Today I found myself once again trying to run a pull request that was created in a different GitHub repository. If it was in the same repo, it would be a simple git checkout, however because GitHub insisted on the creation of "forks" I now need to go through the extra steps of fetching that specific ref. Here's what I do:

git fetch${team}/${respository}.git refs/pull/${prId}/head
git checkout FETCH_HEAD
git rebase master # or main

Just replace team, repository and prId and you're all set.

I picked this script up from The Lounge: It's a more complete example if you want to start using a script like this. Notice the shift and bash substitution ($@) at the end of it — these allow you to pass in command-line arguments to yarn start (or whatever your run script is).