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Diffstat (limited to 'Documentation/git-merge-base.txt')
| -rw-r--r-- | Documentation/git-merge-base.txt | 76 |
1 files changed, 65 insertions, 11 deletions
diff --git a/Documentation/git-merge-base.txt b/Documentation/git-merge-base.txt index 1a7ecbf8f3..767486c770 100644 --- a/Documentation/git-merge-base.txt +++ b/Documentation/git-merge-base.txt @@ -8,26 +8,80 @@ git-merge-base - Find as good common ancestors as possible for a merge SYNOPSIS -------- -'git merge-base' [--all] <commit> <commit> +'git merge-base' [--all] <commit> <commit>... DESCRIPTION ----------- -'git-merge-base' finds as good a common ancestor as possible between -the two commits. That is, given two commits A and B, `git merge-base A -B` will output a commit which is reachable from both A and B through -the parent relationship. +'git-merge-base' finds best common ancestor(s) between two commits to use +in a three-way merge. One common ancestor is 'better' than another common +ancestor if the latter is an ancestor of the former. A common ancestor +that does not have any better common ancestor is a 'best common +ancestor', i.e. a 'merge base'. Note that there can be more than one +merge base for a pair of commits. -Given a selection of equally good common ancestors it should not be -relied on to decide in any particular way. - -The 'git-merge-base' algorithm is still in flux - use the source... +Among the two commits to compute the merge base from, one is specified by +the first commit argument on the command line; the other commit is a +(possibly hypothetical) commit that is a merge across all the remaining +commits on the command line. As the most common special case, specifying only +two commits on the command line means computing the merge base between +the given two commits. OPTIONS ------- --all:: - Output all common ancestors for the two commits instead of - just one. + Output all merge bases for the commits, instead of just one. + +DISCUSSION +---------- + +Given two commits 'A' and 'B', `git merge-base A B` will output a commit +which is reachable from both 'A' and 'B' through the parent relationship. + +For example, with this topology: + + o---o---o---B + / + ---o---1---o---o---o---A + +the merge base between 'A' and 'B' is '1'. + +Given three commits 'A', 'B' and 'C', `git merge-base A B C` will compute the +merge base between 'A' and a hypothetical commit 'M', which is a merge +between 'B' and 'C'. For example, with this topology: + + o---o---o---o---C + / + / o---o---o---B + / / + ---2---1---o---o---o---A + +the result of `git merge-base A B C` is '1'. This is because the +equivalent topology with a merge commit 'M' between 'B' and 'C' is: + + + o---o---o---o---o + / \ + / o---o---o---o---M + / / + ---2---1---o---o---o---A + +and the result of `git merge-base A M` is '1'. Commit '2' is also a +common ancestor between 'A' and 'M', but '1' is a better common ancestor, +because '2' is an ancestor of '1'. Hence, '2' is not a merge base. + +When the history involves criss-cross merges, there can be more than one +'best' common ancestor for two commits. For example, with this topology: + + ---1---o---A + \ / + X + / \ + ---2---o---o---B + +both '1' and '2' are merge-bases of A and B. Neither one is better than +the other (both are 'best' merge bases). When the `--all` option is not given, +it is unspecified which best one is output. Author ------ |