writing-github-actions

name	writing-github-actions
description	Write GitHub Actions workflows with proper syntax, reusable workflows, composite actions, matrix builds, caching, and security best practices. Use when creating CI/CD workflows for GitHub-hosted projects or automating GitHub repository tasks.

Writing GitHub Actions

Create GitHub Actions workflows for CI/CD pipelines, automated testing, deployments, and repository automation using YAML-based configuration with native GitHub integration.

Purpose

GitHub Actions is the native CI/CD platform for GitHub repositories. This skill covers workflow syntax, triggers, job orchestration, reusable patterns, optimization techniques, and security practices specific to GitHub Actions.

Core Focus:

• Workflow YAML syntax and structure
• Reusable workflows and composite actions
• Matrix builds and parallel execution
• Caching and optimization strategies
• Secrets management and OIDC authentication
• Concurrency control and artifact management

Not Covered:

• CI/CD pipeline design strategy → See building-ci-pipelines
• GitOps deployment patterns → See gitops-workflows
• Infrastructure as code → See infrastructure-as-code
• Testing frameworks → See testing-strategies

When to Use This Skill

Trigger this skill when:

• Creating CI/CD workflows for GitHub repositories
• Automating tests, builds, and deployments via GitHub Actions
• Setting up reusable workflows across multiple repositories
• Optimizing workflow performance with caching and parallelization
• Implementing security best practices for GitHub Actions
• Troubleshooting GitHub Actions YAML syntax or behavior

Workflow Fundamentals

Basic Workflow Structure

name: CI
on:
  push:
    branches: [main]
  pull_request:
    branches: [main]

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - uses: actions/setup-node@v4
        with:
          node-version: '20'
          cache: 'npm'
      - run: npm ci
      - run: npm test

Key Components:

• name: Workflow display name
• on: Trigger events (push, pull_request, schedule, workflow_dispatch)
• jobs: Job definitions (run in parallel by default)
• runs-on: Runner type (ubuntu-latest, windows-latest, macos-latest)
• steps: Sequential operations (uses actions or run commands)

Common Triggers

# Code events
on:
  push:
    branches: [main, develop]
    paths: ['src/**']
  pull_request:
    types: [opened, synchronize, reopened]

# Manual trigger
on:
  workflow_dispatch:
    inputs:
      environment:
        type: choice
        options: [dev, staging, production]

# Scheduled
on:
  schedule:
    - cron: '0 2 * * *'  # Daily at 2 AM UTC

For complete trigger reference, see references/triggers-events.md.

Decision Frameworks

Reusable Workflow vs Composite Action

Use Reusable Workflow when:

• Standardizing entire CI/CD jobs across repositories
• Need complete job replacement with inputs/outputs
• Want secrets to inherit by default
• Orchestrating multiple steps with job-level configuration

Use Composite Action when:

• Packaging 5-20 step sequences for reuse
• Need step-level abstraction within jobs
• Want to distribute via marketplace or private repos
• Require local file access without artifacts

Feature	Reusable Workflow	Composite Action
Scope	Complete job	Step sequence
Trigger	workflow_call	uses: in step
Secrets	Inherit by default	Must pass explicitly
File Sharing	Requires artifacts	Same runner/workspace

For detailed patterns, see references/reusable-workflows.md and references/composite-actions.md.

Caching Strategy

Use Built-in Setup Action Caching (Recommended):

- uses: actions/setup-node@v4
  with:
    node-version: '20'
    cache: 'npm'  # or 'yarn', 'pnpm'

Available for: Node.js, Python (pip), Java (maven/gradle), .NET, Go

Use Manual Caching when:

• Need custom cache keys
• Caching build outputs or non-standard paths
• Implementing multi-layer cache strategies

- uses: actions/cache@v4
  with:
    path: ~/.npm
    key: ${{ runner.os }}-deps-${{ hashFiles('**/package-lock.json') }}
    restore-keys: ${{ runner.os }}-deps-

For optimization techniques, see references/caching-strategies.md.

Self-Hosted vs GitHub-Hosted Runners

Use GitHub-Hosted Runners when:

• Standard build environments sufficient
• No private network access required
• Within budget or free tier limits

Use Self-Hosted Runners when:

• Need specific hardware (GPU, ARM, high memory)
• Require private network/VPN access
• High usage volume (cost optimization)
• Custom software must be pre-installed

Common Patterns

Multi-Job Workflow with Dependencies

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v5
      - run: npm run build
      - uses: actions/upload-artifact@v4
        with:
          name: dist
          path: dist/

  test:
    needs: build
    runs-on: ubuntu-latest
    steps:
      - uses: actions/download-artifact@v5
        with:
          name: dist
      - run: npm test

  deploy:
    needs: [build, test]
    if: github.ref == 'refs/heads/main'
    runs-on: ubuntu-latest
    environment: production
    steps:
      - uses: actions/download-artifact@v5
      - run: ./deploy.sh

Key Elements:

• needs: creates job dependencies (sequential execution)
• Artifacts pass data between jobs
• if: enables conditional execution
• environment: enables protection rules and environment secrets

Matrix Builds

jobs:
  test:
    runs-on: ${{ matrix.os }}
    strategy:
      fail-fast: false
      matrix:
        os: [ubuntu-latest, windows-latest, macos-latest]
        node: [18, 20, 22]
    steps:
      - uses: actions/checkout@v5
      - uses: actions/setup-node@v4
        with:
          node-version: ${{ matrix.node }}
      - run: npm test

Result: 9 jobs (3 OS × 3 Node versions)

For advanced matrix patterns, see examples/matrix-build.yml.

Concurrency Control

# Cancel in-progress runs on new push
concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

# Single deployment per environment
jobs:
  deploy:
    concurrency:
      group: production-deployment
      cancel-in-progress: false
    steps: [...]

Reusable Workflows

Defining a Reusable Workflow

File: .github/workflows/reusable-build.yml

name: Reusable Build
on:
  workflow_call:
    inputs:
      node-version:
        type: string
        default: '20'
    secrets:
      NPM_TOKEN:
        required: false
    outputs:
      artifact-name:
        value: ${{ jobs.build.outputs.artifact }}

jobs:
  build:
    runs-on: ubuntu-latest
    outputs:
      artifact: build-output
    steps:
      - uses: actions/checkout@v5
      - uses: actions/setup-node@v4
        with:
          node-version: ${{ inputs.node-version }}
      - run: npm ci && npm run build
      - uses: actions/upload-artifact@v4
        with:
          name: build-output
          path: 

Calling a Reusable Workflow

jobs:
  build:
    uses: ./.github/workflows/reusable-build.yml
    with:
      node-version: '20'
    secrets: inherit  # Same org only

For complete reusable workflow guide, see references/reusable-workflows.md.

Composite Actions

Defining a Composite Action

File: .github/actions/setup-project/action.yml

name: 'Setup Project'
description: 'Install dependencies and setup environment'

inputs:
  node-version:
    description: 'Node.js version'
    default: '20'

outputs:
  cache-hit:
    value: ${{ steps.cache.outputs.cache-hit }}

runs:
  using: "composite"
  steps:
    - uses: actions/setup-node@v4
      with:
        node-version: ${{ inputs.node-version }}
        cache: 'npm'

    - id: cache
      uses: actions/cache@v4
      with:
        path: node_modules
        key: ${{ runner.os }}-deps-${{ hashFiles('**/package-lock.json') }}

    - if: steps.cache.outputs.cache-hit != 'true'
      shell: bash
      run: npm ci

Key Requirements:

• runs.using: "composite" marks action type
• shell: required for all run steps
• Access inputs via ${{ inputs.name }}

Using a Composite Action

steps:
  - uses: actions/checkout@v5
  - uses: ./.github/actions/setup-project
    with:
      node-version: '20'
  - run: npm run build

For detailed composite action patterns, see references/composite-actions.md.

Security Best Practices

Secrets Management

jobs:
  deploy:
    runs-on: ubuntu-latest
    environment: production  # Uses environment secrets
    steps:
      - env:
          API_KEY: ${{ secrets.API_KEY }}
        run: ./deploy.sh

OIDC Authentication (No Long-Lived Credentials)

jobs:
  deploy:
    runs-on: ubuntu-latest
    permissions:
      id-token: write  # Required for OIDC
      contents: read
    steps:
      - uses: aws-actions/configure-aws-credentials@v4
        with:
          role-to-assume: arn:aws:iam::123456789012:role/GitHubActionsRole
          aws-region: us-east-1
      - run: aws s3 sync ./dist s3://my-bucket

Minimal Permissions

# Workflow-level
permissions:
  contents: read
  pull-requests: write

# Job-level
jobs:
  deploy:
    permissions:
      contents: write
      deployments: write
    steps: [...]

Action Pinning

# Pin to commit SHA (not tags)
- uses: actions/checkout@8ade135a41bc03ea155e62e844d188df1ea18608  # v5.0.0

Enable Dependabot:

File: .github/dependabot.yml

version: 2
updates:
  - package-ecosystem: "github-actions"
    directory: "/"
    schedule:
      interval: "weekly"

For comprehensive security guide, see references/security-practices.md.

Optimization Techniques

Use built-in caching in setup actions (cache: 'npm'), run independent jobs in parallel, add conditional execution with if:, and minimize checkout depth (fetch-depth: 1).

For detailed optimization strategies, see references/caching-strategies.md.

Context Variables

Common contexts: github.*, secrets.*, inputs.*, matrix.*, runner.*

- run: echo "Branch: ${{ github.ref }}, Event: ${{ github.event_name }}"

For complete syntax reference, see references/workflow-syntax.md.

Progressive Disclosure

Detailed References

For comprehensive coverage of specific topics:

• references/workflow-syntax.md - Complete YAML syntax reference
• references/reusable-workflows.md - Advanced reusable workflow patterns
• references/composite-actions.md - Composite action deep dive
• references/caching-strategies.md - Optimization and caching techniques
• references/security-practices.md - Comprehensive security guide
• references/triggers-events.md - All trigger types and event filters
• references/marketplace-actions.md - Recommended actions catalog

Working Examples

Complete workflow templates ready to use:

• examples/basic-ci.yml - Simple CI workflow
• examples/matrix-build.yml - Matrix strategy examples
• examples/reusable-deploy.yml - Reusable deployment workflow
• examples/composite-setup/ - Composite action template
• examples/monorepo-workflow.yml - Monorepo with path filters
• examples/security-scan.yml - Security scanning workflow

Validation Scripts

• scripts/validate-workflow.sh - Validate YAML syntax

Related Skills

• building-ci-pipelines - CI/CD pipeline design strategy
• gitops-workflows - GitOps deployment patterns
• infrastructure-as-code - Terraform/Pulumi integration
• testing-strategies - Test frameworks and coverage
• security-hardening - SAST/DAST tools
• git-workflows - Understanding branches and PRs