How to Drive Conditional Jobs From Job Outputs in CI

Job outputs are strings, so incremental gating hinges on comparing them exactly: needs.detect.outputs.flag == the string true, not a bare truthiness check.

Job outputs cross job boundaries only as strings. Compute a flag in a detect job, promote it to outputs:, and compare it as == 'true' in each dependent job. Getting the string comparison right is the whole trick.

Steps

• In the detect job, write flag=true (or false) to $GITHUB_OUTPUT.
• Map it under the job outputs: key.
• In consumers, compare needs.detect.outputs.flag == 'true'.

Workflow

jobs:
  detect:
    runs-on: ubuntu-latest
    outputs:
      run_e2e: ${{ steps.decide.outputs.run_e2e }}
    steps:
      - uses: actions/checkout@v4
        with: { fetch-depth: 0 }
      - id: decide
        run: |
          if git diff --name-only origin/main...HEAD | grep -q '^src/'; then
            echo "run_e2e=true" >> "$GITHUB_OUTPUT"
          else
            echo "run_e2e=false" >> "$GITHUB_OUTPUT"
          fi
  e2e:
    needs: detect
    if: needs.detect.outputs.run_e2e == 'true'
    runs-on: ubuntu-latest
    steps: [{ run: npm run test:e2e }]

Gotchas

• Outputs are always strings; if: needs.detect.outputs.run_e2e (no comparison) is truthy for any non-empty string including "false".
• An empty output is treated as false, which is easy to trip over if the producing step failed silently.

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