Using Bash Parameter Expansion for Conditionals in GitHub Actions

I am late to using GitHub Actions for CI, and immediately ran into an issue trying to figure out how to conditionally install a different version of Bundler for a certain version of Rails in radar/distance_of_time_in_words#104. Bundler 2.x doesn’t work with Rails 4, and needs to be downgraded.

Fetching gem metadata from https://rubygems.org/.............
Fetching gem metadata from https://rubygems.org/.
Resolving dependencies...
Bundler could not find compatible versions for gem "bundler":
  In rails_4.gemfile:
    bundler

    rails (~> 4.0) was resolved to 4.2.11.3, which depends on
      bundler (>= 1.3.0, < 2.0)

  Current Bundler version:
    bundler (2.1.4)

I decided to use default Bundler as much as possible, and run gem install bundler -v 1.17.3 for a certain version of Rails.

Use `if:`

My first solution was to use an if: step.

jobs:
  build:
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        include:
          - ruby-version: 2.6.x
          - ruby-version: 2.4.x
            bundler-version: 1.17.3
            gemfile: gemfiles/rails_4.gemfile
    steps:
      - uses: actions/checkout@v2
      - name: Set up Ruby
        uses: actions/setup-ruby@v1
        with:
          ruby-version: ${{ matrix.ruby-version }}
      - name: Install Bundler
        if: ${{ matrix.bundler-version }}
        run: |
          gem uninstall bundler
          gem install bundler -v ${{ matrix.bundler-version }}
      - name: Build and test
        env:
          BUNDLE_GEMFILE: ${{ matrix.gemfile }}
        run: |
          bundle install
          bundle exec rake

This worked because bundler was installed by default.

Use another `if:`

Using if: to install a non-default version of Bundler will suddenly break when some future run uses a newer, default, version of Bundler that breaks everything. We can craft another if: to install a specific version or, for now, display the default version of Bundler.

      - name: Use Default Bundler
        if: ${{ !matrix.bundler-version }}
        run: |
          bundler --version
      - name: Install Custom Bundler
        if: ${{ matrix.bundler-version }}
        run: |
          gem uninstall bundler
          gem install bundler -v ${{ matrix.bundler-version }}

This worked using the negation operator !. I do find the lack of else: regrettable.

Use Bash Parameter Expansion

My final and favorite solution is to use bash parameter expansion. In Bash you can write ${BUNDLER:-2.1.4} which uses the value of $BUNDLER when available, and 2.1.4 otherwise.

jobs:
  build:
    runs-on: ubuntu-latest
    strategy:
      fail-fast: false
      matrix:
        include:
          - ruby-version: 2.4.x
          - ruby-version: 2.4.x
            bundler-version: 1.17.3
            gemfile: gemfiles/rails_4.gemfile
    steps:
      - uses: actions/checkout@v2
      - name: Set up Ruby
        uses: actions/setup-ruby@v1
        with:
          ruby-version: ${{ matrix.ruby-version }}
      - name: Build and test
        env:
          BUNDLE_GEMFILE: ${{ matrix.gemfile }}
          BUNDLER: ${{ matrix.bundler-version }}
        run: |
          gem uninstall bundler
          gem install bundler -v ${BUNDLER:-2.1.4}
          bundle install --jobs 4 --retry 3
          bundle exec rake

I find this pattern quite elegant. To summarize.

Set a variable in matrix, e.g. bundler-version: 1.17.3.
Assign it to an environment variable via env, e.g. BUNDLER: $.
Use it with a default value in run, e.g. gem install bundler -v ${BUNDLER:-2.1.4}.

See radar/distance_of_time_in_words#104 for more details.

Daniel Doubrovkine

Using Bash Parameter Expansion for Conditionals in GitHub Actions github

Use if:

Use another if:

Use Bash Parameter Expansion

Using Bash Parameter Expansion for Conditionals in GitHub Actions

Use `if:`

Use another `if:`