GitHub Action that checks PRs for commits with messages containing `fixup!` and `squash!`

check-commit-messages.yml

name: PR commit checker
on: pull_request
jobs:
  get-commits:
    runs-on: ubuntu-latest
    steps:
    - name: Get PR commits
      run: |
        curl -H "Authorization: token ${{ secrets.GITHUB_TOKEN }}" -o out.json ${{ github.event.pull_request.commits_url }}

    - name: Check PR commit messages
      run: |
        echo -e 'import json\ncommits = set((d["sha"], d["commit"]["message"]) for d in json.load(open("out.json", "r")))\nbad_commits = {s for s in commits if s[1].startswith("fixup!") or s[1].startswith("squash!")}\nif bad_commits:\n  raise ValueError(f"There are some bad commits: {bad_commits}")' | python3