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0.5.2 • Public • Published

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pxi-args is a command-line arguments parser for pxi (pixie), the small, fast, and magical command-line data processor.

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Installation is done using npm.

$ npm i --save pxi-args

Work in Progress

This project is work in progress. Use it at your own risk!


pxi-args is the command-line argument parser used by pxi.

Defining Command-Line Arguments

The following definitions of answerCmd are identical:

const answerCmd = {key: 'answer', args: ['--answer', '-a'], types: ['number'], only: [42]}
const answerCmd = number('answer', ['--answer', '-a'], {only: [42]})
const questionCmd = string('question', ['--question'])

You may either describe command-line arguments using a plain object or a type function like number or string from the functional DSL.

The definition of answerCmd reads as follows:

answerCmd is a command-line argument that is read to the answer key and is set using either --answer or -a. It must be followed by exactly one number that can only be 42.

The object syntax takes key, args, and types as keys, while the type function syntax takes key and args as the first two arguments and sets types depending on the type function. Additional fields are passed as additional keys in the object syntax or in an object as the third parameter in the type function syntax.


The following command-line argument fields are available:

Field Value Default Description
key string null The command-line argument's value is assigned to a key of this name.
args array of strings [] A list of options that may be used to set the command-line option.
desc string '' Description of the command-line argument for use in the usage text.
only array of values null The command-line argument's value can only be one of the values in this list. If only is null, the value may be set freely.
opts command-line option object null This field is only used if the command-line argument is a command (if types is null).
types ['number'] This command-line argument takes exactly one number.
types ['string'] This command-line argument takes exactly one string.
types ['bool'] This command-line argument takes exactly one boolean, true or false.
types ['number','string'] This command-line argument takes exactly two values, a number and a string.
types [] This command-line argument takes no value. It is a flag that is true if used and false if not used.
types null This command-line argument is a command. It may have its own list of arguments (see opts) and is terminated by either -- or a line ending.

Functional DSL

The following type functions are available to generate command-line arguments:

Function Description
array(types)(key, args, fields) Assigns types, key, and args to fields.
bool(key, args, fields) Assigns types: ['bool'], key and args to fields.
command(key, args, fields) Assigns types: null, key and args to fields.
flag(key, args, fields) Assigns types: [], key and args to fields.
number(key, args, fields) Assigns types: ['number'], key and args to fields.
string(key, args, fields) Assigns types: ['string'], key and args to fields.

Defining Command-Line Options

Command-line options are very similar to command-line arguments. In fact, most command-line parsers use the option function to generate them based on command-line arguments.

The following definitions of answerOpt are identical:

const answerOpt = {
  errs: [],
  args: {
    '--answer': [{key: 'answer', types: ['number'], only: [42]}],
    '-a':       [{key: 'answer', types: ['number'], only: [42]}]
const answerOpt = option(answerCmd)

You may either describe command-line options using a plain object or the option function that takes a command-line argument.

The definition of answerOpt reads as follows:

answerOpt is a command-line option that has no errors and the following arguments: --answer is an argument that is read to the answer key and must be followed by exactly one number that can only be 42. -a is an argument that is read to the answer key and must be followed by exactly one number that can only be 42.

The reason why options are defined redundant is because that makes it easier for parsers to look them up. Command-line arguments exist, because their syntax is easier to parse for usage generators.

Several Command-line options may be combined:

const questionOpt = option(questionCmd)
const combinedOpt = combine(questionOpt, answerOpt)
const combinedOpt = {
  errs: [],
  args: {
    '--question': [{key: 'question', types: ['string']}],
    '--answer':   [{key: 'answer',   types: ['number'], only: [42]}],
    '-a':         [{key: 'answer',   types: ['number'], only: [42]}]

The combinedOpt includes questionOpt as well as answerOpt. If either one has errors, they are gathered in the errs array. Setting errors explicitly does not make much sense. However, option uses errs if e.g. a command-line argument does not set an arg or has an empty args.

In some cases (most cases actually), an answer is not a number but a string.

const answerStrCmd = string('answerStr', ['-a']})
const answerStrOpt = option(answerStrCmd)
const combinedOpt  = combine(answerCmd, answerStrCmd)
const combinedOpt  = {
  errs: [],
  args: {
    '--answer': [
      {key: 'answer',    types: ['number'], only: [42]}
    '-a': [
      {key: 'answer',    types: ['number'], only: [42]}
      {key: 'answerStr', types: ['string']}

Here, a new command-line argument answerStr is defined that has the same argument -a as answer. If options of both command-line arguments are combined, the -a argument is interpreted twice: By answer as a number that can only be 42, as well as by answerStr as a string without restrictions. An option can only be written two several keys, if the number of its arguments match. If combine tries to adds another interpretation to an argument that does not have the same number of keys, an error is recorded instead.

Defining Command-Line Parsers

Command-line parsers combine command-line options with parser plugins to process the command-line string given to a command-line program. The following parser uses combinedOpts:

const deepThought = opts => parser(
const parse = deepThought(combinedOpt)
const result = parse(sliceArgv({argv: process.argv}))

The deepThought parser lets you parse command line input of the following kind:

$ node index.js --question "What is the answer to everything?" -a 42

Which would be equivalent to the following:

const result = {
  errs: [],
  argv: {
    "_": [],
    question: "What is the answer to everything?",
    answer: 42

Parser Plugins

The following parser plugins are available:

Plugin Description
sliceArgv({errs, argv}) Removes the first two elements from a process.argv array, as they are meta data.
splitShortOptions({errs, argv}) Splits argument groups of shape -vs to -v -s. Only works if the arguments are preceded by a single dash.
parseArgs({args})({errs, argv}) Groups arguments together with their values, types, and options if an argument is a command.
mergeArgs(parser)({errs, argv}) Casts values to their types, validates values, and outputs the parsing result in a hierarchical JSON structure.

Note that not every combination of plugins produces a valid parser.

Reporting Issues

Please report issues in the tracker!


We are open to, and grateful for, any contributions made by the community. By contributing to pixie, you agree to abide by the code of conduct. Please read the contributing guide.


pxi-args is MIT licensed.


npm i pxi-args

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