github actions: run on merge only

Github actions are a convenient way to embed automation directly into your project’s repository.

I was working on automating a version bump for my project recently and I thought this was actually a nice fit for a Github action (my solution here).

The only problem is that my initial script was aware when the branch was main and so would do something different if it was and when it wasn’t. I wanted that same degree of control.

To do that, I needed a way to target my Github action to only run on “merge to main“.

Unfortunately, that isn’t actually available.

Github has special events for push and pull_request.

on:
  ? [push, pull_request]

Both of which are too generic.

We can narrow it down to just closed pull requests:

on:
  pull_request:
    types: [closed]

However, this would mean that the workflow is run even if the pull request is not merged.

For specific actions, we can narrow it down with github.event.pull_request.merged == true, however for workflows with multiple actions, this would be redundant.

The best solution I’ve found (h/t jenschelkopf) combines protected branches with the fact that a merged pull request results in a push event.

Once a Github Branch Protection rule has been set up to prevent pushes directly to main, we can run the workflow on pushes to main since we’ll know that only occurs on a successful merge of pull request:

on:
  push:
    branches:
      - main

And just like that, I have a Github action that only runs on merges to main - exactly what I wanted!

In fact, this is even better than targeting a pull request event because if it was a pull request, the targeted branch would the branch with the changes, not the target - which means that a workflow may run even if the target isn’t main. While that may be desireable, it’s definitely not in my case.