Unable to assign output of command to $GITHUB_OUTPUT in github actions

This github action aims to convert the output of ls command into input [json] for the strategy matrix in github actions.

Below, workflow attempt fails on a UNIX github runner.

name: Convert LS Output to Strategy Matrix

on: push: branches: - main

jobs: convert-to-matrix: runs-on: ubuntu-latest

steps:
  - name: Get LS Output
    id: matrix_step
    run: |
      list=$(ls -1 /etc/*.conf);
      matrix_values=$(echo '[ "'"$(echo "$list" | sed ':a;N;$!ba;s/\n/", "/g')"'" ]')
      echo "Matrix values: $matrix_values"
      echo "matrix_values=$matrix_values" >> $GITHUB_ENV
      

build: runs-on: ubuntu-latest needs: convert-to-matrix

strategy:
  matrix: 
    filename: ${{ needs.setup-matrix.outputs.matrix-combinations }}

steps:

  - name: Build the file
    run: |
      echo "Building ${{ matrix.filename }}"
      # Add your build steps here using the ${{ matrix.filename }} variable

convert-to-matrix job is successful and prints this value

Matrix values: [ "/etc/adduser.conf", "/etc/apt-fast.conf", "/etc/ca-certificates.conf", "/etc/cgconfig.conf", "/etc/cgrules.conf", "/etc/debconf.conf", "/etc/sudo_logsrvd.conf", "/etc/sysctl.conf", "/etc/ucf.conf", "/etc/usb_modeswitch.conf", "/etc/waagent.conf", "/etc/xattr.conf" ]

But the build job fails with the following error:

Convert LS Output to Strategy Matrix: .github#L1 Error when evaluating 'strategy' for job 'build'. .github/workflows/generateinput.yml (Line: 27, Col: 19): Unexpected value ''

Can you please suggest how can I convert the output of ls to something reasonable input to a strategy matrix.

The issues with the code example above were:

• job-level outputs
  • When using outputs across different jobs, the required outputs should be defined on job level and not step level only.
• fromJSON
  • matrix_values output then needs to be used in strategy.matrix which requires fromJSON to convert the value to an array.

Detailed discussion and generic example available in this answer: Github Actions: How use strategy/matrix with script

Based on the url provided in comments, here is the updated version with correctly setting and exporting the required output for matrix usage:

jobs:
  convert-to-matrix:
    runs-on: ubuntu-latest

    steps:
      - name: Get LS Output
        id: matrix_step
        run: |
          list=$(ls -1 /etc/*.conf);
          matrix_values=$(echo '[ "'"$(echo "$list" | sed ':a;N;$!ba;s/\n/", "/g')"'" ]')
          echo "Matrix values: $matrix_values"
          echo "matrix_values=$matrix_values" >> $GITHUB_OUTPUT
    outputs:
      matrix-combination: ${{ steps.matrix_step.output.matrix_values }}   
  build:
    runs-on: ubuntu-latest
    needs: convert-to-matrix
    
    strategy:
      matrix: 
        filename: ${{ fromJSON(needs.setup-matrix.outputs.matrix-combinations) }}