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Diffstat (limited to 'Documentation/git-merge-base.txt')
| -rw-r--r-- | Documentation/git-merge-base.txt | 66 |
1 files changed, 64 insertions, 2 deletions
diff --git a/Documentation/git-merge-base.txt b/Documentation/git-merge-base.txt index b295bf8330..808426faac 100644 --- a/Documentation/git-merge-base.txt +++ b/Documentation/git-merge-base.txt @@ -11,7 +11,9 @@ 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 ----------- @@ -23,8 +25,8 @@ 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 MODE --------------- +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. @@ -50,6 +52,20 @@ from linkgit:git-show-branch[1] when used with the `--merge-base` option. 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:: @@ -110,6 +126,52 @@ 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 checkout -b +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---B1 + / + ---o---o---B2--o---o---o---B (origin/master) + \ + B3 + \ + Derived (topic) + +where `origin/master` used to point at commits B3, B2, B1 and now it +points at B, and your `topic` branch was started on top of it back +when `origin/master` was at B3. This mode uses the reflog of +`origin/master` to find B3 as the fork point, so that the `topic` +can be rebased on top of the updated `origin/master` by: + + $ fork_point=$(git merge-base --fork-point origin/master topic) + $ git rebase --onto origin/master $fork_point topic + + See also -------- linkgit:git-rev-list[1], |