[Github Actions] One-liner solution to Get a Multiline Output

echo "PAYLOAD<<EOF"$'\n'"$payload"$'\n'EOF >> "$GITHUB_OUTPUT".

This snippet was kindly shared with me by tjtharrison a while ago and I really like it.

You can go now, you’re welcome. The rest of this post is for my own reference, how I solved an issue using a multiline output.

The single line problem

Alas, I couldn’t find a way to dynamically build a one-line string that would escape ", escape \n, have a markdown format "Drift detected: ```<escaped_output_here>```" compatible with GITHUB_OUTPUT and work well with toJSON so it properly rendered in Slack. Sigh. Gotta up my bash game.

I got into this because we have a terraform drift detection in Github. It uses slackapi/slack-github-action to Slack us about deviations, adding a summary of affected resources inside a ```markdown``` block.

All was well until we got a payload with unescaped quotes:

{
        "type": "section",
        "text": {
          "type": "mrkdwn",
          "text": "Drift detected in terraform: \n```module.user["maria@example.com"]\n\nDetails:\nhttps://github.com/org/repo/actions/runs/1234567890```"
        }
      }
    ]
  }

What I did

• Got a multiline GITHUB_OUTPUT value
• Replaced ```markdown``` with a Slack block of type context with plain_text inside, to mimic the markdown <code>…