125207ef1edad16503545f457bba8525017c8b1a
1 git-merge-base(1)
2 =================
4 NAME
5 ----
6 git-merge-base - Find as good common ancestors as possible for a merge
9 SYNOPSIS
10 --------
11 'git merge-base' [-a|--all] [--octopus] <commit> <commit>...
13 DESCRIPTION
14 -----------
16 'git merge-base' finds best common ancestor(s) between two commits to use
17 in a three-way merge. One common ancestor is 'better' than another common
18 ancestor if the latter is an ancestor of the former. A common ancestor
19 that does not have any better common ancestor is a 'best common
20 ancestor', i.e. a 'merge base'. Note that there can be more than one
21 merge base for a pair of commits.
23 Unless `--octopus` is given, among the two commits to compute the merge
24 base from, one is specified by the first commit argument on the command
25 line; the other commit is a (possibly hypothetical) commit that is a merge
26 across all the remaining commits on the command line. As the most common
27 special case, specifying only two commits on the command line means
28 computing the merge base between the given two commits.
30 As a consequence, the 'merge base' is not necessarily contained in each of the
31 commit arguments if more than two commits are specified. This is different
32 from linkgit:git-show-branch[1] when used with the `--merge-base` option.
34 OPTIONS
35 -------
36 -a::
37 --all::
38 Output all merge bases for the commits, instead of just one.
40 --octopus::
41 Compute the best common ancestors of all supplied commits,
42 in preparation for an n-way merge. This mimics the behavior
43 of 'git show-branch --merge-base'.
45 DISCUSSION
46 ----------
48 Given two commits 'A' and 'B', `git merge-base A B` will output a commit
49 which is reachable from both 'A' and 'B' through the parent relationship.
51 For example, with this topology:
53 o---o---o---B
54 /
55 ---o---1---o---o---o---A
57 the merge base between 'A' and 'B' is '1'.
59 Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the
60 merge base between 'A' and a hypothetical commit 'M', which is a merge
61 between 'B' and 'C'. For example, with this topology:
63 o---o---o---o---C
64 /
65 / o---o---o---B
66 / /
67 ---2---1---o---o---o---A
69 the result of `git merge-base A B C` is '1'. This is because the
70 equivalent topology with a merge commit 'M' between 'B' and 'C' is:
73 o---o---o---o---o
74 / \
75 / o---o---o---o---M
76 / /
77 ---2---1---o---o---o---A
79 and the result of `git merge-base A M` is '1'. Commit '2' is also a
80 common ancestor between 'A' and 'M', but '1' is a better common ancestor,
81 because '2' is an ancestor of '1'. Hence, '2' is not a merge base.
83 When the history involves criss-cross merges, there can be more than one
84 'best' common ancestor for two commits. For example, with this topology:
86 ---1---o---A
87 \ /
88 X
89 / \
90 ---2---o---o---B
92 both '1' and '2' are merge-bases of A and B. Neither one is better than
93 the other (both are 'best' merge bases). When the `--all` option is not given,
94 it is unspecified which best one is output.
96 Author
97 ------
98 Written by Linus Torvalds <torvalds@osdl.org>
100 Documentation
101 --------------
102 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
104 See also
105 --------
106 linkgit:git-rev-list[1],
107 linkgit:git-show-branch[1],
108 linkgit:git-merge[1]
110 GIT
111 ---
112 Part of the linkgit:git[1] suite