HEAD~ for a merge commit

Question

In Git, you can refer to the commit before HEAD using the shorthand HEAD~, and two before using HEAD~2, etc.

I have a repository that has a merge commit like the following:

A----B-------------F
      \           /
       C----D----E

HEAD = F, HEAD~ points to B, and HEAD~2 points to A. With a merge commit like this, is there a shorthand that would point to E?

Answer 1

Yes; the ~ specifies the generation, but you can use ^ to specify the parent number, in the case of a merge.

git show HEAD^2

Will show the 2nd parent.

(The distinction is especially subtle since without a numeric argument both ~ and ^ show the same thing, the first parent of HEAD. This is because both ~ and ^ default to 1 without a numeric parameter. So they show the first parent (by depth) and the first parent (by breadth) which are of course the same.)

Answer 2

I know you asked for commit E, but the shorthand for commit C is an even better example for understanding the difference between ^ and ~.

Here is the shorthand for C:

HEAD^2~2

e.g. 2nd parent of HEAD and then 2 jumps down the first-parent chain from there.

Try cloning this repo to see for yourself (look for the "clone" link):

http://vm.bit-booster.com/bitbucket/plugins/servlet/bb_net/projects/BB/repos/a/commits

And here's how the various shorthands covered in this answer resolve:

git show --no-patch --oneline HEAD
1286a9a F

git show --no-patch --oneline HEAD^2
9640db6 E

git show --no-patch --oneline HEAD^2~2
506916b C

Exercise for the reader: what's another way (using this repo) to write HEAD^2~3?