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