How to break up a monorepo :)

monorepo.md

How to break up a monorepo using git subtree:

0. Set up some basic stuff:
   • These should be independent, not nested

$ NEW_REPO_PATH=
$ OLD_REPO_PATH=

1. isolate the files inside a branch
   • make your preferred file tree
   • choose a folder to isolate (e.g. my-cool-crate)
   • for convenience, you can make the branch name the same as the folder :)

$ FOLDER_TO_ISOLATE=my-cool-crate
$ git subtree split -P $FOLDER_TO_ISOLATE -b $FOLDER_TO_ISOLATE

2. add the old monorepo as a remote in your new repo

$ mkdir -p $NEW_REPO_PATH
$ cd $NEW_REPO_PATH
$ git init
$ git remote add old_repo $OLD_REPO_PATH 

3. Pull the monorepo branch into your new repo on main

$ git pull old_repo $FOLDER_TO_ISOLATE

4. Push the contents to github

$ git push -u origin main

5. Clean up the isolated branch

$ cd $OLD_REPO
$ git branch -D $FOLDER_TO_ISOLATE

---

to port a PR easily:

0. perform the steps above to create the new isolated repo

1. check out your PR branch

$ cd $OLD_REPO_PATH
$ PR_BRANCH=prestwich/my-pr-branch
$ git checkout $PR_BRANCH

2. push the subtree to the new repo
   • the arguments are isolated folder name, new repo location, branch name

$ git subtree push --prefix $FOLDER_TO_ISOLATE $NEW_REPO_PATH $PR_BRANCH

3. Push the new branch to the new github repo

$ cd $NEW_REPO_PATH
$ git switch $PR_BRANCH
$ git push -u origin $PR_BRANCH

4. Open the PR on in the new repo on github as normal