Using output from a previous job in a new one in a GitHub Action

In my case I wanted to pass an entire build/artifact, not just a string:

name: Build something on Ubuntu then use it on MacOS

on:
  workflow_dispatch:
    # Allows for manual build trigger

jobs:
  buildUbuntuProject:
    name: Builds the project on Ubuntu (Put your stuff here)
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: some/compile-action@v99
      - uses: actions/upload-artifact@v2
        # Upload the artifact so the MacOS runner do something with it
        with:
          name: CompiledProject
          path: pathToCompiledProject
  doSomethingOnMacOS:
    name: Runs the program on MacOS or something
    runs-on: macos-latest
    needs: buildUbuntuProject # Needed so the job waits for the Ubuntu job to finish
    steps:
      - uses: actions/download-artifact@master
        with:
          name: CompiledProject
          path: somewhereToPutItOnMacOSRunner
      - run: ls somewhereToPutItOnMacOSRunner # See the artifact on the MacOS runner

Check the "GitHub Actions: New workflow features" from April 2020, which could help in your case (to reference step outputs from previous jobs)

Job outputs

You can specify a set of outputs that you want to pass to subsequent jobs and then access those values from your needs context.

See documentation:

jobs.<jobs_id>.outputs

A map of outputs for a job.

Job outputs are available to all downstream jobs that depend on this job.
For more information on defining job dependencies, see jobs.<job_id>.needs.

Job outputs are strings, and job outputs containing expressions are evaluated on the runner at the end of each job. Outputs containing secrets are redacted on the runner and not sent to GitHub Actions.

To use job outputs in a dependent job, you can use the needs context.
For more information, see "Context and expression syntax for GitHub Actions."

To use job outputs in a dependent job, you can use the needs context.

Example

jobs:
  job1:
    runs-on: ubuntu-latest
    # Map a step output to a job output
    outputs:
      output1: ${{ steps.step1.outputs.test }}
      output2: ${{ steps.step2.outputs.test }}
    steps:
    - id: step1
      run: echo "::set-output name=test::hello"
    - id: step2
      run: echo "::set-output name=test::world"
  job2:
    runs-on: ubuntu-latest
    needs: job1
    steps:
    - run: echo ${{needs.job1.outputs.output1}} ${{needs.job1.outputs.output2}}

---

Jesse Adelman adds in the comments:

This seems to not work well for anything beyond a static string.
How, for example, would I take a multiline text output of step (say, I'm running a pytest or similar) and use that output in another job?

• either write the multi-line text to a file (jschmitter's comment)
• or base64-encode the output and then decode it in the next job (Nate Karasch's comment)

Update: It's now possible to set job outputs that can be used to transfer string values to downstream jobs. See this answer.

What follows is the original answer. These techniques might still be useful for some use cases.

1. Write the data to file and use actions/upload-artifact and actions/download-artifact. A bit awkward, but it works.
2. Create a repository dispatch event and send the data to a second workflow. I prefer this method personally, but the downside is that it needs a repo scoped PAT.

Here is an example of how the second way could work. It uses repository-dispatch action.

name: "We 🎔 Perl"
on:
  issues:
    types: [opened, edited, milestoned]

jobs:
  seasonal_greetings:
    runs-on: windows-latest
    steps:
      - name: Maybe greet
        id: maybe-greet
        env:
          HEY: "Hey you!"
          GREETING: "Merry Xmas to you too!"
          BODY: ${{ github.event.issue.body }}
        run: |
          $output=(perl -e 'print ($ENV{BODY} =~ /Merry/)?$ENV{GREETING}:$ENV{HEY};')
          Write-Output "::set-output name=GREET::$output"
      - name: Repository Dispatch
        uses: peter-evans/repository-dispatch@v1
        with:
          token: ${{ secrets.REPO_ACCESS_TOKEN }}
          event-type: my-event
          client-payload: '{"greet": "${{ steps.maybe-greet.outputs.GREET }}"}'

This triggers a repository dispatch workflow in the same repository.

name: Repository Dispatch
on:
  repository_dispatch:
    types: [my-event]
jobs:
  myEvent:
    runs-on: ubuntu-latest
    steps:
      - run: echo ${{ github.event.client_payload.greet }}

It is possible to capture the entire output (and return code) of a command within a run step, which I've written up here to hopefully save someone else the headache. Fair warning, it requires a lot of shell trickery and a multiline run to ensure everything happens within a single shell instance.

In my case, I needed to invoke a script and capture the entirety of its stdout for use in a later step, as well as preserve its outcome for error checking:

# capture stdout from script 
SCRIPT_OUTPUT=$(./do-something.sh)

# capture exit code as well
SCRIPT_RC=$?

# FYI, this would get stdout AND stderr
SCRIPT_ALL_OUTPUT=$(./do-something.sh 2>&1)

Since Github's job outputs only seem to be able to capture a single line of text, I also had to escape any newlines for the output:

echo "::set-output name=stdout::${SCRIPT_OUTPUT//$'\n'/\\n}"

Additionally, I needed to ultimately return the script's exit code to correctly indicate whether it failed. The whole shebang ends up looking like this:

- name: A run step with stdout as a captured output
  id: myscript
  run: |
    # run in subshell, capturiing stdout to var
    SCRIPT_OUTPUT=$(./do-something.sh)
    # capture exit code too
    SCRIPT_RC=$?
    # print a single line output for github
    echo "::set-output name=stdout::${SCRIPT_OUTPUT//$'\n'/\\n}"
    # exit with the script status
    exit $SCRIPT_RC
  continue-on-error: true
- name: Add above outcome and output as an issue comment
  uses: actions/github-script@v5
  env:
    STEP_OUTPUT: ${{ steps.myscript.outputs.stdout }}
  with:
    github-token: ${{ secrets.GITHUB_TOKEN }}
    script: |
      // indicates whather script succeeded or not
      let comment = `Script finished with \`${{ steps.myscript.outcome }}\`\n`;

      // adds stdout, unescaping newlines again to make it readable
      comment += `<details><summary>Show Output</summary>

      \`\`\`
      ${process.env.STEP_OUTPUT.replace(/\\n/g, '\n')}
      \`\`\`

      </details>`;

      // add the whole damn thing as an issue comment
      github.rest.issues.createComment({
        issue_number: context.issue.number,
        owner: context.repo.owner,
        repo: context.repo.repo,
        body: comment
      })

Edit: there is also an action to accomplish this with much less bootstrapping, which I only just found.