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diff --git a/Documentation/git-merge-base.txt b/Documentation/git-merge-base.txt new file mode 100644 index 000000000000..261d5c116454 --- /dev/null +++ b/Documentation/git-merge-base.txt @@ -0,0 +1,231 @@ +git-merge-base(1) +================= + +NAME +---- +git-merge-base - Find as good common ancestors as possible for a merge + + +SYNOPSIS +-------- +[verse] +'git merge-base' [-a|--all] <commit> <commit>... +'git merge-base' [-a|--all] --octopus <commit>... +'git merge-base' --is-ancestor <commit> <commit> +'git merge-base' --independent <commit>... +'git merge-base' --fork-point <ref> [<commit>] + +DESCRIPTION +----------- + +'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. + +OPERATION MODES +--------------- + +As the most common special case, specifying only two commits on the +command line means computing the merge base between the given two commits. + +More generally, 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 a consequence, the 'merge base' is not necessarily contained in each of the +commit arguments if more than two commits are specified. This is different +from linkgit:git-show-branch[1] when used with the `--merge-base` option. + +--octopus:: + Compute the best common ancestors of all supplied commits, + in preparation for an n-way merge. This mimics the behavior + of 'git show-branch --merge-base'. + +--independent:: + Instead of printing merge bases, print a minimal subset of + the supplied commits with the same ancestors. In other words, + among the commits given, list those which cannot be reached + from any other. This mimics the behavior of 'git show-branch + --independent'. + +--is-ancestor:: + Check if the first <commit> is an ancestor of the second <commit>, + and exit with status 0 if true, or with status 1 if not. + Errors are signaled by a non-zero status that is not 1. + +--fork-point:: + Find the point at which a branch (or any history that leads + to <commit>) forked from another branch (or any reference) + <ref>. This does not just look for the common ancestor of + the two commits, but also takes into account the reflog of + <ref> to see if the history leading to <commit> forked from + an earlier incarnation of the branch <ref> (see discussion + on this mode below). + +OPTIONS +------- +-a:: +--all:: + 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. + +The result of `git merge-base --octopus A B C` is '2', because '2' is +the best common ancestor of all commits. + +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. + +A common idiom to check "fast-forward-ness" between two commits A +and B is (or at least used to be) to compute the merge base between +A and B, and check if it is the same as A, in which case, A is an +ancestor of B. You will see this idiom used often in older scripts. + + A=$(git rev-parse --verify A) + if test "$A" = "$(git merge-base A B)" + then + ... A is an ancestor of B ... + fi + +In modern git, you can say this in a more direct way: + + if git merge-base --is-ancestor A B + then + ... A is an ancestor of B ... + fi + +instead. + +Discussion on fork-point mode +----------------------------- + +After working on the `topic` branch created with `git switch -c +topic origin/master`, the history of remote-tracking branch +`origin/master` may have been rewound and rebuilt, leading to a +history of this shape: + + o---B2 + / + ---o---o---B1--o---o---o---B (origin/master) + \ + B0 + \ + D0---D1---D (topic) + +where `origin/master` used to point at commits B0, B1, B2 and now it +points at B, and your `topic` branch was started on top of it back +when `origin/master` was at B0, and you built three commits, D0, D1, +and D, on top of it. Imagine that you now want to rebase the work +you did on the topic on top of the updated origin/master. + +In such a case, `git merge-base origin/master topic` would return the +parent of B0 in the above picture, but B0^..D is *not* the range of +commits you would want to replay on top of B (it includes B0, which +is not what you wrote; it is a commit the other side discarded when +it moved its tip from B0 to B1). + +`git merge-base --fork-point origin/master topic` is designed to +help in such a case. It takes not only B but also B0, B1, and B2 +(i.e. old tips of the remote-tracking branches your repository's +reflog knows about) into account to see on which commit your topic +branch was built and finds B0, allowing you to replay only the +commits on your topic, excluding the commits the other side later +discarded. + +Hence + + $ fork_point=$(git merge-base --fork-point origin/master topic) + +will find B0, and + + $ git rebase --onto origin/master $fork_point topic + +will replay D0, D1 and D on top of B to create a new history of this +shape: + + o---B2 + / + ---o---o---B1--o---o---o---B (origin/master) + \ \ + B0 D0'--D1'--D' (topic - updated) + \ + D0---D1---D (topic - old) + +A caveat is that older reflog entries in your repository may be +expired by `git gc`. If B0 no longer appears in the reflog of the +remote-tracking branch `origin/master`, the `--fork-point` mode +obviously cannot find it and fails, avoiding to give a random and +useless result (such as the parent of B0, like the same command +without the `--fork-point` option gives). + +Also, the remote-tracking branch you use the `--fork-point` mode +with must be the one your topic forked from its tip. If you forked +from an older commit than the tip, this mode would not find the fork +point (imagine in the above sample history B0 did not exist, +origin/master started at B1, moved to B2 and then B, and you forked +your topic at origin/master^ when origin/master was B1; the shape of +the history would be the same as above, without B0, and the parent +of B1 is what `git merge-base origin/master topic` correctly finds, +but the `--fork-point` mode will not, because it is not one of the +commits that used to be at the tip of origin/master). + + +See also +-------- +linkgit:git-rev-list[1], +linkgit:git-show-branch[1], +linkgit:git-merge[1] + +GIT +--- +Part of the linkgit:git[1] suite |