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] <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 23Among the two commits to compute the merge base from, 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, specifying only 27two commits on the command line means computing the merge base between 28the given two commits. 29 30OPTIONS 31------- 32-a:: 33--all:: 34 Output all merge bases for the commits, instead of just one. 35 36DISCUSSION 37---------- 38 39Given two commits 'A' and 'B', `git merge-base A B` will output a commit 40which is reachable from both 'A' and 'B' through the parent relationship. 41 42For example, with this topology: 43 44 o---o---o---B 45 / 46 ---o---1---o---o---o---A 47 48the merge base between 'A' and 'B' is '1'. 49 50Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the 51merge base between 'A' and a hypothetical commit 'M', which is a merge 52between 'B' and 'C'. For example, with this topology: 53 54 o---o---o---o---C 55 / 56 / o---o---o---B 57 / / 58 ---2---1---o---o---o---A 59 60the result of `git merge-base A B C` is '1'. This is because the 61equivalent topology with a merge commit 'M' between 'B' and 'C' is: 62 63 64 o---o---o---o---o 65 / \ 66 / o---o---o---o---M 67 / / 68 ---2---1---o---o---o---A 69 70and the result of `git merge-base A M` is '1'. Commit '2' is also a 71common ancestor between 'A' and 'M', but '1' is a better common ancestor, 72because '2' is an ancestor of '1'. Hence, '2' is not a merge base. 73 74When the history involves criss-cross merges, there can be more than one 75'best' common ancestor for two commits. For example, with this topology: 76 77 ---1---o---A 78 \ / 79 X 80 / \ 81 ---2---o---o---B 82 83both '1' and '2' are merge-bases of A and B. Neither one is better than 84the other (both are 'best' merge bases). When the `--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