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CS156

You start a feature branch with a fresh copy of main.

But by the time you are ready to do your pull request, you may find that your branch is a bit behind the times.

You may even find that GitHub is showing that your branch cannot be merged becuase it is so far behind.

In that case rebasing on main can be useful.

What does it mean to rebase on main

The idea is this: assume that the last three commits on main before you made your feature branch were A->B->C

Then, you make a feature branch, and you add commits X->Y->Z

Your branch now looks like this:

A->B->C->X->Y->Z

But in the meantime, others have made commits J->K->L and merged them into main. So main now looks like this:

A->B->C->J->K->L

Rebasing on main means that you’ll take all of the commits that you made on top of the old main (X->Y->Z) and replay them on top of the new main.

The result is this:

A->B->C->J->K->L->X->Y->Z

Keep in mind, though that this is a “rewriting of history’. So when you do it, you need to do something a little bit scary called a force push.

Now that we understand what’s going on, here’s how to do it.

Doing a rebase on main

Doing a rebase on main is only possible if you are on a branch other than main. We’ll assume your feature branch is called xy-my-branch

There are many ways to accomplish the task; here’s one:

Start by doing:

git pull --rebase origin main

This starts a rebase.

Note: this is the stage where there can be merge conflicts:

  • If there are merge conflicts, a rebase is a multi-step process
  • If you get back a message indicating there are merge conflicts, you are not finished. You need to read the message and do what it says.
  • Once you start, you have to either see it through to completion, or you have to abort.
  • If you want to abort, the command is git rebase --abort; this will put things back as they were.

If you get back an immediate response, though, and no errors show, then the rebase was successful. You can check by doing git log to see that the commits you did show up on top of the other ones.

Keep in mind though that this rebased branch stil exists only on your local system. To update GitHub, you now need to do a force push:

git push origin xy-my-branch -f

This final step rewrites the branch on github with your commits as the last thing on the branch, with them replayed on top of the most up-to-date version of the main branch.