Merge branch 'maint-1.7.3' into maint-1.7.4
[git/git.git] / Documentation / git-merge-base.txt
1 git-merge-base(1)
2 =================
3
4 NAME
5 ----
6 git-merge-base - Find as good common ancestors as possible for a merge
7
8
9 SYNOPSIS
10 --------
11 [verse]
12 'git merge-base' [-a|--all] [--octopus] <commit> <commit>...
13 'git merge-base' --independent <commit>...
14
15 DESCRIPTION
16 -----------
17
18 'git merge-base' finds best common ancestor(s) between two commits to use
19 in a three-way merge. One common ancestor is 'better' than another common
20 ancestor if the latter is an ancestor of the former. A common ancestor
21 that does not have any better common ancestor is a 'best common
22 ancestor', i.e. a 'merge base'. Note that there can be more than one
23 merge base for a pair of commits.
24
25 Unless `--octopus` is given, among the two commits to compute the merge
26 base from, one is specified by the first commit argument on the command
27 line; the other commit is a (possibly hypothetical) commit that is a merge
28 across all the remaining commits on the command line. As the most common
29 special case, specifying only two commits on the command line means
30 computing the merge base between the given two commits.
31
32 As a consequence, the 'merge base' is not necessarily contained in each of the
33 commit arguments if more than two commits are specified. This is different
34 from linkgit:git-show-branch[1] when used with the `--merge-base` option.
35
36 OPTIONS
37 -------
38 -a::
39 --all::
40 Output all merge bases for the commits, instead of just one.
41
42 --octopus::
43 Compute the best common ancestors of all supplied commits,
44 in preparation for an n-way merge. This mimics the behavior
45 of 'git show-branch --merge-base'.
46
47 --independent::
48 Instead of printing merge bases, print a minimal subset of
49 the supplied commits with the same ancestors. In other words,
50 among the commits given, list those which cannot be reached
51 from any other. This mimics the behavior of 'git show-branch
52 --independent'.
53
54 DISCUSSION
55 ----------
56
57 Given two commits 'A' and 'B', `git merge-base A B` will output a commit
58 which is reachable from both 'A' and 'B' through the parent relationship.
59
60 For example, with this topology:
61
62 o---o---o---B
63 /
64 ---o---1---o---o---o---A
65
66 the merge base between 'A' and 'B' is '1'.
67
68 Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the
69 merge base between 'A' and a hypothetical commit 'M', which is a merge
70 between 'B' and 'C'. For example, with this topology:
71
72 o---o---o---o---C
73 /
74 / o---o---o---B
75 / /
76 ---2---1---o---o---o---A
77
78 the result of `git merge-base A B C` is '1'. This is because the
79 equivalent topology with a merge commit 'M' between 'B' and 'C' is:
80
81
82 o---o---o---o---o
83 / \
84 / o---o---o---o---M
85 / /
86 ---2---1---o---o---o---A
87
88 and the result of `git merge-base A M` is '1'. Commit '2' is also a
89 common ancestor between 'A' and 'M', but '1' is a better common ancestor,
90 because '2' is an ancestor of '1'. Hence, '2' is not a merge base.
91
92 When the history involves criss-cross merges, there can be more than one
93 'best' common ancestor for two commits. For example, with this topology:
94
95 ---1---o---A
96 \ /
97 X
98 / \
99 ---2---o---o---B
100
101 both '1' and '2' are merge-bases of A and B. Neither one is better than
102 the other (both are 'best' merge bases). When the `--all` option is not given,
103 it is unspecified which best one is output.
104
105 Author
106 ------
107 Written by Linus Torvalds <torvalds@osdl.org>
108
109 Documentation
110 --------------
111 Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>.
112
113 See also
114 --------
115 linkgit:git-rev-list[1],
116 linkgit:git-show-branch[1],
117 linkgit:git-merge[1]
118
119 GIT
120 ---
121 Part of the linkgit:git[1] suite