git: moving branch head

Question

I have two git branches, "A" and "B", and commits numbered 1 thru 8. My history looks like this

1 -> 2 -> 3 -> 4[A] -> 5 -> 6 -> 7 -> 8[B]

I want to change it so my history looks like this:

1 -> 2 -> 3 -> 4 -> 5 -> 6[A] -> 7 -> 8[B]

That is, I want to move the head of branch A from commit 4 to commit 6.

What commands do I use to do this?

Answer 1

You can run:

git branch -f A 6

Answer 2

git checkout A
git reset --hard 6

Answer 3

This is a special case of rebase, just that the branch is empty:

git checkout A
git rebase B

rebase is more general; it handles this case also:

Before:

                  A1 -> A2 -> [A]
                /
1 -> 2 -> 3 -> 4  -> 5 -> 6 -> 7 -> 8[B]

After:

                                      A1' -> A2' -> [A]
                                     /
1 -> 2 -> 3 -> 4  -> 5 -> 6 -> 7 -> 8[B]

A1' and A2' are merged to account for the delta between 4 and 8 on the parent branch.

Git rebase handles this trivial case without a hassle. I created a repo with two commits on master and a branch br pointing to the first commit.

$ git checkout br
Switched to branch 'br'
$ git rebase master
First, rewinding head to replay your work on top of it...
Fast-forwarded br to master.

Poof, done. The log now shows the branch pointing to the second commit.

We can also achieve this "After": [thanks to M. Flaschen for pointing out this was missing]:

                            A1'' -> A2'' -> [A]
                           /
1 -> 2 -> 3 -> 4  -> 5 -> 6 -> 7 -> 8[B]

Instead of rebasing to the branch B, we would name the specific commit 6, e.g.

git checkout A
git rebase 6   # rather than rebase B

When there are no A1 and A2 commits, this reduces to the original question: moving the [A] pointer from 4 to 6.