yargs

What is yargs?

The yargs npm package is a command-line argument parser that helps in building interactive command line tools, by parsing arguments and generating an elegant user interface. It provides a simple and efficient way to handle command line arguments for Node.js applications.

What are yargs's main functionalities?

Command Parsing

Yargs allows you to define commands and associated options. This feature is useful for CLI applications that perform different actions based on the command provided.

const yargs = require('yargs/yargs')(process.argv.slice(2));
yargs.command('get', 'make a get HTTP request', () => {}, (argv) => {
  console.log(`Request made to URL: ${argv.url}`);
}).argv;

Option Parsing

Yargs can parse options (also known as flags or switches) with additional configuration such as aliases, types, and descriptions.

const yargs = require('yargs/yargs')(process.argv.slice(2));
yargs.option('verbose', {
  alias: 'v',
  type: 'boolean',
  description: 'Run with verbose logging'
}).argv;

Default Values

Yargs allows setting default values for options, which will be used if no value is provided by the user.

const yargs = require('yargs/yargs')(process.argv.slice(2));
yargs.default('port', 8080).argv;

Automatic Help and Version Information

Yargs can automatically generate help and version information for the CLI tool, making it easier for users to understand how to use the application.

const yargs = require('yargs/yargs')(process.argv.slice(2));
yargs.help().version().argv;

Custom Validation

Yargs provides a way to define custom validation rules for the provided arguments, ensuring that the input meets certain criteria before the application proceeds.

const yargs = require('yargs/yargs')(process.argv.slice(2));
yargs.option('port', {
  describe: 'The port to bind on',
  demandOption: true,
  number: true
}).check((argv, options) => {
  if (argv.port < 1024) {
    throw new Error('Port must be at least 1024');
  }
  return true;
}).argv;

Other packages similar to yargs

commander

Commander is another popular npm package for parsing command-line options. It provides a high-level API for defining commands and options, similar to yargs. Commander is known for its simplicity and declarative approach to command-line arguments.

minimist

Minimist is a minimalistic command-line argument parser. It is more lightweight than yargs and focuses on parsing a list of arguments into an object, without the additional features like command handling, help text generation, or validation.

meow

Meow is a CLI helper for creating Node.js command-line apps. It provides a simpler and more opinionated API compared to yargs, with built-in help text, version output, and flag aliasing. Meow is suitable for smaller projects that require less customization.

caporal

Caporal is a full-featured framework for building command-line applications. It offers a rich set of features including argument parsing, validation, autocomplete, and more. Caporal is more framework-like compared to yargs, which might be more suitable for complex CLI tools.

README

yargs

Yargs be a node.js library fer hearties tryin' ter parse optstrings.

With yargs, ye be havin' a map that leads straight to yer treasure! Treasure of course, being a simple option hash.

Yargs is the official successor to optimist. Please feel free to submit issues and pull requests. If you'd like to contribute and don't know where to start, have a look at the issue list :)

examples

With yargs, the options be just a hash!

plunder.js:

#!/usr/bin/env node
var argv = require('yargs').argv;

if (argv.ships > 3 && argv.distance < 53.5) {
    console.log('Plunder more riffiwobbles!');
}
else {
    console.log('Retreat from the xupptumblers!');
}

---

$ ./plunder.js --ships=4 --distance=22
Plunder more riffiwobbles!

$ ./plunder.js --ships 12 --distance 98.7
Retreat from the xupptumblers!

But don't walk the plank just yet! There be more! You can do short options:

short.js:

#!/usr/bin/env node
var argv = require('yargs').argv;
console.log('(%d,%d)', argv.x, argv.y);

---

$ ./short.js -x 10 -y 21
(10,21)

And booleans, both long, short, and even grouped:

bool.js:

#!/usr/bin/env node
var util = require('util');
var argv = require('yargs').argv;

if (argv.s) {
    process.stdout.write(argv.fr ? 'Le perroquet dit: ' : 'The parrot says: ');
}
console.log(
    (argv.fr ? 'couac' : 'squawk') + (argv.p ? '!' : '')
);

---

$ ./bool.js -s
The parrot says: squawk

$ ./bool.js -sp
The parrot says: squawk!

$ ./bool.js -sp --fr
Le perroquet dit: couac!

And non-hyphenated options too! Just use argv._!

nonopt.js:

#!/usr/bin/env node
var argv = require('yargs').argv;
console.log('(%d,%d)', argv.x, argv.y);
console.log(argv._);

---

$ ./nonopt.js -x 6.82 -y 3.35 rum
(6.82,3.35)
[ 'rum' ]

$ ./nonopt.js "me hearties" -x 0.54 yo -y 1.12 ho
(0.54,1.12)
[ 'me hearties', 'yo', 'ho' ]

Yargs even counts your booleans!

count.js:

#!/usr/bin/env node
var argv = require('yargs')
    .count('verbose')
    .alias('v', 'verbose')
    .argv;

VERBOSE_LEVEL = argv.verbose;

function WARN()  { VERBOSE_LEVEL >= 0 && console.log.apply(console, arguments); }
function INFO()  { VERBOSE_LEVEL >= 1 && console.log.apply(console, arguments); }
function DEBUG() { VERBOSE_LEVEL >= 2 && console.log.apply(console, arguments); }

WARN("Showing only important stuff");
INFO("Showing semi-important stuff too");
DEBUG("Extra chatty mode");

---

$ node count.js
Showing only important stuff

$ node count.js -v
Showing only important stuff
Showing semi-important stuff too

$ node count.js -vv
Showing only important stuff
Showing semi-important stuff too
Extra chatty mode

$ node count.js -v --verbose
Showing only important stuff
Showing semi-important stuff too
Extra chatty mode

Tell users how to use yer options and make demands.

area.js:

#!/usr/bin/env node
var argv = require('yargs')
    .usage('Usage: $0 -w [num] -h [num]')
    .demand(['w','h'])
    .argv;

console.log("The area is:", argv.w * argv.h);

---

$ ./area.js -w 55 -h 11
The area is: 605

$ node ./area.js -w 4.91 -w 2.51
Usage: area.js -w [num] -h [num]

Options:
  -w  [required]
  -h  [required]

Missing required arguments: h

After yer demands have been met, demand more! Ask for non-hypenated arguments!

demand_count.js:

#!/usr/bin/env node
var argv = require('yargs')
    .demand(2)
    .argv;
console.dir(argv);

---

$ ./demand_count.js a

Not enough non-option arguments: got 1, need at least 2

$ ./demand_count.js a b
{ _: [ 'a', 'b' ], '$0': 'demand_count.js' }

$ ./demand_count.js a b c
{ _: [ 'a', 'b', 'c' ], '$0': 'demand_count.js' }

EVEN MORE SHIVER ME TIMBERS!

default_singles.js:

#!/usr/bin/env node
var argv = require('yargs')
    .default('x', 10)
    .default('y', 10)
    .argv
;
console.log(argv.x + argv.y);

---

$ ./default_singles.js -x 5
15

default_hash.js:

#!/usr/bin/env node
var argv = require('yargs')
    .default({ x : 10, y : 10 })
    .argv
;
console.log(argv.x + argv.y);

---

$ ./default_hash.js -y 7
17

And if you really want to get all descriptive about it...

boolean_single.js:

#!/usr/bin/env node
var argv = require('yargs')
    .boolean('v')
    .argv
;
console.dir(argv.v);
console.dir(argv._);

---

$ ./boolean_single.js -v "me hearties" yo ho
true
[ 'me hearties', 'yo', 'ho' ]

boolean_double.js:

#!/usr/bin/env node
var argv = require('yargs')
    .boolean(['x','y','z'])
    .argv
;
console.dir([ argv.x, argv.y, argv.z ]);
console.dir(argv._);

---

$ ./boolean_double.js -x -z one two three
[ true, false, true ]
[ 'one', 'two', 'three' ]

Yargs is here to help you...

Ye can describe parameters fer help messages and set aliases. Yargs figures out how ter format a handy help string automatically.

line_count.js:

#!/usr/bin/env node
var argv = require('yargs')
    .usage('Usage: $0 <command> [options]')
    .command('count', 'Count the lines in a file')
    .demand(1)
    .example('$0 count -f foo.js', 'count the lines in the given file')
    .demand('f')
    .alias('f', 'file')
    .nargs('f', 1)
    .describe('f', 'Load a file')
    .help('h')
    .alias('h', 'help')
    .epilog('copyright 2015')
    .argv;

var fs = require('fs');
var s = fs.createReadStream(argv.file);

var lines = 0;
s.on('data', function (buf) {
    lines += buf.toString().match(/\n/g).length;
});

s.on('end', function () {
    console.log(lines);
});

---

$ node line_count.js count
Usage: line_count.js <command> [options]

Commands:
  count    Count the lines in a file

Options:
  -f, --file  Load a file        [required]
  -h, --help  Show help           [boolean]

Examples:
  line_count.js count -f foo.js  count the lines in the given file

copyright 2015

Missing required arguments: f

$ node line_count.js count --file line_count.js
26

$ node line_count.js count -f line_count.js
26

methods

By itself,

require('yargs').argv

will use the process.argv array to construct the argv object.

You can pass in the process.argv yourself:

require('yargs')([ '-x', '1', '-y', '2' ]).argv

or use .parse() to do the same thing:

require('yargs').parse([ '-x', '1', '-y', '2' ])

The rest of these methods below come in just before the terminating .argv.

.alias(key, alias)

Set key names as equivalent such that updates to a key will propagate to aliases and vice-versa.

Optionally .alias() can take an object that maps keys to aliases. Each key of this object should be the canonical version of the option, and each value should be a string or an array of strings.

.argv

Get the arguments as a plain old object.

Arguments without a corresponding flag show up in the argv._ array.

The script name or node command is available at argv.$0 similarly to how $0 works in bash or perl.

If yargs is executed in an environment that embeds node and there's no script name (e.g. [Electron] (http://electron.atom.io/) or nw.js), it will ignore the first parameter since it expects it to be the script name. In order to override this behavior, use .parse(process.argv.slice(1)) instead of .argv and the first parameter won't be ignored.

.array(key)

Tell the parser to interpret key as an array. If .array('foo') is set, --foo foo bar will be parsed as ['foo', 'bar'] rather than as 'foo'.

.boolean(key)

Interpret key as a boolean. If a non-flag option follows key in process.argv, that string won't get set as the value of key.

key will default to false, unless a default(key, undefined) is explicitly set.

If key is an array, interpret all the elements as booleans.

.check(fn)

Check that certain conditions are met in the provided arguments.

fn is called with two arguments, the parsed argv hash and an array of options and their aliases.

If fn throws or returns a non-truthy value, show the thrown error, usage information, and exit.

.choices(key, choices)

Limit valid values for key to a predefined set of choices, given as an array or as an individual value.

var argv = require('yargs')
  .alias('i', 'ingredient')
  .describe('i', 'choose your sandwich ingredients')
  .choices('i', ['peanut-butter', 'jelly', 'banana', 'pickles'])
  .help('help')
  .argv

If this method is called multiple times, all enumerated values will be merged together. Choices are generally strings or numbers, and value matching is case-sensitive.

Optionally .choices() can take an object that maps multiple keys to their choices.

Choices can also be specified as choices in the object given to option().

var argv = require('yargs')
  .option('size', {
    alias: 's',
    describe: 'choose a size',
    choices: ['xs', 's', 'm', 'l', 'xl']
  })
  .argv

.command(cmd, desc, [fn])

Document the commands exposed by your application.

Use desc to provide a description for each command your application accepts (the values stored in argv._). Set desc to false to create a hidden command. Hidden commands don't show up in the help output and aren't available for completion.

Optionally, you can provide a handler fn which will be executed when a given command is provided. The handler will be executed with an instance of yargs, which can be used to compose nested commands.

Here's an example of top-level and nested commands in action:

var argv = require('yargs')
  .usage('npm <command>')
  .command('install', 'tis a mighty fine package to install')
  .command('publish', 'shiver me timbers, should you be sharing all that', function (yargs) {
    argv = yargs.option('f', {
      alias: 'force',
      description: 'yar, it usually be a bad idea'
    })
    .help('help')
    .argv
  })
  .help('help')
  .argv;

.completion(cmd, [description], [fn]);

Enable bash-completion shortcuts for commands and options.

cmd: When present in argv._, will result in the .bashrc completion script being outputted. To enable bash completions, concat the generated script to your .bashrc or .bash_profile.

description: Provide a description in your usage instructions for the command that generates bash completion scripts.

fn: Rather than relying on yargs' default completion functionality, which shiver me timbers is pretty awesome, you can provide your own completion method.

var argv = require('yargs')
  .completion('completion', function(current, argv) {
    // 'current' is the current command being completed.
    // 'argv' is the parsed arguments so far.
    // simply return an array of completions.
    return [
      'foo',
      'bar'
    ];
  })
  .argv;

But wait, there's more! You can provide asynchronous completions.

var argv = require('yargs')
  .completion('completion', function(current, argv, done) {
    setTimeout(function() {
      done([
        'apple',
        'banana'
      ]);
    }, 500);
  })
  .argv;

.config(key, [description])

Tells the parser that if the option specified by key is passed in, it should be interpreted as a path to a JSON config file. The file is loaded and parsed, and its properties are set as arguments. If present, the description parameter customizes the description of the config (key) option in the usage string.

.default(key, value, [description])

Set argv[key] to value if no option was specified in process.argv.

Optionally .default() can take an object that maps keys to default values.

But wait, there's more! The default value can be a function which returns a value. The name of the function will be used in the usage string:

var argv = require('yargs')
  .default('random', function randomValue() {
    return Math.random() * 256;
  }).argv;

Optionally, description can also be provided and will take precedence over displaying the value in the usage instructions:

.default('timeout', 60000, '(one-minute)')

.demand(key, [msg | boolean])

.demand(count, [max], [msg])

If key is a string, show the usage information and exit if key wasn't specified in process.argv.

If key is a number, demand at least as many non-option arguments, which show up in argv._. A second number can also optionally be provided, which indicates the maximum number of non-option arguments.

If key is an array, demand each element.

If a msg string is given, it will be printed when the argument is missing, instead of the standard error message. This is especially helpful for the non-option arguments in argv._.

If a boolean value is given, it controls whether the option is demanded; this is useful when using .options() to specify command line parameters.

.describe(key, desc)

Describe a key for the generated usage information.

Optionally .describe() can take an object that maps keys to descriptions.

.epilog(str)

.epilogue(str)

A message to print at the end of the usage instructions, e.g.

var argv = require('yargs')
  .epilogue('for more information, find our manual at http://example.com');

.example(cmd, desc)

Give some example invocations of your program. Inside cmd, the string $0 will get interpolated to the current script name or node command for the present script similar to how $0 works in bash or perl. Examples will be printed out as part of the help message.

.exitProcess(enable)

By default, yargs exits the process when the user passes a help flag, uses the .version functionality, or when validation fails. Calling .exitProcess(false) disables this behavior, enabling further actions after yargs have been validated.

.fail(fn)

Method to execute when a failure occurs, rather than printing the failure message.

fn is called with the failure message that would have been printed.

.help([option, [description]])

Add an option (e.g. --help) that displays the usage string and exits the process. If present, the description parameter customizes the description of the help option in the usage string.

If invoked without parameters, .help() returns the generated usage string.

Example:

var yargs = require("yargs")
  .usage("$0 -operand1 number -operand2 number -operation [add|subtract]");
console.log(yargs.help());

Later on, argv can be retrieved with yargs.argv.

.implies(x, y)

Given the key x is set, it is required that the key y is set.

Optionally .implies() can accept an object specifying multiple implications.

.locale(locale)

Set a locale other than the default en locale:

var argv = require('yargs')
  .usage('./$0 - follow ye instructions true')
  .option('option', {
    alias: 'o',
    describe: "'tis a mighty fine option",
    demand: true
  })
  .command('run', "Arrr, ya best be knowin' what yer doin'")
  .example('$0 run foo', "shiver me timbers, here's an example for ye")
  .help('help')
  .wrap(70)
  .locale('pirate')
  .argv

---

./test.js - follow ye instructions true

Choose yer command:
  run  Arrr, ya best be knowin' what yer doin'

Options for me hearties!
  --option, -o  'tis a mighty fine option               [requi-yar-ed]
  --help        Parlay this here code of conduct             [boolean]

Ex. marks the spot:
  test.js run foo  shiver me timbers, here's an example for ye

Ye be havin' to set the followin' argument land lubber: option

Locales currently supported:

• en: American English.
• es: Spanish.
• fr: French.
• pirate: American Pirate.

To submit a new translation for yargs:

1. use ./locales/en.json as a starting point.
2. submit a pull request with the new locale file.

.nargs(key, count)

The number of arguments that should be consumed after a key. This can be a useful hint to prevent parsing ambiguity. For example:

var argv = require('yargs')
  .nargs('token', 1)
  .parse(['--token', '-my-token']);

parses as:

{ _: [], token: '-my-token', '$0': 'node test' }

Optionally .nargs() can take an object of key/narg pairs.

.option(key, opt)

.options(key, opt)

Instead of chaining together .alias().demand().default().describe().string(), you can specify keys in opt for each of the chainable methods.

For example:

var argv = require('yargs')
    .option('f', {
        alias: 'file',
        demand: true,
        default: '/etc/passwd',
        describe: 'x marks the spot',
        type: 'string'
    })
    .argv
;

is the same as

var argv = require('yargs')
    .alias('f', 'file')
    .demand('f')
    .default('f', '/etc/passwd')
    .describe('f', 'x marks the spot')
    .string('f')
    .argv
;

Optionally .options() can take an object that maps keys to opt parameters.

var argv = require('yargs')
    .options({
      'f': {
        alias: 'file',
        demand: true,
        default: '/etc/passwd',
        describe: 'x marks the spot',
        type: 'string'
      }
    })
    .argv
;

.parse(args)

Parse args instead of process.argv. Returns the argv object.

.require(key, [msg | boolean])

.required(key, [msg | boolean])

An alias for demand(). See docs there.

.requiresArg(key)

Specifies either a single option key (string), or an array of options that must be followed by option values. If any option value is missing, show the usage information and exit.

The default behavior is to set the value of any key not followed by an option value to true.

.reset()

Reset the argument object built up so far. This is useful for creating nested command line interfaces.

var yargs = require('yargs')
  .usage('$0 command')
  .command('hello', 'hello command')
  .command('world', 'world command')
  .demand(1, 'must provide a valid command'),
  argv = yargs.argv,
  command = argv._[0];

if (command === 'hello') {
  yargs.reset()
    .usage('$0 hello')
    .help('h')
    .example('$0 hello', 'print the hello message!')
    .argv

  console.log('hello!');
} else if (command === 'world'){
  yargs.reset()
    .usage('$0 world')
    .help('h')
    .example('$0 world', 'print the world message!')
    .argv

  console.log('world!');
} else {
  yargs.showHelp();
}

.showCompletionScript()

Generate a bash completion script. Users of your application can install this script in their .bashrc, and yargs will provide completion shortcuts for commands and options.

.showHelp(consoleLevel='error')

Print the usage data using the console function consoleLevel for printing.

Example:

var yargs = require("yargs")
  .usage("$0 -operand1 number -operand2 number -operation [add|subtract]");
yargs.showHelp(); //prints to stderr using console.error()

Or, to print the usage data to stdout instead, you can specify the use of console.log:

yargs.showHelp("log"); //prints to stdout using console.log()

Later on, argv can be retrieved with yargs.argv.

.showHelpOnFail(enable, [message])

By default, yargs outputs a usage string if any error is detected. Use the .showHelpOnFail() method to customize this behavior. If enable is false, the usage string is not output. If the message parameter is present, this message is output after the error message.

line_count.js:

#!/usr/bin/env node
var argv = require('yargs')
    .usage('Count the lines in a file.\nUsage: $0 -f <file>')
    .demand('f')
    .alias('f', 'file')
    .describe('f', 'Load a file')
    .string('f')
    .showHelpOnFail(false, 'Specify --help for available options')
    .help('help')
    .argv;

// etc.

---

$ node line_count.js
Missing argument value: f

Specify --help for available options

.strict()

Any command-line argument given that is not demanded, or does not have a corresponding description, will be reported as an error.

.string(key)

Tell the parser logic not to interpret key as a number or boolean. This can be useful if you need to preserve leading zeros in an input.

If key is an array, interpret all the elements as strings.

.string('_') will result in non-hyphenated arguments being interpreted as strings, regardless of whether they resemble numbers.

.updateLocale(obj)

.updateStrings(obj)

Override the default strings used by yargs with the key/value pairs provided in obj:

var argv = require('yargs')
  .command('run', 'the run command')
  .help('help')
  .updateStrings({
    'Commands:': 'My Commands -->\n'
  })
  .wrap(null)
  .argv

---

My Commands -->

  run  the run command

Options:
  --help  Show help  [boolean]

If you explicitly specify a locale(), you should do so before calling updateStrings().

.usage(message, [opts])

Set a usage message to show which commands to use. Inside message, the string $0 will get interpolated to the current script name or node command for the present script similar to how $0 works in bash or perl.

opts is optional and acts like calling .options(opts).

.version(version, [option], [description])

Add an option (e.g. --version) that displays the version number (given by the version parameter) and exits the process. If present, the description parameter customizes the description of the version option in the usage string.

You can provide a function for version, rather than a string. This is useful if you want to use the version from your package.json:

var argv = require('yargs')
  .version(function() {
    return require('../package').version;
  })
  .argv;

.wrap(columns)

Format usage output to wrap at columns many columns.

By default wrap will be set to Math.min(80, windowWidth). Use .wrap(null) to specify no column limit (no right-align). Use .wrap(yargs.terminalWidth()) to maximize the width of yargs' usage instructions.

parsing tricks

stop parsing

Use -- to stop parsing flags and stuff the remainder into argv._.

$ node examples/reflect.js -a 1 -b 2 -- -c 3 -d 4
{ _: [ '-c', '3', '-d', '4' ],
  a: 1,
  b: 2,
  '$0': 'examples/reflect.js' }

negate fields

If you want to explicity set a field to false instead of just leaving it undefined or to override a default you can do --no-key.

$ node examples/reflect.js -a --no-b
{ _: [], a: true, b: false, '$0': 'examples/reflect.js' }

numbers

Every argument that looks like a number (!isNaN(Number(arg))) is converted to one. This way you can just net.createConnection(argv.port) and you can add numbers out of argv with + without having that mean concatenation, which is super frustrating.

duplicates

If you specify a flag multiple times it will get turned into an array containing all the values in order.

$ node examples/reflect.js -x 5 -x 8 -x 0
{ _: [], x: [ 5, 8, 0 ], '$0': 'examples/reflect.js' }

dot notation

When you use dots (.s) in argument names, an implicit object path is assumed. This lets you organize arguments into nested objects.

$ node examples/reflect.js --foo.bar.baz=33 --foo.quux=5
{ _: [],
  foo: { bar: { baz: 33 }, quux: 5 },
  '$0': 'examples/reflect.js' }

short numbers

Short numeric -n5 style arguments work too:

$ node examples/reflect.js -n123 -m456
{ _: [], n: 123, m: 456, '$0': 'examples/reflect.js' }

installation

With npm, just do:

npm install yargs

or clone this project on github:

git clone http://github.com/bcoe/yargs.git

To run the tests with npm, just do:

npm test

inspired by

This module is loosely inspired by Perl's Getopt::Casual.