Running GitHub Actions When a Specific Branch Is Deleted

ToC

• Conclusion
  • Specifying the branch name with on: delete and if
  • on.delete becomes available only after you push to the default branch
• Summary

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I tried out a configuration for triggering and running GitHub Actions when a specific branch is deleted, so here is a summary.

Below is the sample configuration.
https://github.com/kenzo0107/tutorial-delete-event-on-github-actions/blob/develop/.github/workflows/delete.yml

Conclusion

I implemented it with the following configuration.

• .github/workflows/delete.yml

on: delete

jobs:
  run:
    if: ${{ (github.event.ref_type == 'branch' && startsWith(github.event.ref, 'tmp/')) }}
    runs-on: ubuntu-latest

    steps:
      - uses: actions/checkout@v2
      - run: echo "delete event ${{ github.event.ref }}"

Here is a summary of the key points.

Specifying the branch name with on: delete and if

This is triggered when a branch or tag is deleted.

Unlike on.push shown below, on.delete cannot currently filter branches via branches.

on:
  push:
    branches:
      - tmp/**

For that reason, the configuration only runs the following when what was deleted via on.delete is a branch and that branch name starts with tmp/ (e.g. tmp/dummy).

jobs:
  run:
    if: ${{ (github.event.ref_type == 'branch' && startsWith(github.event.ref, 'tmp/')) }}

Because of the configuration above, the delete trigger fires every time a branch is deleted (such as deleting a branch after merging), but in practice it stopped in about 1 second.

Since these small costs add up over time, if you want to avoid this, you might use the GitHub Webhook with Lambda’s free tier… but that’s the gist of it, and my impression is that there is no fundamental solution.

I really (sincerely) wish on.delete supported a branches filter.

on.delete becomes available only after you push to the default branch

on.push can be triggered even on the very first push to any branch.

on.delete becomes available only after you push to the default branch.
I was getting impatient because it just wouldn’t trigger, but it turns out it was properly documented in the official docs.

https://docs.github.com/en/actions/using-workflows/events-that-trigger-workflows#delete

Note: This event will only trigger a workflow run if the workflow file is on the default branch.

Summary

• on.delete cannot use a branches filter
• on.delete will not trigger unless there is a workflow file used on the default branch

That’s all.
I hope this is helpful.