1git-merge-base(1) 2================= 3 4NAME 5---- 6git-merge-base - Find as good common ancestors as possible for a merge 7 8 9SYNOPSIS 10-------- 11[verse] 12'git merge-base' [-a|--all] <commit> <commit>... 13'git merge-base' [-a|--all] --octopus <commit>... 14'git merge-base' --independent <commit>... 15 16DESCRIPTION 17----------- 18 19'git merge-base' finds best common ancestor(s) between two commits to use 20in a three-way merge. One common ancestor is 'better' than another common 21ancestor if the latter is an ancestor of the former. A common ancestor 22that does not have any better common ancestor is a 'best common 23ancestor', i.e. a 'merge base'. Note that there can be more than one 24merge base for a pair of commits. 25 26Unless `--octopus` is given, among the two commits to compute the merge 27base from, one is specified by the first commit argument on the command 28line; the other commit is a (possibly hypothetical) commit that is a merge 29across all the remaining commits on the command line. As the most common 30special case, specifying only two commits on the command line means 31computing the merge base between the given two commits. 32 33As a consequence, the 'merge base' is not necessarily contained in each of the 34commit arguments if more than two commits are specified. This is different 35from linkgit:git-show-branch[1] when used with the `--merge-base` option. 36 37OPTIONS 38------- 39-a:: 40--all:: 41 Output all merge bases for the commits, instead of just one. 42 43--octopus:: 44 Compute the best common ancestors of all supplied commits, 45 in preparation for an n-way merge. This mimics the behavior 46 of 'git show-branch --merge-base'. 47 48--independent:: 49 Instead of printing merge bases, print a minimal subset of 50 the supplied commits with the same ancestors. In other words, 51 among the commits given, list those which cannot be reached 52 from any other. This mimics the behavior of 'git show-branch 53 --independent'. 54 55DISCUSSION 56---------- 57 58Given two commits 'A' and 'B', `git merge-base A B` will output a commit 59which is reachable from both 'A' and 'B' through the parent relationship. 60 61For example, with this topology: 62 63 o---o---o---B 64 / 65 ---o---1---o---o---o---A 66 67the merge base between 'A' and 'B' is '1'. 68 69Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the 70merge base between 'A' and a hypothetical commit 'M', which is a merge 71between 'B' and 'C'. For example, with this topology: 72 73 o---o---o---o---C 74 / 75 / o---o---o---B 76 / / 77 ---2---1---o---o---o---A 78 79the result of `git merge-base A B C` is '1'. This is because the 80equivalent topology with a merge commit 'M' between 'B' and 'C' is: 81 82 83 o---o---o---o---o 84 / \ 85 / o---o---o---o---M 86 / / 87 ---2---1---o---o---o---A 88 89and the result of `git merge-base A M` is '1'. Commit '2' is also a 90common ancestor between 'A' and 'M', but '1' is a better common ancestor, 91because '2' is an ancestor of '1'. Hence, '2' is not a merge base. 92 93The result of `git merge-base --octopus A B C` is '2', because '2' is 94the best common ancestor of all commits. 95 96When the history involves criss-cross merges, there can be more than one 97'best' common ancestor for two commits. For example, with this topology: 98 99 ---1---o---A 100 \ / 101 X 102 / \ 103 ---2---o---o---B 104 105both '1' and '2' are merge-bases of A and B. Neither one is better than 106the other (both are 'best' merge bases). When the `--all` option is not given, 107it is unspecified which best one is output. 108 109Author 110------ 111Written by Linus Torvalds <torvalds@osdl.org> 112 113Documentation 114-------------- 115Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. 116 117See also 118-------- 119linkgit:git-rev-list[1], 120linkgit:git-show-branch[1], 121linkgit:git-merge[1] 122 123GIT 124--- 125Part of the linkgit:git[1] suite