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2cf565c5 DG |
1 | git-merge-base(1) |
2 | ================= | |
2cf565c5 DG |
3 | |
4 | NAME | |
5 | ---- | |
c3f0baac | 6 | git-merge-base - Find as good common ancestors as possible for a merge |
2cf565c5 DG |
7 | |
8 | ||
9 | SYNOPSIS | |
10 | -------- | |
995bdc73 | 11 | 'git merge-base' [-a|--all] <commit> <commit>... |
2cf565c5 DG |
12 | |
13 | DESCRIPTION | |
14 | ----------- | |
2aa83961 | 15 | |
995bdc73 | 16 | 'git merge-base' finds best common ancestor(s) between two commits to use |
99f1c04b JH |
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 | |
29b802aa | 19 | that does not have any better common ancestor is a 'best common |
99f1c04b | 20 | ancestor', i.e. a 'merge base'. Note that there can be more than one |
29b802aa | 21 | merge base for a pair of commits. |
2aa83961 | 22 | |
29b802aa | 23 | Among the two commits to compute the merge base from, one is specified by |
99f1c04b JH |
24 | the first commit argument on the command line; the other commit is a |
25 | (possibly hypothetical) commit that is a merge across all the remaining | |
29b802aa RW |
26 | commits on the command line. As the most common special case, specifying only |
27 | two commits on the command line means computing the merge base between | |
99f1c04b | 28 | the given two commits. |
2cf565c5 | 29 | |
f621a845 MG |
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. | |
33 | ||
2aa83961 FK |
34 | OPTIONS |
35 | ------- | |
995bdc73 | 36 | -a:: |
2aa83961 | 37 | --all:: |
99f1c04b JH |
38 | Output all merge bases for the commits, instead of just one. |
39 | ||
40 | DISCUSSION | |
41 | ---------- | |
42 | ||
43 | Given two commits 'A' and 'B', `git merge-base A B` will output a commit | |
44 | which is reachable from both 'A' and 'B' through the parent relationship. | |
45 | ||
46 | For example, with this topology: | |
47 | ||
48 | o---o---o---B | |
49 | / | |
50 | ---o---1---o---o---o---A | |
51 | ||
52 | the merge base between 'A' and 'B' is '1'. | |
53 | ||
54 | Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the | |
29b802aa | 55 | merge base between 'A' and a hypothetical commit 'M', which is a merge |
99f1c04b JH |
56 | between 'B' and 'C'. For example, with this topology: |
57 | ||
58 | o---o---o---o---C | |
59 | / | |
60 | / o---o---o---B | |
61 | / / | |
62 | ---2---1---o---o---o---A | |
63 | ||
64 | the result of `git merge-base A B C` is '1'. This is because the | |
65 | equivalent topology with a merge commit 'M' between 'B' and 'C' is: | |
66 | ||
67 | ||
68 | o---o---o---o---o | |
69 | / \ | |
70 | / o---o---o---o---M | |
71 | / / | |
72 | ---2---1---o---o---o---A | |
73 | ||
74 | and the result of `git merge-base A M` is '1'. Commit '2' is also a | |
75 | common ancestor between 'A' and 'M', but '1' is a better common ancestor, | |
76 | because '2' is an ancestor of '1'. Hence, '2' is not a merge base. | |
77 | ||
78 | When the history involves criss-cross merges, there can be more than one | |
29b802aa | 79 | 'best' common ancestor for two commits. For example, with this topology: |
99f1c04b JH |
80 | |
81 | ---1---o---A | |
82 | \ / | |
83 | X | |
84 | / \ | |
85 | ---2---o---o---B | |
86 | ||
29b802aa RW |
87 | both '1' and '2' are merge-bases of A and B. Neither one is better than |
88 | the other (both are 'best' merge bases). When the `--all` option is not given, | |
99f1c04b | 89 | it is unspecified which best one is output. |
2cf565c5 DG |
90 | |
91 | Author | |
92 | ------ | |
93 | Written by Linus Torvalds <torvalds@osdl.org> | |
94 | ||
95 | Documentation | |
96 | -------------- | |
97 | Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. | |
98 | ||
99 | GIT | |
100 | --- | |
9e1f0a85 | 101 | Part of the linkgit:git[1] suite |