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Advanced Topics

Commands

Yargs provides a powerful set of tools for composing modular command-driven-applications. In this section we cover some of the advanced features available in this API:

Default Commands

To specify a default command use the string * or $0. A default command will be run if the positional arguments provided match no known commands. tldr; default commands allow you to define the entry point to your application using a similar API to subcommands.

const argv = require('yargs')
  .command('$0', 'the default command', () => {}, (argv) => {
    console.log('this command will be run by default')
  })

The command defined above will be executed if the program is run with ./my-cli.js --x=22.

Default commands can also be used as a command alias, like so:

const argv = require('yargs')
  .command(['serve', '$0'], 'the serve command', () => {}, (argv) => {
    console.log('this command will be run by default')
  })

The command defined above will be executed if the program is run with ./my-cli.js --x=22, or with ./my-cli.js serve --x=22.

Positional Arguments

Commands can accept optional and required positional arguments. Required positional arguments take the form <foo>, and optional arguments take the form [bar]. The parsed positional arguments will be populated in argv:

yargs.command('get <source> [proxy]', 'make a get HTTP request')
  .help()
  .argv

Positional Argument Aliases

Aliases can be provided for positional arguments using the | character. As an example, suppose our application allows either a username or an email as the first argument:

yargs.command('get <username|email> [password]', 'fetch a user by username or email.')
  .help()
  .argv

In this way, both argv.username and argv.email would be populated with the same value when the command is executed.

Variadic Positional Arguments

The last positional argument can optionally accept an array of values, by using the .. operator:

yargs.command('download <url> [files..]', 'download several files')
  .help()
  .argv

Describing Positional Arguments

You can use the method .positional() in a command's builder function to describe and configure a positional argument:

yargs.command('get <source> [proxy]', 'make a get HTTP request', (yargs) => {
  yargs.positional('source', {
    describe: 'URL to fetch content from',
    type: 'string',
    default: 'http://www.google.com'
  }).positional('proxy', {
    describe: 'optional proxy URL'
  })
})
.help()
.argv

Command Execution

When a command is given on the command line, yargs will execute the following:

  1. push the command into the current context
  2. reset non-global configuration
  3. apply command configuration via the builder, if given
  4. parse and validate args from the command line, including positional args
  5. if validation succeeds, run the handler function, if given
  6. pop the command from the current context

Command Aliases

You can define aliases for a command by putting the command and all of its aliases into an array.

Alternatively, a command module may specify an aliases property, which may be a string or an array of strings. All aliases defined via the command property and the aliases property will be concatenated together.

The first element in the array is considered the canonical command, which may define positional arguments, and the remaining elements in the array are considered aliases. Aliases inherit positional args from the canonical command, and thus any positional args defined in the aliases themselves are ignored.

If either the canonical command or any of its aliases are given on the command line, the command will be executed.

#!/usr/bin/env node
require('yargs')
  .command(['start [app]', 'run', 'up'], 'Start up an app', {}, (argv) => {
    console.log('starting up the', argv.app || 'default', 'app')
  })
  .command({
    command: 'configure <key> [value]',
    aliases: ['config', 'cfg'],
    desc: 'Set a config variable',
    builder: (yargs) => yargs.default('value', 'true'),
    handler: (argv) => {
      console.log(`setting ${argv.key} to ${argv.value}`)
    }
  })
  .demandCommand()
  .help()
  .wrap(72)
  .argv
$ ./svc.js help
Commands:
  start [app]              Start up an app            [aliases: run, up]
  configure <key> [value]  Set a config variable  [aliases: config, cfg]

Options:
  --help  Show help                                            [boolean]

$ ./svc.js cfg concurrency 4
setting concurrency to 4

$ ./svc.js run web
starting up the web app

Providing a Command Module

For complicated commands you can pull the logic into a module. A module simply needs to export:

  • exports.command: string (or array of strings) that executes this command when given on the command line, first string may contain positional args
  • exports.aliases: array of strings (or a single string) representing aliases of exports.command, positional args defined in an alias are ignored
  • exports.describe: string used as the description for the command in help text, use false for a hidden command
  • exports.builder: object declaring the options the command accepts, or a function accepting and returning a yargs instance
  • exports.handler: a function which will be passed the parsed argv.
// my-module.js
exports.command = 'get <source> [proxy]'

exports.describe = 'make a get HTTP request'

exports.builder = {
  banana: {
    default: 'cool'
  },
  batman: {
    default: 'sad'
  }
}

exports.handler = function (argv) {
  // do something with argv.
}

You then register the module like so:

yargs.command(require('my-module'))
  .help()
  .argv

Or if the module does not export command and describe (or if you just want to override them):

yargs.command('get <source> [proxy]', 'make a get HTTP request', require('my-module'))
  .help()
  .argv

.commandDir(directory, [opts])

Apply command modules from a directory relative to the module calling this method.

This allows you to organize multiple commands into their own modules under a single directory and apply all of them at once instead of calling .command(require('./dir/module')) multiple times.

By default, it ignores subdirectories. This is so you can use a directory structure to represent your command hierarchy, where each command applies its subcommands using this method in its builder function. See the example below.

Note that yargs assumes all modules in the given directory are command modules and will error if non-command modules are encountered. In this scenario, you can either move your module to a different directory or use the exclude or visit option to manually filter it out. More on that below.

directory is a relative directory path as a string (required).

opts is an options object (optional). The following options are valid:

  • recurse: boolean, default false

    Look for command modules in all subdirectories and apply them as a flattened (non-hierarchical) list.

  • extensions: array of strings, default ['js']

    The types of files to look for when requiring command modules.

  • visit: function

    A synchronous function called for each command module encountered. Accepts commandObject, pathToFile, and filename as arguments. Returns commandObject to include the command; any falsy value to exclude/skip it.

  • include: RegExp or function

    Whitelist certain modules. See require-directory whitelisting for details.

  • exclude: RegExp or function

    Blacklist certain modules. See require-directory blacklisting for details.

Example command hierarchy using .commandDir()

Desired CLI:

$ myapp --help
$ myapp init
$ myapp remote --help
$ myapp remote add base http://yargs.js.org
$ myapp remote prune base
$ myapp remote prune base fork whatever

Directory structure:

myapp/
├─ cli.js
└─ cmds/
   ├─ init.js
   ├─ remote.js
   └─ remote_cmds/
      ├─ add.js
      └─ prune.js

cli.js:

#!/usr/bin/env node
require('yargs')
  .commandDir('cmds')
  .demandCommand()
  .help()
  .argv

cmds/init.js:

exports.command = 'init [dir]'
exports.desc = 'Create an empty repo'
exports.builder = {
  dir: {
    default: '.'
  }
}
exports.handler = function (argv) {
  console.log('init called for dir', argv.dir)
}

cmds/remote.js:

exports.command = 'remote <command>'
exports.desc = 'Manage set of tracked repos'
exports.builder = function (yargs) {
  return yargs.commandDir('remote_cmds')
}
exports.handler = function (argv) {}

cmds/remote_cmds/add.js:

exports.command = 'add <name> <url>'
exports.desc = 'Add remote named <name> for repo at url <url>'
exports.builder = {}
exports.handler = function (argv) {
  console.log('adding remote %s at url %s', argv.name, argv.url)
}

cmds/remote_cmds/prune.js:

exports.command = 'prune <name> [names..]'
exports.desc = 'Delete tracked branches gone stale for remotes'
exports.builder = {}
exports.handler = function (argv) {
  console.log('pruning remotes %s', [].concat(argv.name).concat(argv.names).join(', '))
}

Building Configurable CLI Apps

One of the goals of yargs has been to examine practices common in the JavaScript CLI community, and to make it easy to apply these conventions to your own application.

One useful set of conventions that has emerged is around how applications allow users to extend and customize their functionality.

.rc files

It's common for libraries, e.g., Babel, ESLint, to allow you to provide configuration by populating a .rc file.

Yargs' config(), combined with the module find-up, makes it easy to implement .rc functionality:

const findUp = require('find-up')
const fs = require('fs')
const configPath = findUp.sync(['.myapprc', '.myapprc.json'])
const config = configPath ? JSON.parse(fs.readFileSync(configPath)) : {}
const argv = require('yargs')
  .config(config)
  .argv

Providing Configuration in Your package.json

Another common practice is to allow users to provide configuration via a reserved field in the package.json. You can configure nyc or babel, for instance, using the nyc and babel key respectively:

{
  "nyc": {
    "watermarks": {
      "lines": [80, 95],
      "functions": [80, 95],
      "branches": [80, 95],
      "statements": [80, 95]
    }
  }
}

Yargs gives you this functionality using the pkgConf() method:

const argv = require('yargs')
  .pkgConf('nyc')
  .argv

Creating a Plugin Architecture

Both pkgConf() and config() support the extends keyword. extends allows you to inherit configuration from other npm modules, making it possible to build plugin architectures similar to Babel's presets:

{
  "nyc": {
    "extends": "@istanbuljs/nyc-config-babel"
  }
}

Customizing Yargs' Parser

Not everyone always agrees on how process.argv should be interpreted; using the yargs stanza in your package.json you can turn on and off some of yargs' parsing features:

{
  "yargs": {
    "short-option-groups": true,
    "camel-case-expansion": true,
    "dot-notation": true,
    "parse-numbers": true,
    "boolean-negation": true
  }
}

See the yargs-parser module for detailed documentation of this feature.

Middleware

Sometimes you might want to transform arguments before they reach the command handler. For example, perhaps you want to validate that credentials have been provided and otherwise load credentials from a file.

Middleware is simply a stack of functions, each of which is passed the the current parsed arguments, which it can in turn update by adding values, removing values, or overwriting values.

Diagram:

                        --------------         --------------        ---------
stdin ----> argv ----> | Middleware 1 | ----> | Middleware 2 | ---> | Command |
                        --------------         --------------        ---------

Example Credentials Middleware

In this example, our middleware will check if the username and password is provided. If not, it will load them from ~/.credentials, and fill in the argv.username and argv.password values.

Middleware function

const normalizeCredentials = (argv) => {
  if (!argv.username || !argv.password) {
    const credentials = JSON.parse(fs.readSync('~/.credentials'))
    return credentials
  }
  return {}
}

yargs parsing configuration

var argv = require('yargs')
  .usage('Usage: $0 <command> [options]')
  .command('login', 'Authenticate user', (yargs) =>{
        return yargs.option('username')
                    .option('password')
      } ,(argv) => {
        authenticateUser(argv.username, argv.password)
      }, 
      [normalizeCredentials]
     )
  .argv;