Github Actions, set shell via matrix variable

I have a job in Github Actions that uses a matrix. I want to run some of the job steps on a custom shell that depends on a matrix variable.

Specifically, the image has a foreign chroot so I can emulate any hardware architecture and run native binaries for it. The custom shell is just a wrapper script around bash which runs the commands in the chroot and with the emulator for the desired target architecture.

The matrix looks about like this:

cross_build:
  strategy:
    matrix:
      job:
      - { release: bullseye  , arch: armhf  , ocaml-version: 4.14.0  , publish: true }

I have tried

shell: bash-${{ matrix.job.arch }} {0}

(bash-$ARCH being the wrapper script), but get an error:

The workflow is not valid. .github/workflows/CI.yml (Line: mmm, Col: nn): Unrecognized named-value: 'matrix'.

If I hardcode the architecture in the shell, everything works:

shell: bash-armhf {0}

So the issue really is about resolving variables in the shell element.

Is there a way to pass a variable in this place so that different instances of the job run on different custom shells?

If not, I would need to have one bash-wrapper script and somehow pass it the information about the target platform I would like to use. A command line argument is presumably not an option if I can’t pass variables in shell – so what is the standard way of passing that information to the custom shell?

As pointed out by Azeem, variables in shell are currently not supported.

This works:

In the workflow definition, add the following to the job:

env:
  CROSS_ARCH: ${{ matrix.job.arch }}

Then, for the custom shell, specify the path to the wrapper script – one single script regardless of target architecture, thus no use of variable in shell.

In the wrapper script, evaluate CROSS_ARCH and choose the emulator and chroot based on that.

Since I already had multiple copies of a single script, which would determine the architecture by the name with which it was invoked, this was very easy to implement and is even cleaner than converting /bin/bash-ARCH into ARCH by stripping the beginning.