How to define GitHub Actions multiline environment variable or output - CI/CD

I'm not sure if that was a hack or undocumented feature, but I can find it now in the GitHub Actions docs.

But in the past, I needed to copy a short multiline file between GitHub Actions jobs, and I didn't want to bother with extra steps of stash/unstash stuff, so I found that you can define a multiline GitHub Actions variable!

It was as easy as this:

jobs:
  job1:
    runs-on: ubuntu-latest
    steps:
      - name: Set multiline value in bash
        run: |
          # The curly brackets are just Bash syntax to group commands
          # and are not mandatory.
          {
              echo 'JSON_RESPONSE<<EOF'
              cat my-file.json
              echo EOF
          } >> "$GITHUB_OUTPUT"

Of course, you need to be sure that the delimiter EOF doesn't occure within the value.

Then you can call that again as:

[...]
  job2:
    needs: job1
    runs-on: ubuntu-latest
    steps:
      - name: Get multiline value in bash
        run: |
          echo "${{ needs.job1.outputs.JSON_RESPONSE }}"

That's it! Enjoy! ♾️