How to use env variable as default value for input in github actions?

I have a github action that has an input which should have a default value based on an env.variable. Because github actions doesn't support env variables in the default field, I was wondering if I could reassign the inputs.variable in the steps portion of my action.yml file.

Here's what I've tried so far:

Doesn't work:

...
inputs:
  ...
  mono-build-tag:
    description: Release tag to download from the mono-build-repo
    # Github Actions complains that env is being used here
    default: "${{ env.GODOT_MONO_BUILD_TAG }}" 
runs:
  using: "composite"
  steps:
    - ...
  ...

Doesn't work:

...
inputs:
  ...
  mono-build-tag:
    description: Release tag to download from the mono-build-repo
    default: ""  
runs:
  using: "composite"
  steps:
    - name: Setup default inputs
      run: |
        if ${{ inputs.mono-build-tag == '' }}; then
          # How do I set inputs.mono-build-tag here???
        fi
  ...

Solution

You could define the env variable as follow (remember to put string literal in single quotes):


env:
  GODOT_MONO_BUILD_TAG: ${{ github.event.inputs.mono-build-repo || 'latest' }}

where latest should be the default value for the env var