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GIT-MERGE-BASE(1)		  Git Manual		     GIT-MERGE-BASE(1)

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

SEE ALSO
       git-rev-list(1),	git-show-branch(1), git-merge(1)

GIT
       Part of the git(1) suite

Git 2.13.2			  06/24/2017		     GIT-MERGE-BASE(1)

NAME | SYNOPSIS | DESCRIPTION | OPERATION MODES | OPTIONS | DISCUSSION | DISCUSSION ON FORK-POINT MODE | SEE ALSO | GIT

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