Martin von Zweigbergk <> writes:

>> +       if (argc != 2)
>> +               die("--is-ancestor takes exactly two commits");
> I think git merge-base shows the usage message regardless if argc < 2,
> so this only happens when more than two arguments are given.

Yes, but it is a good discipline not to depend too much on what
other parts of the code may have done, when you do not have to.

> Maybe
> include --is-ancestor in the usage message?

I think "merge-base -h" will get this for free thanks to parseopt.

>> +       if (is_ancestor && (show_all | octopus | reduce))
>> +               die("--is-reachable cannot be used with other options");
> I suppose --is-reachable should be --is-ancestor.

Thanks, that was a renaming error.

I'll squash the fix in with documentation.

 Documentation/git-merge-base.txt | 28 ++++++++++++++++++++++++++++
 builtin/merge-base.c             |  6 +++---
 2 files changed, 31 insertions(+), 3 deletions(-)

diff --git i/Documentation/git-merge-base.txt w/Documentation/git-merge-base.txt
index b295bf8..87842e3 100644
--- i/Documentation/git-merge-base.txt
+++ w/Documentation/git-merge-base.txt
@@ -11,6 +11,7 @@ SYNOPSIS
 '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>...
@@ -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
+       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.
@@ -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
 See also
diff --git i/builtin/merge-base.c w/builtin/merge-base.c
index 615aa04..447ab7c 100644
--- i/builtin/merge-base.c
+++ w/builtin/merge-base.c
@@ -70,7 +70,7 @@ static int handle_octopus(int count, const char **args, int 
reduce, int show_all
        return 0;
-static int show_is_ancestor(int argc, const char **argv)
+static int handle_is_ancestor(int argc, const char **argv)
        struct commit *one, *two;
@@ -107,9 +107,9 @@ int cmd_merge_base(int argc, const char **argv, const char 
        if (!octopus && !reduce && argc < 2)
                usage_with_options(merge_base_usage, options);
        if (is_ancestor && (show_all | octopus | reduce))
-               die("--is-reachable cannot be used with other options");
+               die("--is-ancestor cannot be used with other options");
        if (is_ancestor)
-               return show_is_ancestor(argc, argv);
+               return handle_is_ancestor(argc, argv);
        if (reduce && (show_all || octopus))
                die("--independent cannot be used with other options");
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