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 is a node.js library fer option parsin' fer hearties who hate option parsin'. This here module is for hearties who like all the --bells and -whistlz of program usage but think optstrings are a waste of yer time.

With yargs, option parsing doesn't have to suck (as much).

examples

With yargs, the options are just a hash! No optstrings attached.

xup.js:

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

if (argv.rif - 5 * argv.xup > 7.138) {
    console.log('Plunder more riffiwobbles!');
}
else {
    console.log('Drop the xupptumblers!');
}

---

$ ./xup.js --rif=55 --xup=9.52
Plunder more riffiwobbles!

$ ./xup.js --rif 12 --xup 8.1
Drop the xupptumblers!

But wait! Ther's more! Ye 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 and short (and grouped):

bool.js:

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

if (argv.s) {
    util.print(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 yer gold doubooleans!

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-mportant 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

Plus, yargs comes with .usage() and .demand()!

divide.js:

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

console.log(argv.x / argv.y);

---

$ ./divide.js -x 55 -y 11
5

$ node ./divide.js -x 4.91 -z 2.51
Usage: node ./divide.js -x [num] -y [num]

Options:
  -x  [required]
  -y  [required]

Missing required arguments: y

Then demand some non-hypenated arguments!

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

---

$ ./demand_count.js a
Not enough arguments, expected 2, but only found 1
$ ./demand_count.js a b
{ _: [ 'a', 'b' ], '$0': 'node ./demand_count.js' }
$ ./demand_count.js a b c
{ _: [ 'a', 'b', 'c' ], '$0': 'node ./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 ye really want ter get all descriptive abouts 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 ter help ye...

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('Count the lines in a file.\nUsage: $0')
    .example('$0 -f', 'count the lines in the given file')
    .demand('f')
    .alias('f', 'file')
    .describe('f', 'Load a file')
    .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 the lines in a file.
Usage: node ./line_count.js

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

Options:
  -f, --file  Load a file  [required]

Missing required arguments: f

$ node line_count.js --file line_count.js 
20

$ node line_count.js -f line_count.js 
20

methods

By itself,

require('yargs').argv

will use 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.

.default(key, value)

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

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

.demand(key, [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._.

If key is an Array, demand each element.

If msg is supplied, 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._.

.describe(key, desc)

Describe a key for the generated usage information.

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

.options(key, opt)

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

For example:

var argv = require('yargs')
    .options('f', {
        alias : 'file',
        default : '/etc/passwd',
    })
    .argv
;

is the same as

var argv = require('yargs')
    .alias('f', 'file')
    .default('f', '/etc/passwd')
    .argv
;

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

.usage(message)

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.

.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.

.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 false, show the thrown error, usage information, and exit.

.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.

If key never shows up as a flag in process.arguments, argv[key] will be false.

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

.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.

.config(key)

Tells the parser to interpret key as a path to a JSON config file. The file is loaded and parsed, and its properties are set as arguments.

.wrap(columns)

Format usage output to wrap at columns many columns.

.help()

Return 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 retrived with yargs.argv

.showHelp(fn=console.error)

Print the usage data using fn for printing.

Example:

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

Later on, argv can be retrived with yargs.argv

.parse(args)

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

.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.

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' ],
  '$0': 'node ./examples/reflect.js',
  a: 1,
  b: 2 }

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
{ _: [],
  '$0': 'node ./examples/reflect.js',
  a: true,
  b: false }

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

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

short numbers

Short numeric head -n5 style argument work too:

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

installation

With npm, just do:

npm install yargs

or clone this project on github:

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

To run the tests with expresso, just do:

expresso

inspired By

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