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' [--all] <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 than it is a 'best common 20ancestor', i.e. a 'merge base'. Note that there can be more than one 21merge bases between two commits. 22 23Among the two commits to compute their merge bases, one is specified by 24the first commit argument on the command line; the other commit is a 25(possibly hypothetical) commit that is a merge across all the remaining 26commits on the command line. As the most common special case, giving only 27two commits from the command line means computing the merge base between 28the given two commits. 29 30OPTIONS 31------- 32--all:: 33 Output all merge bases for the commits, instead of just one. 34 35DISCUSSION 36---------- 37 38Given two commits 'A' and 'B', `git merge-base A B` will output a commit 39which is reachable from both 'A' and 'B' through the parent relationship. 40 41For example, with this topology: 42 43 o---o---o---B 44 / 45 ---o---1---o---o---o---A 46 47the merge base between 'A' and 'B' is '1'. 48 49Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the 50merge base between 'A' and an hypothetical commit 'M', which is a merge 51between 'B' and 'C'. For example, with this topology: 52 53 o---o---o---o---C 54 / 55 / o---o---o---B 56 / / 57 ---2---1---o---o---o---A 58 59the result of `git merge-base A B C` is '1'. This is because the 60equivalent topology with a merge commit 'M' between 'B' and 'C' is: 61 62 63 o---o---o---o---o 64 / \ 65 / o---o---o---o---M 66 / / 67 ---2---1---o---o---o---A 68 69and the result of `git merge-base A M` is '1'. Commit '2' is also a 70common ancestor between 'A' and 'M', but '1' is a better common ancestor, 71because '2' is an ancestor of '1'. Hence, '2' is not a merge base. 72 73When the history involves criss-cross merges, there can be more than one 74'best' common ancestors between two commits. For example, with this 75topology: 76 77 ---1---o---A 78 \ / 79 X 80 / \ 81 ---2---o---o---B 82 83both '1' and '2' are merge-base of A and B. Neither one is better than 84the other (both are 'best' merge base). When `--all` option is not given, 85it is unspecified which best one is output. 86 87Author 88------ 89Written by Linus Torvalds <torvalds@osdl.org> 90 91Documentation 92-------------- 93Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. 94 95GIT 96--- 97Part of the linkgit:git[1] suite