Example showing how to set a dynamic matrix sequence based on input

dynamic-matrix-ghaction.yml

name: Dynamic Matrix
on:
  push:
  workflow_dispatch:
    inputs:
      someInput:
        description: 'Some input'
        required: true
        type: choice
        options: [ one, two, three ]

jobs:
  set-matrix:
    runs-on: ubuntu-22.04
    outputs:
      matrix: ${{ steps.set-matrix.outputs.matrix }}
    steps:
      - name: Set matrix in output
        id: set-matrix
        run: |
          if [ "${{ inputs.someInput }}" ]; then
           echo "someInput is set"
           echo 'matrix=[ "${{ inputs.someInput }}" ]' >> $GITHUB_OUTPUT
          else
           echo "someInput is empty"
           echo 'matrix=[ "one", "two", "three", "four", "five" ]' >> $GITHUB_OUTPUT
          fi

  simple-print-job:
    runs-on: ubuntu-22.04
    needs: [ set-matrix ]
    strategy:
      matrix:
        value: ${{ fromJson(needs.set-matrix.outputs.matrix) }}
    steps:
      - name: Print matrix value
        run: echo "${{ matrix.value }}"