For example, with this topology:
- o---o---o---B
- /
- ---o---1---o---o---o---A
+....
+ o---o---o---B
+ /
+---o---1---o---o---o---A
+....
the merge base between 'A' and 'B' is '1'.
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
+....
+ 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
+....
+ 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,
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
+....
+---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,
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" = "$(git merge-base A B)"
- then
- ... A is an ancestor of B ...
- fi
+....
+A=$(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
+....
+if git merge-base --is-ancestor A B
+then
+ ... A is an ancestor of B ...
+fi
+....
instead.
`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)
+....
+ 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
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)
+....
+ 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