git-merge-base(1)

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

6       git-merge-base - Find as good common ancestors as possible for a merge
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SYNOPSIS

9       git merge-base [-a|--all] <commit> <commit>...
10       git merge-base [-a|--all] --octopus <commit>...
11       git merge-base --is-ancestor <commit> <commit>
12       git merge-base --independent <commit>...
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DESCRIPTION

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 merge
21       base for a pair of commits.
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OPERATION MODE

24       As the most common special case, specifying only two commits on the
25       command line means computing the merge base between the given two
26       commits.
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28       More generally, among the two commits to compute the merge base from,
29       one is specified by the first commit argument on the command line; the
30       other commit is a (possibly hypothetical) commit that is a merge across
31       all the remaining commits on the command line.
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33       As a consequence, the merge base is not necessarily contained in each
34       of the commit arguments if more than two commits are specified. This is
35       different from git-show-branch(1) when used with the --merge-base
36       option.
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38       --octopus
39           Compute the best common ancestors of all supplied commits, in
40           preparation for an n-way merge. This mimics the behavior of git
41           show-branch --merge-base.
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43       --independent
44           Instead of printing merge bases, print a minimal subset of the
45           supplied commits with the same ancestors. In other words, among the
46           commits given, list those which cannot be reached from any other.
47           This mimics the behavior of git show-branch --independent.
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49       --is-ancestor
50           Check if the first <commit> is an ancestor of the second <commit>,
51           and exit with status 0 if true, or with status 1 if not. Errors are
52           signaled by a non-zero status that is not 1.
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OPTIONS

55       -a, --all
56           Output all merge bases for the commits, instead of just one.
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DISCUSSION

59       Given two commits A and B, git merge-base A B will output a commit
60       which is reachable from both A and B through the parent relationship.
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62       For example, with this topology:
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64                    o---o---o---B
65                   /
66           ---o---1---o---o---o---A
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68       the merge base between A and B is 1.
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70       Given three commits A, B and C, git merge-base A B C will compute the
71       merge base between A and a hypothetical commit M, which is a merge
72       between B and C. For example, with this topology:
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74                  o---o---o---o---C
75                 /
76                /   o---o---o---B
77               /   /
78           ---2---1---o---o---o---A
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80       the result of git merge-base A B C is 1. This is because the equivalent
81       topology with a merge commit M between B and C is:
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83                  o---o---o---o---o
84                 /                 \
85                /   o---o---o---o---M
86               /   /
87           ---2---1---o---o---o---A
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89       and the result of git merge-base A M is 1. Commit 2 is also a common
90       ancestor between A and M, but 1 is a better common ancestor, because 2
91       is an ancestor of 1. Hence, 2 is not a merge base.
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93       The result of git merge-base --octopus A B C is 2, because 2 is the
94       best common ancestor of all commits.
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96       When the history involves criss-cross merges, there can be more than
97       one best common ancestor for two commits. For example, with this
98       topology:
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100           ---1---o---A
101               \ /
102                X
103               / \
104           ---2---o---o---B
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106       both 1 and 2 are merge-bases of A and B. Neither one is better than the
107       other (both are best merge bases). When the --all option is not given,
108       it is unspecified which best one is output.
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110       A common idiom to check "fast-forward-ness" between two commits A and B
111       is (or at least used to be) to compute the merge base between A and B,
112       and check if it is the same as A, in which case, A is an ancestor of B.
113       You will see this idiom used often in older scripts.
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115           A=$(git rev-parse --verify A)
116           if test "$A" = "$(git merge-base A B)"
117           then
118                   ... A is an ancestor of B ...
119           fi
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121       In modern git, you can say this in a more direct way:
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123           if git merge-base --is-ancestor A B
124           then
125                   ... A is an ancestor of B ...
126           fi
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128       instead.
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GIT

134       Part of the git(1) suite
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138Git 1.8.3.1                       11/19/2018                 GIT-MERGE-BASE(1)