matrix jobs: how to conditionally use either shell: wsl-bash {0} or just shell

[ssbarnea]

When doing matrix testing on multiple platforms one may want to test on a matrix like below:

matrix:
  os:
  - ubuntu-latest
  - macos-11
  shell:
  - bash
  include:
  - os: windows-latest
    shell: wsl-bash {0}
steps:
- name: Activate WSL
  if: runner.os == 'Windows'
  uses: Vampire/setup-wsl@v1
- name: run something
  run: cat /etc/os-release
  shell: ${{ matrix.shell }} # <-- apparently GHA issue

The problem is that apparently GHA does not allow you use shell: ${{ matrix.shell }}, giving an unexpected error

The workflow is not valid. .github/workflows/ci.yaml (Line: 77, Col: 16): Unrecognized named-value: 'matrix'. Located at position 1 within expression: matrix.shell .github/workflows/ci.yaml (Line: 124, Col: 16): Unrecognized named-value: 'matrix'. Located at position 1 within expression: matrix.shell

The error above makes me think that GHA is not really able to use dynamic expressions on shell:, while it is able to do the same thing on if:, name: or run:.

It would be very useful to have an example where something like this works cross-platforms, especially as in real life the number of steps is considerably bigger and duplicating each "run" and adding conditions makes the workflow files reach an unmaintainable size. Even splitting them into two and using different jobs for windows and non windows does create similar maintenance challenges.

Also posted at https://github.saobby.my.eu.orgmunity/t/unable-to-use-expressions-as-argument-to-shell/226230

[ssbarnea]

In case someone else gets into the same issue, I found a solution, using defaults with fallback values can help you switch the default shell. Look at ansible/team-devtools#29 (comment)

[Vampire]

This is more a GitHub issue.
I played around and guess this is a GitHub action issue that you should report to GitHub as the expression gets evaluated but the matrix variable is not available.
As I don't think it is in the scope of my actions documentation to document this, I moved the issue to the discussions for others as reference.
Glad that you found a solution, that's also what I would have suggested.

Another possible option would be to use as shell a custom script that dynamically decides from RUNNER_OS environment variable or similar whether to use bash or wsl-bash and use that as the shell in the workflow file.

[Vampire]

At least GitHub 1st level supports it is a bug.
I submitted a support request (you should maybe still do it too to show that more people have the problem) and 1st level forwarded it as bug issue to the engineering team.