# GitHub Actions Quick Reference

*Workflows, triggers, jobs, secrets, caching, artifacts*

> Source: GitHub Actions Documentation (docs.github.com/actions) · MIT

## Workflow Basics

### Minimal Workflow

```
# .github/workflows/ci.yml
name: CI
on: push
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4
      - run: echo "Hello from CI"
```

### Key Concepts

| Command | Description |
|---------|-------------|
| `Workflow` | YAML file in `.github/workflows/` that defines automation |
| `Event` | Trigger that starts a workflow (push, PR, schedule, etc.) |
| `Job` | Set of steps that run on the same runner |
| `Step` | Individual task — runs a command or uses an action |
| `Runner` | VM that executes jobs (`ubuntu-latest`, `macos-latest`, `windows-latest`) |
| `Action` | Reusable unit of code referenced with `uses:` |

## Triggers

### Common Events

```
on:
  push:
    branches: [main]
  pull_request:
    branches: [main]
  schedule:
    - cron: "0 6 * * 1"    # every Monday 6 AM UTC
  workflow_dispatch:          # manual trigger
```

### Event Filters

| Command | Description |
|---------|-------------|
| `branches:` | Trigger only for specific branches |
| `paths:` | Trigger only when matching files change |
| `tags:` | Trigger on tag pushes (`v*`) |
| `types: [opened, synchronize]` | Filter PR activity types |
| `branches-ignore:` | Exclude specific branches |
| `paths-ignore:` | Exclude specific file paths |

## Jobs & Steps

### Job Configuration

```
jobs:
  test:
    runs-on: ubuntu-latest
    needs: build            # depends on build job
    if: github.ref == 'refs/heads/main'
    timeout-minutes: 10
    steps:
      - uses: actions/checkout@v4
      - run: npm test
```

### Step Types

| Command | Description |
|---------|-------------|
| `run:` | Execute a shell command |
| `uses:` | Use a published action |
| `with:` | Pass inputs to an action |
| `name:` | Display name in the UI |
| `id:` | Reference step outputs via `steps.<id>.outputs` |
| `if:` | Conditional execution |
| `continue-on-error: true` | Don't fail the job if step fails |

## Actions

### Using Actions

```
steps:
  - uses: actions/checkout@v4
  - uses: actions/setup-node@v4
    with:
      node-version: 20
  - uses: ./.github/actions/my-action  # local action
```

### Popular Actions

| Command | Description |
|---------|-------------|
| `actions/checkout@v4` | Check out repository code |
| `actions/setup-node@v4` | Install Node.js |
| `actions/setup-python@v5` | Install Python |
| `actions/upload-artifact@v4` | Upload build artifacts |
| `actions/download-artifact@v4` | Download artifacts from another job |
| `actions/cache@v4` | Cache dependencies between runs |
| `actions/github-script@v7` | Run JS with the GitHub API client |

## Environment Variables

### Setting Variables

```
env:                          # workflow-level
  NODE_ENV: production
jobs:
  build:
    env:                      # job-level
      CI: true
    steps:
      - run: echo "$MY_VAR"
        env:                  # step-level
          MY_VAR: hello
```

### Default Variables

| Command | Description |
|---------|-------------|
| `github.sha` | Commit SHA that triggered the workflow |
| `github.ref` | Branch or tag ref (`refs/heads/main`) |
| `github.repository` | Owner/repo name |
| `github.actor` | User who triggered the workflow |
| `github.event_name` | Event that triggered the workflow |
| `runner.os` | Runner OS (`Linux`, `macOS`, `Windows`) |

## Secrets

### Using Secrets

```
steps:
  - run: deploy --token "$TOKEN"
    env:
      TOKEN: ${{ secrets.DEPLOY_TOKEN }}
  - uses: some/action@v1
    with:
      api-key: ${{ secrets.API_KEY }}
```

### Secret Rules

| Command | Description |
|---------|-------------|
| `secrets.GITHUB_TOKEN` | Auto-generated token scoped to the repo |
| `Settings → Secrets` | Add secrets in repo or org settings |
| `Masking` | Secret values are masked in logs automatically |
| `Environment secrets` | Scoped to a deployment environment |
| `Org secrets` | Shared across repos in an organization |

## Matrix Strategy

### Matrix Builds

```
jobs:
  test:
    strategy:
      matrix:
        os: [ubuntu-latest, macos-latest]
        node: [18, 20]
    runs-on: ${{ matrix.os }}
    steps:
      - uses: actions/setup-node@v4
        with:
          node-version: ${{ matrix.node }}
```

### Matrix Options

| Command | Description |
|---------|-------------|
| `matrix:` | Define variable combinations to expand |
| `include:` | Add extra combinations to the matrix |
| `exclude:` | Remove specific combinations |
| `fail-fast: false` | Continue other jobs if one fails |
| `max-parallel: 2` | Limit concurrent matrix jobs |

## Caching

### Cache Dependencies

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

### Built-in Caching

```
- uses: actions/setup-node@v4
  with:
    node-version: 20
    cache: npm            # auto-cache for npm/yarn/pnpm
- uses: actions/setup-python@v5
  with:
    python-version: "3.12"
    cache: pip            # auto-cache for pip
```

## Artifacts

### Upload & Download

```
- uses: actions/upload-artifact@v4
  with:
    name: build-output
    path: dist/
    retention-days: 7
- uses: actions/download-artifact@v4
  with:
    name: build-output
```

### Artifact Notes

| Command | Description |
|---------|-------------|
| `retention-days` | Auto-delete after N days (default 90) |
| `path` | File or directory to upload (supports globs) |
| `Cross-job` | Upload in one job, download in another with `needs:` |
| `compression-level` | 0 (none) to 9 (max), default 6 |

## Common Patterns

### Conditional Deployment

```
- name: Deploy to production
  if: github.ref == 'refs/heads/main'
  run: ./deploy.sh
- name: Post PR comment
  if: github.event_name == 'pull_request'
  run: gh pr comment $PR --body "Build passed"
```

### Useful Expressions

| Command | Description |
|---------|-------------|
| `success()` | True if all previous steps passed |
| `failure()` | True if any previous step failed |
| `always()` | Run regardless of status (cleanup) |
| `cancelled()` | True if workflow was cancelled |
| `contains(github.ref, 'release')` | String contains check |
| `startsWith(github.ref, 'refs/tags')` | String prefix check |
| `hashFiles('**/lock*')` | SHA-256 of files (for cache keys) |