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