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 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 40DISCUSSION 41---------- 42 43Given two commits 'A' and 'B', `git merge-base A B` will output a commit 44which is reachable from both 'A' and 'B' through the parent relationship. 45 46For example, with this topology: 47 48 o---o---o---B 49 / 50 ---o---1---o---o---o---A 51 52the merge base between 'A' and 'B' is '1'. 53 54Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the 55merge base between 'A' and a hypothetical commit 'M', which is a merge 56between 'B' and 'C'. For example, with this topology: 57 58 o---o---o---o---C 59 / 60 / o---o---o---B 61 / / 62 ---2---1---o---o---o---A 63 64the result of `git merge-base A B C` is '1'. This is because the 65equivalent topology with a merge commit 'M' between 'B' and 'C' is: 66 67 68 o---o---o---o---o 69 / \ 70 / o---o---o---o---M 71 / / 72 ---2---1---o---o---o---A 73 74and the result of `git merge-base A M` is '1'. Commit '2' is also a 75common ancestor between 'A' and 'M', but '1' is a better common ancestor, 76because '2' is an ancestor of '1'. Hence, '2' is not a merge base. 77 78When the history involves criss-cross merges, there can be more than one 79'best' common ancestor for two commits. For example, with this topology: 80 81 ---1---o---A 82 \ / 83 X 84 / \ 85 ---2---o---o---B 86 87both '1' and '2' are merge-bases of A and B. Neither one is better than 88the other (both are 'best' merge bases). When the `--all` option is not given, 89it is unspecified which best one is output. 90 91Author 92------ 93Written by Linus Torvalds <torvalds@osdl.org> 94 95Documentation 96-------------- 97Documentation by David Greaves, Junio C Hamano and the git-list <git@vger.kernel.org>. 98 99GIT 100--- 101Part of the linkgit:git[1] suite