`git rebase --onto` for switching target branches

git-rebase-onto-for-switching-target-branches.md

git rebase --onto for switching target branches

Git rebase with --onto implements the idea:

cut and paste from start commit B to end commit E and put it "onto" commit O.

nixpkgs example

If you have a situation where somebody made a somebranch PR against master but it should have been against another branch, like this:

              o---o---o  somebranch
             / 
o---o---o---B---o  master
     \
      o---o---o---o---o  haskell-updates

(Note the commit where somebranch branches off master is called B for "merge base").

Then you can switch the the somebranch to be based on haskell-updates instead by using git rebase with --onto.

In this case, it is:

cut and paste from start commit B to end commit somebranch and put it "onto" commit haskell-updates.

So we run (while being on somebranch):

git rebase B somebranch --onto haskell-updates

Result

o---o---o---B---o  master
     \
      o---o---o---o---o  haskell-updates
                       \
                        o---o---o  somebranch

Then you can switch the base in Github. (You can also do this before.)

Interesting to know

This approach is a short-cut for going onto haskell-updates and using git cherry-pick for every single commit in somebranch manually (and then renaming the result of that back to somebranch).