1GIT-MERGE-BASE(1) Git Manual GIT-MERGE-BASE(1)
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6 git-merge-base - Find as good common ancestors as possible for a merge
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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>...
13 git merge-base --fork-point <ref> [<commit>]
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17 git merge-base finds best common ancestor(s) between two commits to use
18 in a three-way merge. One common ancestor is better than another common
19 ancestor if the latter is an ancestor of the former. A common ancestor
20 that does not have any better common ancestor is a best common
21 ancestor, i.e. a merge base. Note that there can be more than one merge
22 base for a pair of commits.
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25 As the most common special case, specifying only two commits on the
26 command line means computing the merge base between the given two
27 commits.
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29 More generally, among the two commits to compute the merge base from,
30 one is specified by the first commit argument on the command line; the
31 other commit is a (possibly hypothetical) commit that is a merge across
32 all the remaining commits on the command line.
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34 As a consequence, the merge base is not necessarily contained in each
35 of the commit arguments if more than two commits are specified. This is
36 different from git-show-branch(1) when used with the --merge-base
37 option.
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39 --octopus
40 Compute the best common ancestors of all supplied commits, in
41 preparation for an n-way merge. This mimics the behavior of git
42 show-branch --merge-base.
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44 --independent
45 Instead of printing merge bases, print a minimal subset of the
46 supplied commits with the same ancestors. In other words, among the
47 commits given, list those which cannot be reached from any other.
48 This mimics the behavior of git show-branch --independent.
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50 --is-ancestor
51 Check if the first <commit> is an ancestor of the second <commit>,
52 and exit with status 0 if true, or with status 1 if not. Errors are
53 signaled by a non-zero status that is not 1.
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55 --fork-point
56 Find the point at which a branch (or any history that leads to
57 <commit>) forked from another branch (or any reference) <ref>. This
58 does not just look for the common ancestor of the two commits, but
59 also takes into account the reflog of <ref> to see if the history
60 leading to <commit> forked from an earlier incarnation of the
61 branch <ref> (see discussion on this mode below).
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64 -a, --all
65 Output all merge bases for the commits, instead of just one.
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68 Given two commits A and B, git merge-base A B will output a commit
69 which is reachable from both A and B through the parent relationship.
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71 For example, with this topology:
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73 o---o---o---B
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75 ---o---1---o---o---o---A
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77 the merge base between A and B is 1.
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79 Given three commits A, B and C, git merge-base A B C will compute the
80 merge base between A and a hypothetical commit M, which is a merge
81 between B and C. For example, with this topology:
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83 o---o---o---o---C
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85 / o---o---o---B
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87 ---2---1---o---o---o---A
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89 the result of git merge-base A B C is 1. This is because the equivalent
90 topology with a merge commit M between B and C is:
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92 o---o---o---o---o
93 / \
94 / o---o---o---o---M
95 / /
96 ---2---1---o---o---o---A
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98 and the result of git merge-base A M is 1. Commit 2 is also a common
99 ancestor between A and M, but 1 is a better common ancestor, because 2
100 is an ancestor of 1. Hence, 2 is not a merge base.
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102 The result of git merge-base --octopus A B C is 2, because 2 is the
103 best common ancestor of all commits.
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105 When the history involves criss-cross merges, there can be more than
106 one best common ancestor for two commits. For example, with this
107 topology:
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109 ---1---o---A
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111 X
112 / \
113 ---2---o---o---B
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115 both 1 and 2 are merge-bases of A and B. Neither one is better than the
116 other (both are best merge bases). When the --all option is not given,
117 it is unspecified which best one is output.
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119 A common idiom to check "fast-forward-ness" between two commits A and B
120 is (or at least used to be) to compute the merge base between A and B,
121 and check if it is the same as A, in which case, A is an ancestor of B.
122 You will see this idiom used often in older scripts.
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124 A=$(git rev-parse --verify A)
125 if test "$A" = "$(git merge-base A B)"
126 then
127 ... A is an ancestor of B ...
128 fi
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130 In modern git, you can say this in a more direct way:
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132 if git merge-base --is-ancestor A B
133 then
134 ... A is an ancestor of B ...
135 fi
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137 instead.
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140 After working on the topic branch created with git switch -c topic
141 origin/master, the history of remote-tracking branch origin/master may
142 have been rewound and rebuilt, leading to a history of this shape:
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144 o---B2
145 /
146 ---o---o---B1--o---o---o---B (origin/master)
147 \
148 B0
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150 D0---D1---D (topic)
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152 where origin/master used to point at commits B0, B1, B2 and now it
153 points at B, and your topic branch was started on top of it back when
154 origin/master was at B0, and you built three commits, D0, D1, and D, on
155 top of it. Imagine that you now want to rebase the work you did on the
156 topic on top of the updated origin/master.
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158 In such a case, git merge-base origin/master topic would return the
159 parent of B0 in the above picture, but B0^..D is not the range of
160 commits you would want to replay on top of B (it includes B0, which is
161 not what you wrote; it is a commit the other side discarded when it
162 moved its tip from B0 to B1).
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164 git merge-base --fork-point origin/master topic is designed to help in
165 such a case. It takes not only B but also B0, B1, and B2 (i.e. old tips
166 of the remote-tracking branches your repository’s reflog knows about)
167 into account to see on which commit your topic branch was built and
168 finds B0, allowing you to replay only the commits on your topic,
169 excluding the commits the other side later discarded.
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171 Hence
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173 $ fork_point=$(git merge-base --fork-point origin/master topic)
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175 will find B0, and
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177 $ git rebase --onto origin/master $fork_point topic
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179 will replay D0, D1 and D on top of B to create a new history of this
180 shape:
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182 o---B2
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184 ---o---o---B1--o---o---o---B (origin/master)
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186 B0 D0'--D1'--D' (topic - updated)
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188 D0---D1---D (topic - old)
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190 A caveat is that older reflog entries in your repository may be expired
191 by git gc. If B0 no longer appears in the reflog of the remote-tracking
192 branch origin/master, the --fork-point mode obviously cannot find it
193 and fails, avoiding to give a random and useless result (such as the
194 parent of B0, like the same command without the --fork-point option
195 gives).
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197 Also, the remote-tracking branch you use the --fork-point mode with
198 must be the one your topic forked from its tip. If you forked from an
199 older commit than the tip, this mode would not find the fork point
200 (imagine in the above sample history B0 did not exist, origin/master
201 started at B1, moved to B2 and then B, and you forked your topic at
202 origin/master^ when origin/master was B1; the shape of the history
203 would be the same as above, without B0, and the parent of B1 is what
204 git merge-base origin/master topic correctly finds, but the
205 --fork-point mode will not, because it is not one of the commits that
206 used to be at the tip of origin/master).
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209 git-rev-list(1), git-show-branch(1), git-merge(1)
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212 Part of the git(1) suite
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216Git 2.24.1 12/10/2019 GIT-MERGE-BASE(1)