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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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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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.
27 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.
32 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.
37 --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.
42 --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.
48 --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.
53 --fork-point
55 Find the point at which a branch (or any history that leads to
56 <commit>) forked from another branch (or any reference) <ref>. This
57 does not just look for the common ancestor of the two commits, but
58 also takes into account the reflog of <ref> to see if the history
59 leading to <commit> forked from an earlier incarnation of the
60 branch <ref> (see discussion on this mode below).
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63 -a, --all
64 Output all merge bases for the commits, instead of just one.
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67 Given two commits A and B, git merge-base A B will output a commit
68 which is reachable from both A and B through the parent relationship.
69 For example, with this topology:
71 o---o---o---B
73 /
74 ---o---1---o---o---o---A
75 the merge base between A and B is 1.
77 Given three commits A, B and C, git merge-base A B C will compute the
79 merge base between A and a hypothetical commit M, which is a merge
80 between B and C. For example, with this topology:
81 o---o---o---o---C
83 /
84 / o---o---o---B
85 / /
86 ---2---1---o---o---o---A
87 the result of git merge-base A B C is 1. This is because the equivalent
89 topology with a merge commit M between B and C is:
90 o---o---o---o---o
92 / \
93 / o---o---o---o---M
94 / /
95 ---2---1---o---o---o---A
96 and the result of git merge-base A M is 1. Commit 2 is also a common
98 ancestor between A and M, but 1 is a better common ancestor, because 2
99 is an ancestor of 1. Hence, 2 is not a merge base.
100 The result of git merge-base --octopus A B C is 2, because 2 is the
102 best common ancestor of all commits.
103 When the history involves criss-cross merges, there can be more than
105 one best common ancestor for two commits. For example, with this
106 topology:
107 ---1---o---A
109 \ /
110 X
111 / \
112 ---2---o---o---B
113 both 1 and 2 are merge-bases of A and B. Neither one is better than the
115 other (both are best merge bases). When the --all option is not given,
116 it is unspecified which best one is output.
117 A common idiom to check "fast-forward-ness" between two commits A and B
119 is (or at least used to be) to compute the merge base between A and B,
120 and check if it is the same as A, in which case, A is an ancestor of B.
121 You will see this idiom used often in older scripts.
122 A=$(git rev-parse --verify A)
124 if test "$A" = "$(git merge-base A B)"
125 then
126 ... A is an ancestor of B ...
127 fi
128 In modern git, you can say this in a more direct way:
130 if git merge-base --is-ancestor A B
132 then
133 ... A is an ancestor of B ...
134 fi
135 instead.
137
139 After working on the topic branch created with git switch -c topic
140 origin/master, the history of remote-tracking branch origin/master may
141 have been rewound and rebuilt, leading to a history of this shape:
142 o---B2
144 /
145 ---o---o---B1--o---o---o---B (origin/master)
146 \
147 B0
148 \
149 D0---D1---D (topic)
150 where origin/master used to point at commits B0, B1, B2 and now it
152 points at B, and your topic branch was started on top of it back when
153 origin/master was at B0, and you built three commits, D0, D1, and D, on
154 top of it. Imagine that you now want to rebase the work you did on the
155 topic on top of the updated origin/master.
156 In such a case, git merge-base origin/master topic would return the
158 parent of B0 in the above picture, but B0^..D is not the range of
159 commits you would want to replay on top of B (it includes B0, which is
160 not what you wrote; it is a commit the other side discarded when it
161 moved its tip from B0 to B1).
162 git merge-base --fork-point origin/master topic is designed to help in
164 such a case. It takes not only B but also B0, B1, and B2 (i.e. old tips
165 of the remote-tracking branches your repository’s reflog knows about)
166 into account to see on which commit your topic branch was built and
167 finds B0, allowing you to replay only the commits on your topic,
168 excluding the commits the other side later discarded.
169 Hence
171 $ fork_point=$(git merge-base --fork-point origin/master topic)
173 will find B0, and
175 $ git rebase --onto origin/master $fork_point topic
177 will replay D0, D1 and D on top of B to create a new history of this
179 shape:
180 o---B2
182 /
183 ---o---o---B1--o---o---o---B (origin/master)
184 \ \
185 B0 D0'--D1'--D' (topic - updated)
186 \
187 D0---D1---D (topic - old)
188 A caveat is that older reflog entries in your repository may be expired
190 by git gc. If B0 no longer appears in the reflog of the remote-tracking
191 branch origin/master, the --fork-point mode obviously cannot find it
192 and fails, avoiding to give a random and useless result (such as the
193 parent of B0, like the same command without the --fork-point option
194 gives).
195 Also, the remote-tracking branch you use the --fork-point mode with
197 must be the one your topic forked from its tip. If you forked from an
198 older commit than the tip, this mode would not find the fork point
199 (imagine in the above sample history B0 did not exist, origin/master
200 started at B1, moved to B2 and then B, and you forked your topic at
201 origin/master^ when origin/master was B1; the shape of the history
202 would be the same as above, without B0, and the parent of B1 is what
203 git merge-base origin/master topic correctly finds, but the
204 --fork-point mode will not, because it is not one of the commits that
205 used to be at the tip of origin/master).
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208 git-rev-list(1), git-show-branch(1), git-merge(1)
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211 Part of the git(1) suite
212Git 2.26.2 2020-04-20 GIT-MERGE-BASE(1)