Building a CLI with a GitHub App

Follow this tutorial to write a CLI in Ruby that generates a user access token for a GitHub App via the device flow.

In this article

Introduction

This tutorial demonstrates how to build a command line interface (CLI) backed by a GitHub App, and how to use the device flow to generate a user access token for the app.

The CLI will have three commands:

  • help: Outputs the usage instructions.
  • login: Generates a user access token that the app can use to make API requests on behalf of the user.
  • whoami: Returns information about the logged in user.

This tutorial uses Ruby, but you can write a CLI and use the device flow to generate a user access token with any programming language.

About device flow and user access tokens

The CLI will use the device flow to authenticate a user and generate a user access token. Then, the CLI can use the user access token to make API requests on behalf of the authenticated user.

Your app should use a user access token if you want to attribute the app's actions to a user. For more information, see Authenticating with a GitHub App on behalf of a user.

There are two ways to generate a user access token for a GitHub App: web application flow and device flow. You should use the device flow to generate a user access token if your app is headless or does not have access to a web interface. For example, CLI tools, simple Raspberry Pis, and desktop applications should use the device flow. If your app has access to a web interface, you should use web application flow instead. For more information, see Generating a user access token for a GitHub App and Building a "Login with GitHub" button with a GitHub App.

Prerequisites

This tutorial assumes that you have already registered a GitHub App. For more information about registering a GitHub App, see Registering a GitHub App.

Before following this tutorial, you must enable device flow for your app. For more information about enabling device flow for your app, see Modifying a GitHub App registration.

This tutorial assumes that you have a basic understanding of Ruby. For more information, see Ruby.

Get the client ID

You will need your app's client ID in order to generate a user access token via the device flow.

  1. In the upper-right corner of any page on GitHub, click your profile picture.

  2. Navigate to your account settings.

    • For an app owned by a personal account, click Settings.
    • For an app owned by an organization:
      1. Click Your organizations.
      2. To the right of the organization, click Settings.

  3. In the left sidebar, click Developer settings.

  4. In the left sidebar, click GitHub Apps.

  5. Next to the GitHub App that you want to work with, click Edit.

  6. On the app's settings page, find the client ID for your app. You will use it later in this tutorial. Note that the client ID is different from the app ID.

Write the CLI

These steps lead you through building a CLI and using device flow to get a user access token. To skip ahead to the final code, see Full code example.

Setup

  1. Create a Ruby file to hold the code that will generate a user access token. This tutorial will name the file app_cli.rb.

  2. In your terminal, from the directory where app_cli.rb is stored, run the following command to make app_cli.rb executable:

    Text
    chmod +x app_cli.rb

  3. Add this line to the top of app_cli.rb to indicate that the Ruby interpreter should be used to run the script:

    Ruby
    #!/usr/bin/env ruby

  4. Add these dependencies to the top of app_cli.rb, following #!/usr/bin/env ruby:

    Ruby
    require "net/http"
require "json"
require "uri"
require "fileutils"

    These are all part of the Ruby standard library, so you don't need to install any gems.

  5. Add the following main function that will serve as an entry point. The function includes a case statement to take different actions depending on which command is specified. You will expand this case statement later.

    Ruby
    def main
  case ARGV[0]
  when "help"
    puts "`help` is not yet defined"
  when "login"
    puts "`login` is not yet defined"
  when "whoami"
    puts "`whoami` is not yet defined"
  else
    puts "Unknown command `#{ARGV[0]}`"
  end
end

  6. At the bottom of the file, add the following line to call the entry point function. This function call should remain at the bottom of your file as you add more functions to this file later in the tutorial.

    Ruby
    main

  7. Optionally, check your progress:

    app_cli.rb now looks like this:

    Ruby
    #!/usr/bin/env ruby

require "net/http"
require "json"
require "uri"
require "fileutils"

def main
  case ARGV[0]
  when "help"
    puts "`help` is not yet defined"
  when "login"
    puts "`login` is not yet defined"
  when "whoami"
    puts "`whoami` is not yet defined"
  else
    puts "Unknown command `#{ARGV[0]}`"
  end
end

main

    In your terminal, from the directory where app_cli.rb is stored, run ./app_cli.rb help. You should see this output:

    `help` is not yet defined

    You can also test your script without a command or with an unhandled command. For example, ./app_cli.rb create-issue should output:

    Unknown command `create-issue`

Add a help command

  1. Add the following help function to app_cli.rb. Currently, the help function prints a line to tell users that this CLI takes one command, "help". You will expand this help function later.

    Ruby
    def help
  puts "usage: app_cli <help>"
end

  2. Update the main function to call the help function when the help command is given:

    Ruby
    def main
  case ARGV[0]
  when "help"
    help
  when "login"
    puts "`login` is not yet defined"
  when "whoami"
    puts "`whoami` is not yet defined"
  else
    puts "Unknown command #{ARGV[0]}"
  end
end

  3. Optionally, check your progress:

    app_cli.rb now looks like this. The order of the functions doesn't matter as long as the main function call is at the end of the file.

    Ruby
    #!/usr/bin/env ruby

require "net/http"
require "json"
require "uri"
require "fileutils"

def help
  puts "usage: app_cli <help>"
end

def main
  case ARGV[0]
  when "help"
    help
  when "login"
    puts "`login` is not yet defined"
  when "whoami"
    puts "`whoami` is not yet defined"
  else
    puts "Unknown command #{ARGV[0]}"
  end
end

main

    In your terminal, from the directory where app_cli.rb is stored, run ./app_cli.rb help. You should see this output:

    usage: app_cli <help>

Add a login command

The login command will run the device flow to get a user access token. For more information, see Generating a user access token for a GitHub App.

  1. Near the top of your file, after the require statements, add the CLIENT_ID of your GitHub App as a constant in app_cli.rb. For more information about finding your app's client ID, see Get the client ID. Replace YOUR_CLIENT_ID with the client ID of your app:

    Ruby
    CLIENT_ID="YOUR_CLIENT_ID"

  2. Add the following parse_response function to app_cli.rb. This function parses a response from the GitHub REST API. When the response status is 200 OK or 201 Created, the function returns the parsed response body. Otherwise, the function prints the response and body and exits the program.

    Ruby
    def parse_response(response)
  case response
  when Net::HTTPOK, Net::HTTPCreated
    JSON.parse(response.body)
  else
    puts response
    puts response.body
    exit 1
  end
end

  3. Add the following request_device_code function to app_cli.rb. This function makes a POST request to https://github.com/login/device/code and returns the response.

    Ruby
    def request_device_code
  uri = URI("https://github.com/login/device/code")
  parameters = URI.encode_www_form("client_id" => CLIENT_ID)
  headers = {"Accept" => "application/json"}

  response = Net::HTTP.post(uri, parameters, headers)
  parse_response(response)
end

  4. Add the following request_token function to app_cli.rb. This function makes a POST request to https://github.com/login/oauth/access_token and returns the response.

    Ruby
    def request_token(device_code)
  uri = URI("https://github.com/login/oauth/access_token")
  parameters = URI.encode_www_form({
    "client_id" => CLIENT_ID,
    "device_code" => device_code,
    "grant_type" => "urn:ietf:params:oauth:grant-type:device_code"
  })
  headers = {"Accept" => "application/json"}
  response = Net::HTTP.post(uri, parameters, headers)
  parse_response(response)
end

  5. Add the following poll_for_token function to app_cli.rb. This function polls https://github.com/login/oauth/access_token at the specified interval until GitHub responds with an access_token parameter instead of an error parameter. Then, it writes the user access token to a file and restricts the permissions on the file.

    Ruby
    def poll_for_token(device_code, interval)

  loop do
    response = request_token(device_code)
    error, access_token = response.values_at("error", "access_token")

    if error
      case error
      when "authorization_pending"
        # The user has not yet entered the code.
        # Wait, then poll again.
        sleep interval
        next
      when "slow_down"
        # The app polled too fast.
        # Wait for the interval plus 5 seconds, then poll again.
        sleep interval + 5
        next
      when "expired_token"
        # The `device_code` expired, and the process needs to restart.
        puts "The device code has expired. Please run `login` again."
        exit 1
      when "access_denied"
        # The user cancelled the process. Stop polling.
        puts "Login cancelled by user."
        exit 1
      else
        puts response
        exit 1
      end
    end

    File.write("./.token", access_token)

    # Set the file permissions so that only the file owner can read or modify the file
    FileUtils.chmod(0600, "./.token")

    break
  end
end

  6. Add the following login function.

    This function:

    1. Calls the request_device_code function and gets the verification_uri, user_code, device_code, and interval parameters from the response.
    2. Prompts users to enter the user_code from the previous step.
    3. Calls the poll_for_token to poll GitHub for an access token.
    4. Lets the user know that authentication was successful.
    Ruby
    def login
  verification_uri, user_code, device_code, interval = request_device_code.values_at("verification_uri", "user_code", "device_code", "interval")

  puts "Please visit: #{verification_uri}"
  puts "and enter code: #{user_code}"

  poll_for_token(device_code, interval)

  puts "Successfully authenticated!"
end

  7. Update the main function to call the login function when the login command is given:

    Ruby
    def main
  case ARGV[0]
  when "help"
    help
  when "login"
    login
  when "whoami"
    puts "`whoami` is not yet defined"
  else
    puts "Unknown command #{ARGV[0]}"
  end
end

  8. Update the help function to include the login command:

    Ruby
    def help
  puts "usage: app_cli <login | help>"
end

  9. Optionally, check your progress:

    app_cli.rb now looks something like this, where YOUR_CLIENT_ID is the client ID of your app. The order of the functions doesn't matter as long as the main function call is at the end of the file.

    Ruby
    #!/usr/bin/env ruby

require "net/http"
require "json"
require "uri"
require "fileutils"

CLIENT_ID="YOUR_CLIENT_ID"

def help
  puts "usage: app_cli <login | help>"
end

def main
  case ARGV[0]
  when "help"
    help
  when "login"
    login
  when "whoami"
    puts "`whoami` is not yet defined"
  else
    puts "Unknown command #{ARGV[0]}"
  end
end

def parse_response(response)
  case response
  when Net::HTTPOK, Net::HTTPCreated
    JSON.parse(response.body)
  else
    puts response
    puts response.body
    exit 1
  end
end

def request_device_code
  uri = URI("https://github.com/login/device/code")
  parameters = URI.encode_www_form("client_id" => CLIENT_ID)
  headers = {"Accept" => "application/json"}

  response = Net::HTTP.post(uri, parameters, headers)
  parse_response(response)
end

def