Creating a composite action

In this guide, you'll learn how to build a composite action.

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Introduction

In this guide, you'll learn about the basic components needed to create and use a packaged composite action. To focus this guide on the components needed to package the action, the functionality of the action's code is minimal. The action prints "Hello World" and then "Goodbye", or if you provide a custom name, it prints "Hello [who-to-greet]" and then "Goodbye". The action also maps a random number to the random-number output variable, and runs a script named goodbye.sh.

Once you complete this project, you should understand how to build your own composite action and test it in a workflow.

Warning

When creating workflows and actions, you should always consider whether your code might execute untrusted input from possible attackers. Certain contexts should be treated as untrusted input, as an attacker could insert their own malicious content. For more information, see "Security hardening for GitHub Actions."

Composite actions and reusable workflows

Composite actions allow you to collect a series of workflow job steps into a single action which you can then run as a single job step in multiple workflows. Reusable workflows provide another way of avoiding duplication, by allowing you to run a complete workflow from within other workflows. For more information, see "Avoiding duplication."

Prerequisites

Before you begin, you'll create a repository on GitHub.

  1. Create a new public repository on GitHub. You can choose any repository name, or use the following hello-world-composite-action example. You can add these files after your project has been pushed to GitHub. For more information, see "Creating a new repository."

  2. Clone your repository to your computer. For more information, see "Cloning a repository."

  3. From your terminal, change directories into your new repository.

    Shell
    cd hello-world-composite-action

  4. In the hello-world-composite-action repository, create a new file called goodbye.sh with example code:

    Shell
    echo "echo Goodbye" > goodbye.sh

  5. From your terminal, make goodbye.sh executable.

    Shell
    chmod +x goodbye.sh
    Shell
    chmod +x goodbye.sh
    Shell
    git add --chmod=+x -- goodbye.sh

  6. From your terminal, check in your goodbye.sh file.

    Shell
    git add goodbye.sh
git commit -m "Add goodbye script"
git push
    Shell
    git add goodbye.sh
git commit -m "Add goodbye script"
git push
    Shell
    git commit -m "Add goodbye script"
git push

Creating an action metadata file

  1. In the hello-world-composite-action repository, create a new file called action.yml and add the following example code. For more information about this syntax, see "Metadata syntax for GitHub Actions".

    YAML
    name: 'Hello World'
description: 'Greet someone'
inputs:
  who-to-greet:  # id of input
    description: 'Who to greet'
    required: true
    default: 'World'
outputs:
  random-number:
    description: "Random number"
    value: ${{ steps.random-number-generator.outputs.random-number }}
runs:
  using: "composite"
  steps:
    - name: Set Greeting
      run: echo "Hello $INPUT_WHO_TO_GREET."
      shell: bash
      env:
        INPUT_WHO_TO_GREET: ${{ inputs.who-to-greet }}

    - name: Random Number Generator
      id: random-number-generator
      run: echo "random-number=$(echo $RANDOM)" >> $GITHUB_OUTPUT
      shell: bash

    - name: Set GitHub Path
      run: echo "$GITHUB_ACTION_PATH" >> $GITHUB_PATH
      shell: bash
      env:
        GITHUB_ACTION_PATH: ${{ github.action_path }}

    - name: Run goodbye.sh
      run: goodbye.sh
      shell: bash

    This file defines the who-to-greet input, maps the random generated number to the random-number output variable, adds the action's path to the runner system path (to locate the goodbye.sh script during execution), and runs the goodbye.sh script.

    For more information about managing outputs, see "Metadata syntax for GitHub Actions".

    For more information about how to use github.action_path, see "Accessing contextual information about workflow runs".

  2. From your terminal, check in your action.yml file.

    Shell
    git add action.yml
git commit -m "Add action"
git push

  3. From your terminal, add a tag. This example uses a tag called v1. For more information, see "About custom actions."

    Shell
    git tag -a -m "Description of this release" v1
git push --follow-tags

Testing out your action in a workflow

The following workflow code uses the completed hello world action that you made in "Creating a composite action".

Copy the workflow code into a .github/workflows/main.yml file in another repository, replacing actions and SHA with the repository owner and the SHA of the commit you want to use, respectively. You can also replace the who-to-greet input with your name.

YAML
on: [push]

jobs:
  hello_world_job:
    runs-on: ubuntu-latest
    name: A job to say hello
    steps:
      - uses: actions/checkout@v4
      - id: foo
        uses: OWNER/hello-world-composite-action@SHA
        with:
          who-to-greet: 'Mona the Octocat'
      - run: echo random-number "$RANDOM_NUMBER"
        shell: bash
        env:
          RANDOM_NUMBER: ${{ steps.foo.outputs.random-number }}

From your repository, click the Actions tab, and select the latest workflow run. The output should include: "Hello Mona the Octocat", the result of the "Goodbye" script, and a random number.

Example composite actions on GitHub

You can find many examples of composite actions on GitHub.