Make a monorepo with existing repos

momorepofy.sh

#!/usr/bin/env bash

# Before:
# (Assuming you standard branch is development in the mono repo)
# git checkout -b add-services

githubOrg=FOOBAR
repoList="api backend-service frontend"
repoBranch="development"
serviceDir=services # local directory to pull the other repos in

for repoName in $(echo $repoList); do
  echo "Adding ${repoName}.."
  git subtree add --prefix=${serviceDir}/${repoName} https://github.com/${githubOrg}/${repoName} ${repoBranch}
done

# After:
# git checkout development
# git rebase add-services
# git push --force

There's a surprising amount of outdated or just plain incorrect info on the internet regarding this. The above will preserve the history of each service so git blame/lens continues to work. The heavy lifting is done by git subtree add.