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3º. 2º cuatrimestre. Itinerario de Computación. Grado en Ingeniería Informática. ULL

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Table of Contents

GitHub Actions: An Introduction

Github Actions enables you to create custom software development lifecycle workflows directly in your Github repository. These workflows are made out of different tasks so-called actions that can be run automatically on certain events.

This enables you to include Continues Integration (CI) and continuous deployment (CD) capabilities and many other features directly in your repository.

Here is a brief glossary of terms (for more see Core concepts for GitHub Actions):

Workflow

A Workflow is an automated process that is made up of one or multiple jobs and can be triggered by an event. Workflows are defined using a YAML file in the .github/workflows directory.

Workflows can be created inside the .github/workflows directory by adding a .yml workflow file. Here in the terminal we do:

$ mkdir -p .github/workflows
$ touch .github/workflows/nodejs.yml

and use our favourite editor.

Ejemplo:

name: Node.js Package
on:
  release:
    types: [created]
jobs:
  build:
    ...
  publish-npm:
    ...
  publish-gpr:
    needs: build
    ...

The Editor of Github Actions

We can also use the GitHub Interface.

The Github Actions Editor is quite clever: Auto-complete can be triggered with Ctrl+Space almost anywhere.

Auto-complete works even inside expressions

Job

A job is made up of multiple steps and runs in an instance of the virtual environment. Jobs can

Example:

name: Node.js Package

on:
  release:
    types: [created]

jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: actions/setup-node@v1
        with:
          node-version: 12
      - run: npm ci
      - run: npm test

  publish-npm:
    needs: build
    runs-on: ubuntu-latest
    steps:
      ...
  publish-gpr:
    needs: build
    runs-on: ubuntu-latest
    steps:
      ...

The needs attribute inside the publish-npm job tell us that this job can not start until the build step has finished

Step

A step is a set of tasks that can be executed by a job. Steps can run commands or actions.

Example:

name: Node.js Package
on:
  release:
    types: [created]
jobs:
  build:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2  # An action
      - uses: actions/setup-node@v1
        with:
          node-version: 12
      - run: npm ci      # A command
      - run: npm test

Actions

Actions are the smallest portable building block of a workflow and can be combined as steps to create a job.

There are two types of actions:

  1. Docker container and
  2. JavaScript actions

Docker container actions allow the environment to be packaged with the GitHub Actions code and can only execute in the GitHub-Hosted Linux environment.

JavaScript actions decouple the GitHub Actions code from the environment allowing faster execution but accepting greater dependency management responsibility.

Actions require a metadata file to define the

  1. inputs,
  2. outputs and
  3. main entrypoint

for your action.

The metadata filename must be either action.yml or action.yaml.

Type Operating system
Docker container Linux
JavaScript Linux, MacOS, Windows

Event

Events are specific activities that trigger a workflow run. For example, a workflow is triggered when somebody pushes to the repository or when a pull request is created. Events can also be configured to listen to external events using Webhooks.

The release event

Runner

A runner is a machine with the Github Actions runner application installed. Then runner waits for available jobs it can then execute. After picking up a job they run the job’s actions and report the progress and results back to Github. Runners can be hosted on Github or self-hosted on your own machines/servers.

Syntax of the .yml File

Github Actions files are written using YAML syntax and have either a .yml or .yaml file extension. Here are the most important concepts for the workflow file.

Name:

The name of your workflow that is displayed on the Github actions page. If you omit this field, it is set to the file name.

name: CI for scapegoat module

On:

The on keyword defines the Github events that trigger the workflow. You can provide a single event, array or events or a configuration map that schedules a workflow.

on: push

or

on: [pull_request, issues]

Jobs:

A workflow run is made up of one or more jobs. Jobs define the functionality that will be run in the workflow and run in parallel by default.

jobs: 
    ci-scapegoat:
    # Define the OS our workflow should run on
    runs-on: ubuntu-latest

    strategy:
        # To test across multiple language versions
        matrix:
        node-version: [12.x]

    steps: # Clone the repo. See https://github.com/actions/checkout
    - uses: actions/checkout@v2
    # Example of using an environment variable
    - name: Use Node.js ${{ matrix.node-version }} # Will be: "Use Node.js 12.x"
        uses: actions/setup-node@v1 # Install node. See https://github.com/actions/setup-node
        with:
        node-version: ${{ matrix.node-version }}
    # Install a project with a clean slate
    - run: npm ci
    - run: npm test
        # Environment variables
        env:
        CI: true

Env:

Env defines a map of environment variables that are available to all jobs and steps in the workflow. You can also set environment variables that are only available to a job or step.

env:
CI: true

steps.with

A map of the input parameters defined by the action. Each input parameter is a key/value pair.

Input parameters are set as environment variables.

The variable is prefixed with INPUT_ and converted to upper case.

Example

Defines the three input parameters (first_name, middle_name, and last_name) defined by the hello_world action.

These input variables will be accessible to the hello-world action as INPUT_FIRST_NAME, INPUT_MIDDLE_NAME, and INPUT_LAST_NAME environment variables.

jobs:
  my_first_job:
    steps:
      - name: My first step
        uses: actions/hello_world@master
        with:
          first_name: Mona
          middle_name: The
          last_name: Octocat

Expression Syntax

You can use expressions to programmatically set variables in workflow files and access contexts.

${{ <expression> }}

You can combine literals, context references, and functions using operators.

An expression can be any combination of

literal values,

env:
    myNull: ${ { null }}
    myBoolean: ${ { false }}
    myIntegerNumber: ${ { 711 }}
    myFloatNumber: ${ { -9.2 }}
    myHexNumber: ${ { 0xff }}
    myExponentialNumber: ${ { -2.99-e2 }}
    myString: ${ { 'Mona the Octocat' }}
    myEscapedString: ${ { 'It''s open source!' }}

Operators

Operator Description
( ) Logical grouping
[ ] Index
. Property dereference
! Not
< Less than
<= Less than or equal
> Greater than
>= Greater than or equal
== Equal
!= Not equal
&& And
|| Or

functions

GitHub offers a set of built-in functions

Example:

format('Hello {0} {1} {2}', 'Mona', 'the', 'Octocat')

Returns 'Hello Mona the Octocat'

contains('Hello world', 'llo') returns true

The if Keyword and Functions to Check Job Status

Expressions are commonly used with the conditional if keyword in a workflow file to determine whether a step should run.

When you use expressions in an if conditional, you do not need to use the expression syntax (${ { }}) because GitHub automatically evaluates the if conditional as an expression.

For more information about if conditionals, see “Workflow syntax for GitHub Actions.”

Example expression in an if conditional

steps:
  - name: Git checkout
    if: github.event.check_suite.app.name == 'Netlify' && github.event.check_suite.conclusion == 'success'
    uses: actions/checkout@master

  - name: Install Node
    if: success()
    uses: actions/setup-node@v1
    with:
      node-version: 10.x

  - name: Install npm dependencies
    if: success()
    run: npm install

  - name: Run Audit
    if: success()
    uses: ./.github/actions/run-audit

success() returns true when none of the previous steps have failed or been canceled.

See Job status check functions

Object Filters

You can use the * syntax to apply a filter and select matching items in a collection:

[
  { "name": "apple", "quantity": 1 },
  { "name": "orange", "quantity": 2 },
  { "name": "pear", "quantity": 1 }
]

The filter fruits.*.name returns the array [ "apple", "orange", "pear" ]

Here is another example:

contains(github.event.issue.labels.*.name, 'bug')

will be true if the attribute name of one of the labels of the issue that has triggered the event is 'bug'

Contexts

Contexts are a way to access information about workflow runs, runner environments, jobs, and steps. Contexts use the expression syntax. See Context and expression syntax for GitHub Actions at the GitHub Actions Reference.

${{ <context> }}

Matrix Context

The matrix context enables access to the matrix parameters you configured for the current job.

For example, if you configure a matrix build with the os and node versions, the matrix context object includes the os and node versions of the current job.

GitHub Context

The github context contains information about

You can read most of the github context data in environment variables.

for example, github.refcontains the branch or tag ref that triggered the workflow run

Env Context

The env context contains environment variables that have been set in a workflow, job, or step.

This context changes for each step in a job. You can access this context from any step in a job.

Steps Context

The steps context contains information about the steps in the current job that have already run.

Here is a more complex example using step information and functions

...
- name: save vsix
      uses: actions/upload-artifact@master
      with:
        name: ${ { format('vscode-hugo-{0}-{1}.vsix', steps.build_package.outputs.version, github.sha) }}
        path: ${ { format('vscode-hugo-{0}.vsix', steps.build_package.outputs.version) }}

The Runner Context

The runner context contains information about the runner that is executing the current job.

Examples are runner.os for the Operating System or runner.temp for the path of the temporary directory for the runner. This directory is guaranteed to be empty at the start of each job, even on self-hosted runners.

The Strategy Context

The strategy context enables access to the configured strategy parameters and information about the current job.

Here is an example of use of the strategy context.

The Secrets Context

The secrets context access to secrets set in a repository. See Creating and storing encrypted secrets.

To create a secret:

To use a secret:

steps:
  - name: Hello world action
    with: # Set the secret as an input
      super_secret: ${{ secrets.SuperSecret }}
    env: # Or as an environment variable
      super_secret: ${{ secrets.SuperSecret }}

For example, to write a github action to publish a npm package in the npm registry I surely need to give GitHub a token so that it can work on my name and publish the package. Thus, the procedure will be:

  1. You create a token for npm with read and publish permits:

     [~/.../lexer-generator(master)]$ npm token create
 npm password:
 ┌────────────────┬──────────────────────────────────────┐
 │ token          │ blah-blah-blah-blah-blahblahblah     │
 ├────────────────┼──────────────────────────────────────┤
 │ cidr_whitelist │                                      │
 ├────────────────┼──────────────────────────────────────┤
 │ readonly       │ false                                │
 ├────────────────┼──────────────────────────────────────┤
 │ created        │ 2020-03-30T15:39:01.799Z             │
 │ created        │ 2020-03-30T15:39:01.799Z             │
 └────────────────┴──────────────────────────────────────┘
  2. Set the secret token in the secrets section of your repo with name for example npm_token
  3. Make the secret accesible to the GitHub Action via the secrets context
name: Node.js Package
on:
  release:
    types: [created]
jobs:
  build:
    ...
  publish-npm:
    needs: build
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: actions/setup-node@v1
        with:
          node-version: 12
          registry-url: https://registry.npmjs.org/
      - run: npm ci
      - run: npm publish --access public
        env:
          NODE_AUTH_TOKEN: ${{secrets.npm_token }}

Example printing context information to the log file

To inspect the information that is accessible in each context, you can use this workflow file example.

[~/.../scapegoat(master)]$ cat .github/workflows/debug.yml

name: Debugging contexts
on: push

jobs:
  one:
    runs-on: ubuntu-16.04
    steps:
      - name: Dump GitHub context
        env:
          GITHUB_CONTEXT: ${ { toJson(github) }}
        run: echo "$GITHUB_CONTEXT"
      - name: Dump job context
        env:
          JOB_CONTEXT: ${ { toJson(job) }}
        run: echo "$JOB_CONTEXT"
      - name: Dump steps context
        env:
          STEPS_CONTEXT: ${ { toJson(steps) }}
        run: echo "$STEPS_CONTEXT"
      - name: Dump runner context
        env:
          RUNNER_CONTEXT: ${ { toJson(runner) }}
        run: echo "$RUNNER_CONTEXT"
      - name: Dump strategy context
        env:
          STRATEGY_CONTEXT: ${ { toJson(strategy) }}
        run: echo "$STRATEGY_CONTEXT"
      - name: Dump matrix context
        env:
          MATRIX_CONTEXT: ${ { toJson(matrix) }}
        run: echo "$MATRIX_CONTEXT"

Here is an example of output

GITHUB_TOKEN

GitHub automatically creates a GITHUB_TOKEN secret to use in your workflow. You can use the GITHUB_TOKEN to authenticate in a workflow run.

When you enable GitHub Actions, GitHub installs a GitHub App on your repository.

The GITHUB_TOKEN secret is a GitHub App installation access token.

You can use the installation access token to authenticate on behalf of the GitHub App installed on your repository.

The token’s permissions are limited to the repository that contains your workflow.

For more see Authenticating with the GITHUB_TOKEN

For example, when the repo contains and npm module and we want to write a github action to publish the npm package in the GitHub Package Registry it is enough to use the GITHUB_TOKEN.

Thus, this is enough to do the job:

jobs:
  build:
    ...
  publish-npm:
    ...
  publish-gpr:
    needs: build
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2
      - uses: actions/setup-node@v1
        with:
          node-version: 12
          registry-url: https://npm.pkg.github.com/
          scope: @ULL-ESIT-PL1920
      - run: npm ci
      - run: npm publish
        env:
          NODE_AUTH_TOKEN: ${{secrets.GITHUB_TOKEN }}

Creating a Packaged JavaScript Action

References

Videos about GitHub Actions

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