Having every github action run on every pull request will end up slowing you down and sometimes even discourage you from making changes. For example, if you see an error in the README.md but you know you'll have to wait for the entire test suite to run you may choose not to push the change.

Path Filtering

To address this problem, Github has a feature called path filtering which works really well! You can include and exclude paths with a simple syntax:

on:
  pull_request:
    paths:
      - '*'
      - '*/**'
      - '!README.md'
      - '!.gitignore'

Which will run on any changes except for README.md and .gitignore. There is even a simpler format to this by using paths-ignore:

on:
  pull_request:
    paths-ignore:
      - 'README.md'
      - '.gitignore'

Although the path filtering feature of github works, I do not recommend using it because it has some critical issues that I'll talk about below!

Branch Protection / Required Checks

Path Filtering has one major flaw which is that it skips the run of the job completely which means if you are using branch protection your PR will be stuck in pending state.

To resolve this we will have to run the job, but skip the actions inside it that are slow conditionally. There are a few opensource actions out there that make this easy but the one I recommend is:

https://github.com/tj-actions/changed-files

It creates a lot of useful outputs you can use in your conditional out of the box which makes your action easy to write.

Skipping a required check

So using tj-actions/changed-files to skip a required check, it will look very similar to the path filtering from github. You first define what files you care about:

jobs:
  terraform-lint: 
    runs-on: ubuntu-22.04
    steps:
      - uses: actions/checkout@v3
      - name: Check if files changed
        id: changed-files-yaml
        uses: tj-actions/changed-files@v40
        with:
          files_yaml: |
            src:
              - '*'
              - '*/**'
              - '!README.md'
              - '!.gitignore'

The src is just a key selection I made here. You can have many groups of files like tests, docs, etc. and conditionally do things if each of them changed.

So from our example, this step will generate output we can use steps.changed-files-yaml.outputs.src_any_changed that is either true or false.. To use this we can us the if conditional block on our jobs.

steps:
    - if: steps.changed-files-yaml.outputs.src_any_changed == 'true'
    run: |
        earthly \
        +ci-terraform-lint \
            --TERRAFORM_VERSION=${{ env.RTX_TERRAFORM_VERSION }}

This will then skip linting if we didn't change any terraform and the job will be marked successful!

This can be confusing sometimes and the people you work with (or future you!) may wonder why these critical checks are being skipped. I like to include a message with the decision that was made so its obvious that we wanted to skip them:

- if: steps.changed-files-yaml.outputs.src_any_changed == 'false'
  run: |
    echo "No terraform files changed"

Which will output:

✅ "No terraform files changed"

Right next to the skipped job which is exactly what we wanted to see! So the end result would look something like this:

on:
  pull_request:
    
jobs:
  terraform-lint: 
    runs-on: ubuntu-22.04
    steps:
      - uses: actions/checkout@v3
      - name: Check if real files changed
        id: changed-files-yaml
        uses: tj-actions/changed-files@v40
        with:
          files_yaml: |
            src:
              - '*'
              - '*/**'
              - '!README.md'
              - '!.gitignore'
 
      - if: steps.changed-files-yaml.outputs.src_any_changed == 'true'
        run: |
          earthly \
            +ci-terraform-lint \
              --TERRAFORM_VERSION=${{ env.RTX_TERRAFORM_VERSION }}
 
      - if: steps.changed-files-yaml.outputs.src_any_changed == 'false'
        run: |
          echo "No terraform files changed"

Conclusion

Hopefully with this you can start speeding up your github actions and making the developer experience enjoyable!