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-rw-r--r--Documentation/git-merge-base.txt28
1 files changed, 28 insertions, 0 deletions
diff --git a/Documentation/git-merge-base.txt b/Documentation/git-merge-base.txt
index b295bf8..87842e3 100644
--- a/Documentation/git-merge-base.txt
+++ b/Documentation/git-merge-base.txt
@@ -11,6 +11,7 @@ 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>...
DESCRIPTION
@@ -50,6 +51,12 @@ 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.
+
+
OPTIONS
-------
-a::
@@ -110,6 +117,27 @@ 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.
+
+
See also
--------
linkgit:git-rev-list[1],