits.
+OPERATION MODE
+--------------
+
+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 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.
+
OPTIONS
-------
+-a::
--all::
Output all merge bases for the commits, instead of just one.
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:
the other (both are 'best' merge bases). When the `--all` option is not given,
it is unspecified which best one is output.
-Author
-------
-Written by Linus Torvalds <torvalds@osdl.org>
+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.
-Documentation
---------------
-Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
+ 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.
+
+
+See also
+--------
+linkgit:git-rev-list[1],
+linkgit:git-show-branch[1],
+linkgit:git-merge[1]
GIT
---