tutorial - Documentation for Ruby 4.0
Description: tutorial: Tutorial === Why +OptionParser+? When a Ruby program executes, it captures its command-line arguments and options into variable ARGV. ruby argv.
Keywords: ruby,documentation,tutorial

🔍

No recent searches

## Tutorial

### Why `OptionParser`?

When a Ruby program executes, it captures its command-line arguments and options into variable ARGV. This simple program just prints its `ARGV`:

    p ARGV

Execution, with arguments and options:

    $ ruby argv.rb foo --bar --baz bat bam
    ["foo", "--bar", "--baz", "bat", "bam"]

The executing program is responsible for parsing and handling the command-line options.

`OptionParser` offers methods for parsing and handling those options.

With `OptionParser`, you can define options so that for each option:

- The code that defines the option and code that handles that option are in the same place.

- The option may take no argument, a required argument, or an optional argument.

- The argument may be automatically converted to a specified class.

- The argument may be restricted to specified forms.

- The argument may be restricted to specified values.

The class also has method help, which displays automatically-generated help text.

### Contents

- To Begin With

- Defining Options

- Option Names

  - Short Option Names

  - Long Option Names

  - Mixing Option Names

  - Option Name Abbreviations

- Option Arguments

  - Option with No Argument

  - Option with Required Argument

  - Option with Optional Argument

  - Argument Abbreviations

- Argument Values

  - Explicit Argument Values

    - Explicit Values in Array

    - Explicit Values in Hash

  - Argument Value Patterns

- Keyword Argument into

  - Collecting Options

  - Checking for Missing Options

  - Default Values for Options

- Argument Converters

- Help

- Top List and Base List

- Methods for Defining Options

- Parsing

  - Method parse!

  - Method parse

  - Method order!

  - Method order

  - Method permute!

  - Method permute

### To Begin With

To use `OptionParser`:

1. Require the `OptionParser` code.

2. Create an `OptionParser` object.

3. Define one or more options.

4. Parse the command line.

`File` `basic.rb` defines three options, `-x`, `-y`, and `-z`, each with a descriptive string, and each with a block.

    # Require the OptionParser code.
    require 'optparse'
    # Create an OptionParser object.
    parser = OptionParser.new
    # Define one or more options.
    parser.on('-x', 'Whether to X') do |value|
      p ['x', value]
    end
    parser.on('-y', 'Whether to Y') do |value|
      p ['y', value]
    end
    parser.on('-z', 'Whether to Z') do |value|
      p ['z', value]
    end
    # Parse the command line and return pared-down ARGV.
    p parser.parse!

From these defined options, the parser automatically builds help text:

    $ ruby basic.rb --help
    Usage: basic [options]
        -x                               Whether to X
        -y                               Whether to Y
        -z                               Whether to Z

When an option is found during parsing, the block defined for the option is called with the argument value. An invalid option raises an exception.

Method parse!, which is used most often in this tutorial, removes from `ARGV` the options and arguments it finds, leaving other non-option arguments for the program to handle on its own. The method returns the possibly-reduced `ARGV` array.

Executions:

    $ ruby basic.rb -x -z
    ["x", true]
    ["z", true]
    []
    $ ruby basic.rb -z -y -x
    ["z", true]
    ["y", true]
    ["x", true]
    []
    $ ruby basic.rb -x input_file.txt output_file.txt
    ["x", true]
    ["input_file.txt", "output_file.txt"]
    $ ruby basic.rb -a
    basic.rb:16:in '<main>': invalid option: -a (OptionParser::InvalidOption)

### Defining Options

A common way to define an option in `OptionParser` is with instance method `OptionParser#on`.

The method may be called with any number of arguments (whose order does not matter), and may also have a trailing optional keyword argument `into`.

The given arguments determine the characteristics of the new option. These may include:

- One or more short option names.

- One or more long option names.

- Whether the option takes no argument, an optional argument, or a required argument.

- Acceptable forms for the argument.

- Acceptable values for the argument.

- A proc or method to be called when the parser encounters the option.

- `String` descriptions for the option.

### Option Names

You can give an option one or more names of two types:

- Short (1-character) name, beginning with one hyphen (`-`).

- Long (multi-character) name, beginning with two hyphens (`--`).

#### Short Option Names

A short option name consists of a hyphen and a single character.

`File` `short_names.rb` defines an option with a short name, `-x`, and an option with two short names (aliases, in effect) `-y` and `-z`.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-x', 'Short name') do |value|
      p ['x', value]
    end
    parser.on('-1', '-%', 'Two short names') do |value|
      p ['-1 or -%', value]
    end
    parser.parse!

Executions:

    $ ruby short_names.rb --help
    Usage: short_names [options]
        -x                               Short name
        -1, -%                           Two short names
    $ ruby short_names.rb -x
    ["x", true]
    $ ruby short_names.rb -1
    ["-1 or -%", true]
    $ ruby short_names.rb -%
    ["-1 or -%", true]

Multiple short names can “share” a hyphen:

    $ ruby short_names.rb -x1%
    ["x", true]
    ["-1 or -%", true]
    ["-1 or -%", true]

#### Long Option Names

A long option name consists of two hyphens and a one or more characters (usually two or more characters).

`File` `long_names.rb` defines an option with a long name, `--xxx`, and an option with two long names (aliases, in effect) `--y1%` and `--z2#`.

    require 'optparse'
    parser = OptionParser.new
    parser.on('--xxx', 'Long name') do |value|
      p ['-xxx', value]
    end
    parser.on('--y1%', '--z2#', "Two long names") do |value|
      p ['--y1% or --z2#', value]
    end
    parser.parse!

Executions:

    $ ruby long_names.rb --help
    Usage: long_names [options]
            --xxx                        Long name
            --y1%, --z2#                 Two long names
    $ ruby long_names.rb --xxx
    ["-xxx", true]
    $ ruby long_names.rb --y1%
    ["--y1% or --z2#", true]
    $ ruby long_names.rb --z2#
    ["--y1% or --z2#", true]

A long name may be defined with both positive and negative senses.

`File` `long_with_negation.rb` defines an option that has both senses.

    require 'optparse'
    parser = OptionParser.new
    parser.on('--[no-]binary', 'Long name with negation') do |value|
      p [value, value.class]
    end
    parser.parse!

Executions:

    $ ruby long_with_negation.rb --help
    Usage: long_with_negation [options]
            --[no-]binary                Long name with negation
    $ ruby long_with_negation.rb --binary
    [true, TrueClass]
    $ ruby long_with_negation.rb --no-binary
    [false, FalseClass]

#### Mixing Option Names

Many developers like to mix short and long option names, so that a short name is in effect an abbreviation of a long name.

`File` `mixed_names.rb` defines options that each have both a short and a long name.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-x', '--xxx', 'Short and long, no argument') do |value|
      p ['--xxx', value]
    end
    parser.on('-yYYY', '--yyy', 'Short and long, required argument') do |value|
      p ['--yyy', value]
    end
    parser.on('-z [ZZZ]', '--zzz', 'Short and long, optional argument') do |value|
      p ['--zzz', value]
    end
    parser.parse!

Executions:

    $ ruby mixed_names.rb --help
    Usage: mixed_names [options]
        -x, --xxx                        Short and long, no argument
        -y, --yyyYYY                     Short and long, required argument
        -z, --zzz [ZZZ]                  Short and long, optional argument
    $ ruby mixed_names.rb -x
    ["--xxx", true]
    $ ruby mixed_names.rb --xxx
    ["--xxx", true]
    $ ruby mixed_names.rb -y
    mixed_names.rb:12:in '<main>': missing argument: -y (OptionParser::MissingArgument)
    $ ruby mixed_names.rb -y FOO
    ["--yyy", "FOO"]
    $ ruby mixed_names.rb --yyy
    mixed_names.rb:12:in '<main>': missing argument: --yyy (OptionParser::MissingArgument)
    $ ruby mixed_names.rb --yyy BAR
    ["--yyy", "BAR"]
    $ ruby mixed_names.rb -z
    ["--zzz", nil]
    $ ruby mixed_names.rb -z BAZ
    ["--zzz", "BAZ"]
    $ ruby mixed_names.rb --zzz
    ["--zzz", nil]
    $ ruby mixed_names.rb --zzz BAT
    ["--zzz", "BAT"]

#### Option Name Abbreviations

By default, abbreviated option names on the command-line are allowed. An abbreviated name is valid if it is unique among abbreviated option names.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-n', '--dry-run',) do |value|
      p ['--dry-run', value]
    end
    parser.on('-d', '--draft',) do |value|
      p ['--draft', value]
    end
    parser.parse!

Executions:

    $ ruby name_abbrev.rb --help
    Usage: name_abbrev [options]
        -n, --dry-run
        -d, --draft
    $ ruby name_abbrev.rb -n
    ["--dry-run", true]
    $ ruby name_abbrev.rb --dry-run
    ["--dry-run", true]
    $ ruby name_abbrev.rb -d
    ["--draft", true]
    $ ruby name_abbrev.rb --draft
    ["--draft", true]
    $ ruby name_abbrev.rb --d
    name_abbrev.rb:9:in '<main>': ambiguous option: --d (OptionParser::AmbiguousOption)
    $ ruby name_abbrev.rb --dr
    name_abbrev.rb:9:in '<main>': ambiguous option: --dr (OptionParser::AmbiguousOption)
    $ ruby name_abbrev.rb --dry
    ["--dry-run", true]
    $ ruby name_abbrev.rb --dra
    ["--draft", true]

You can disable abbreviation using method `require_exact`.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-n', '--dry-run',) do |value|
      p ['--dry-run', value]
    end
    parser.on('-d', '--draft',) do |value|
      p ['--draft', value]
    end
    parser.require_exact = true
    parser.parse!

Executions:

    $ ruby no_abbreviation.rb --dry-ru
    no_abbreviation.rb:10:in '<main>': invalid option: --dry-ru (OptionParser::InvalidOption)
    $ ruby no_abbreviation.rb --dry-run
    ["--dry-run", true]

### Option Arguments

An option may take no argument, a required argument, or an optional argument.

#### Option with No Argument

All the examples above define options with no argument.

#### Option with Required Argument

Specify a required argument for an option by adding a dummy word to its name definition.

`File` `required_argument.rb` defines two options; each has a required argument because the name definition has a following dummy word.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-x XXX', '--xxx', 'Required argument via short name') do |value|
      p ['--xxx', value]
    end
    parser.on('-y', '--y YYY', 'Required argument via long name') do |value|
      p ['--yyy', value]
    end
    parser.parse!

When an option is found, the given argument is yielded.

Executions:

    $ ruby required_argument.rb --help
    Usage: required_argument [options]
        -x, --xxx XXX                    Required argument via short name
        -y, --y YYY                      Required argument via long name
    $ ruby required_argument.rb -x AAA
    ["--xxx", "AAA"]
    $ ruby required_argument.rb -y BBB
    ["--yyy", "BBB"]

Omitting a required argument raises an error:

    $ ruby required_argument.rb -x
    required_argument.rb:9:in '<main>': missing argument: -x (OptionParser::MissingArgument)

#### Option with Optional Argument

Specify an optional argument for an option by adding a dummy word enclosed in square brackets to its name definition.

`File` `optional_argument.rb` defines two options; each has an optional argument because the name definition has a following dummy word in square brackets.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-x [XXX]', '--xxx', 'Optional argument via short  name') do |value|
      p ['--xxx', value]
    end
    parser.on('-y', '--yyy [YYY]', 'Optional argument via long name') do |value|
      p ['--yyy', value]
    end
    parser.parse!

When an option with an argument is found, the given argument yielded.

Executions:

    $ ruby optional_argument.rb --help
    Usage: optional_argument [options]
        -x, --xxx [XXX]                  Optional argument via short  name
        -y, --yyy [YYY]                  Optional argument via long name
    $ ruby optional_argument.rb -x AAA
    ["--xxx", "AAA"]
    $ ruby optional_argument.rb -y BBB
    ["--yyy", "BBB"]

Omitting an optional argument does not raise an error.

#### Argument Abbreviations

Specify an argument list as an `Array` or a `Hash`.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-x', '--xxx=VALUE', %w[ABC def], 'Argument abbreviations') do |value|
      p ['--xxx', value]
    end
    parser.on('-y', '--yyy=VALUE', {"abc"=>"XYZ", def: "FOO"}, 'Argument abbreviations') do |value|
      p ['--yyy', value]
    end
    parser.parse!

When an argument is abbreviated, the expanded argument yielded.

Executions:

    $ ruby argument_abbreviation.rb --help
    Usage: argument_abbreviation [options]
    Usage: argument_abbreviation [options]
        -x, --xxx=VALUE                  Argument abbreviations
        -y, --yyy=VALUE                  Argument abbreviations
    $ ruby argument_abbreviation.rb --xxx A
    ["--xxx", "ABC"]
    $ ruby argument_abbreviation.rb --xxx c
    argument_abbreviation.rb:9:in '<main>': invalid argument: --xxx c (OptionParser::InvalidArgument)
    $ ruby argument_abbreviation.rb --yyy a --yyy d
    ["--yyy", "XYZ"]
    ["--yyy", "FOO"]

### Argument Values

Permissible argument values may be restricted either by specifying explicit values or by providing a pattern that the given value must match.

#### Explicit Argument Values

You can specify argument values in either of two ways:

- Specify values an array of strings.

- Specify values a hash.

##### Explicit Values in `Array`

You can specify explicit argument values in an array of strings. The argument value must be one of those strings, or an unambiguous abbreviation.

`File` `explicit_array_values.rb` defines options with explicit argument values.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-xXXX', ['foo', 'bar'], 'Values for required argument' ) do |value|
      p ['-x', value]
    end
    parser.on('-y [YYY]', ['baz', 'bat'], 'Values for optional argument') do |value|
      p ['-y', value]
    end
    parser.parse!

Executions:

    $ ruby explicit_array_values.rb --help
    Usage: explicit_array_values [options]
        -xXXX                            Values for required argument
        -y [YYY]                         Values for optional argument
    $ ruby explicit_array_values.rb -x
    explicit_array_values.rb:9:in '<main>': missing argument: -x (OptionParser::MissingArgument)
    $ ruby explicit_array_values.rb -x foo
    ["-x", "foo"]
    $ ruby explicit_array_values.rb -x f
    ["-x", "foo"]
    $ ruby explicit_array_values.rb -x bar
    ["-x", "bar"]
    $ ruby explicit_array_values.rb -y ba
    explicit_array_values.rb:9:in '<main>': ambiguous argument: -y ba (OptionParser::AmbiguousArgument)
    $ ruby explicit_array_values.rb -x baz
    explicit_array_values.rb:9:in '<main>': invalid argument: -x baz (OptionParser::InvalidArgument)

##### Explicit Values in `Hash`

You can specify explicit argument values in a hash with string keys. The value passed must be one of those keys, or an unambiguous abbreviation; the value yielded will be the value for that key.

`File` `explicit_hash_values.rb` defines options with explicit argument values.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-xXXX', {foo: 0, bar: 1}, 'Values for required argument' ) do |value|
      p ['-x', value]
    end
    parser.on('-y [YYY]', {baz: 2, bat: 3}, 'Values for optional argument') do |value|
      p ['-y', value]
    end
    parser.parse!

Executions:

    $ ruby explicit_hash_values.rb --help
    Usage: explicit_hash_values [options]
        -xXXX                            Values for required argument
        -y [YYY]                         Values for optional argument
    $ ruby explicit_hash_values.rb -x
    explicit_hash_values.rb:9:in '<main>': missing argument: -x (OptionParser::MissingArgument)
    $ ruby explicit_hash_values.rb -x foo
    ["-x", 0]
    $ ruby explicit_hash_values.rb -x f
    ["-x", 0]
    $ ruby explicit_hash_values.rb -x bar
    ["-x", 1]
    $ ruby explicit_hash_values.rb -x baz
    explicit_hash_values.rb:9:in '<main>': invalid argument: -x baz (OptionParser::InvalidArgument)
    $ ruby explicit_hash_values.rb -y
    ["-y", nil]
    $ ruby explicit_hash_values.rb -y baz
    ["-y", 2]
    $ ruby explicit_hash_values.rb -y bat
    ["-y", 3]
    $ ruby explicit_hash_values.rb -y ba
    explicit_hash_values.rb:9:in '<main>': ambiguous argument: -y ba (OptionParser::AmbiguousArgument)
    $ ruby explicit_hash_values.rb -y bam
    ["-y", nil]

#### Argument Value Patterns

You can restrict permissible argument values by specifying a `Regexp` that the given argument must match.

`File` `matched_values.rb` defines options with matched argument values.

    require 'optparse'
    parser = OptionParser.new
    parser.on('--xxx XXX', /foo/i, 'Matched values') do |value|
      p ['--xxx', value]
    end
    parser.on('--yyy YYY', Integer, 'Check by range', 1..3) do |value|
      p ['--yyy', value]
    end
    parser.on('--zzz ZZZ', Integer, 'Check by list', [1, 3, 4]) do |value|
      p ['--zzz', value]
    end
    parser.parse!

Executions:

    $ ruby matched_values.rb --help
    Usage: matched_values [options]
            --xxx XXX                    Matched values
    $ ruby matched_values.rb --xxx foo
    ["--xxx", "foo"]
    $ ruby matched_values.rb --xxx FOO
    ["--xxx", "FOO"]
    $ ruby matched_values.rb --xxx bar
    matched_values.rb:6:in '<main>': invalid argument: --xxx bar (OptionParser::InvalidArgument)

### Keyword Argument `into`

In parsing options, you can add keyword option `into` with a hash-like argument; each parsed option will be added as a name/value pair.

This is useful for:

- Collecting options.

- Checking for missing options.

- Providing default values for options.

#### Collecting Options

Use keyword argument `into` to collect options.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-x', '--xxx', 'Short and long, no argument')
    parser.on('-yYYY', '--yyy', 'Short and long, required argument')
    parser.on('-z [ZZZ]', '--zzz', 'Short and long, optional argument')
    options = {}
    parser.parse!(into: options)
    p options

Executions:

    $ ruby collected_options.rb --help
    Usage: into [options]
        -x, --xxx                        Short and long, no argument
        -y, --yyyYYY                     Short and long, required argument
        -z, --zzz [ZZZ]                  Short and long, optional argument
    $ ruby collected_options.rb --xxx
    {:xxx=>true}
    $ ruby collected_options.rb --xxx --yyy FOO
    {:xxx=>true, :yyy=>"FOO"}
    $ ruby collected_options.rb --xxx --yyy FOO --zzz Bar
    {:xxx=>true, :yyy=>"FOO", :zzz=>"Bar"}
    $ ruby collected_options.rb --xxx --yyy FOO --yyy BAR
    {:xxx=>true, :yyy=>"BAR"}

Note in the last execution that the argument value for option `--yyy` was overwritten.

#### Checking for Missing Options

Use the collected options to check for missing options.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-x', '--xxx', 'Short and long, no argument')
    parser.on('-yYYY', '--yyy', 'Short and long, required argument')
    parser.on('-z [ZZZ]', '--zzz', 'Short and long, optional argument')
    options = {}
    parser.parse!(into: options)
    required_options = [:xxx, :zzz]
    missing_options = required_options - options.keys
    unless missing_options.empty?
      fail "Missing required options: #{missing_options}"
    end

Executions:

    $ ruby missing_options.rb --help
    Usage: missing_options [options]
        -x, --xxx                        Short and long, no argument
        -y, --yyyYYY                     Short and long, required argument
        -z, --zzz [ZZZ]                  Short and long, optional argument
    $ ruby missing_options.rb --yyy FOO
    missing_options.rb:11:in '<main>': Missing required options: [:xxx, :zzz] (RuntimeError)

#### Default Values for Options

Initialize the `into` argument to define default values for options.

    require 'optparse'
    parser = OptionParser.new
    parser.on('-x', '--xxx', 'Short and long, no argument')
    parser.on('-yYYY', '--yyy', 'Short and long, required argument')
    parser.on('-z [ZZZ]', '--zzz', 'Short and long, optional argument')
    options = {yyy: 'AAA', zzz: 'BBB'}
    parser.parse!(into: options)
    p options

Executions:

    $ ruby default_values.rb --help
    Usage: default_values [options]
        -x, --xxx                        Short and long, no argument
        -y, --yyyYYY                     Short and long, required argument
        -z, --zzz [ZZZ]                  Short and long, optional argument
    $ ruby default_values.rb --yyy FOO
    {:yyy=>"FOO", :zzz=>"BBB"}

### Argument Converters

An option can specify that its argument is to be converted from the default `String` to an instance of another class. There are a number of built-in converters.

Example: `File` date.rb defines an option whose argument is to be converted to a `Date` object. The argument is converted by method Date#parse.

    require 'optparse/date'
    parser = OptionParser.new
    parser.on('--date=DATE', Date) do |value|
      p [value, value.class]
    end
    parser.parse!

Executions:

    $ ruby date.rb --date 2001-02-03
    [#<Date: 2001-02-03 ((2451944j,0s,0n),+0s,2299161j)>, Date]
    $ ruby date.rb --date 20010203
    [#<Date: 2001-02-03 ((2451944j,0s,0n),+0s,2299161j)>, Date]
    $ ruby date.rb --date "3rd Feb 2001"
    [#<Date: 2001-02-03 ((2451944j,0s,0n),+0s,2299161j)>, Date]

You can also define custom converters. See Argument Converters for both built-in and custom converters.

### Help

`OptionParser` makes automatically generated help text available.

The help text consists of:

- A banner, showing the usage.

- Option short and long names.

- Option dummy argument names.

- Option descriptions.

Example code:

    require 'optparse'
    parser = OptionParser.new
    parser.on(
      '-x', '--xxx',
      'Adipiscing elit. Aenean commodo ligula eget.',
      'Aenean massa. Cum sociis natoque penatibus',
      )
    parser.on(
      '-y', '--yyy YYY',
      'Lorem ipsum dolor sit amet, consectetuer.'
    )
    parser.on(
      '-z', '--zzz [ZZZ]',
      'Et magnis dis parturient montes, nascetur',
      'ridiculus mus. Donec quam felis, ultricies',
      'nec, pellentesque eu, pretium quis, sem.',
      )
    parser.parse!

The option names and dummy argument names are defined as described above.

The option description consists of the strings that are not themselves option names; An option can have more than one description string. Execution:

    Usage: help [options]
        -x, --xxx                        Adipiscing elit. Aenean commodo ligula eget.
                                         Aenean massa. Cum sociis natoque penatibus
        -y, --yyy YYY                    Lorem ipsum dolor sit amet, consectetuer.
        -z, --zzz [ZZZ]                  Et magnis dis parturient montes, nascetur
                                         ridiculus mus. Donec quam felis, ultricies
                                         nec, pellentesque eu, pretium quis, sem.

The program name is included in the default banner: `Usage: #{program_name} [options]`; you can change the program name.

    require 'optparse'
    parser = OptionParser.new
    parser.program_name = 'help_program_name.rb'
    parser.parse!

Execution:

    $ ruby help_program_name.rb --help
    Usage: help_program_name.rb [options]

You can also change the entire banner.

    require 'optparse'
    parser = OptionParser.new
    parser.banner = "Usage: ruby help_banner.rb"
    parser.parse!

Execution:

    $ ruby help_banner.rb --help
    Usage: ruby help_banner.rb

By default, the option names are indented 4 spaces and the width of the option-names field is 32 spaces.

You can change these values, along with the banner, by passing parameters to `OptionParser.new`.

    require 'optparse'
    parser = OptionParser.new(
      'ruby help_format.rb [options]', # Banner
      20,                               # Width of options field
      ' ' * 2                               # Indentation
    )
    parser.on(
      '-x', '--xxx',
      'Adipiscing elit. Aenean commodo ligula eget.',
      'Aenean massa. Cum sociis natoque penatibus',
      )
    parser.on(
      '-y', '--yyy YYY',
      'Lorem ipsum dolor sit amet, consectetuer.'
    )
    parser.on(
      '-z', '--zzz [ZZZ]',
      'Et magnis dis parturient montes, nascetur',
      'ridiculus mus. Donec quam felis, ultricies',
      'nec, pellentesque eu, pretium quis, sem.',
      )
    parser.parse!

Execution:

    $ ruby help_format.rb --help
    ruby help_format.rb [options]
      -x, --xxx            Adipiscing elit. Aenean commodo ligula eget.
                           Aenean massa. Cum sociis natoque penatibus
      -y, --yyy YYY        Lorem ipsum dolor sit amet, consectetuer.
      -z, --zzz [ZZZ]      Et magnis dis parturient montes, nascetur
                           ridiculus mus. Donec quam felis, ultricies
                           nec, pellentesque eu, pretium quis, sem.

### Top List and Base List

An `OptionParser` object maintains a stack of `OptionParser::List` objects, each of which has a collection of zero or more options. It is unlikely that you’ll need to add or take away from that stack.

The stack includes:

- The top list, given by `OptionParser#top`.

- The base list, given by `OptionParser#base`.

When `OptionParser` builds its help text, the options in the top list precede those in the base list.

### Methods for Defining Options

Option-defining methods allow you to create an option, and also append/prepend it to the top list or append it to the base list.

Each of these next three methods accepts a sequence of parameter arguments and a block, creates an option object using method `OptionParser#make_switch` (see below), and returns the created option:

- Method `OptionParser#define` appends the created option to the top list.

- Method `OptionParser#define_head` prepends the created option to the top list.

- Method `OptionParser#define_tail` appends the created option to the base list.

These next three methods are identical to the three above, except for their return values:

- Method `OptionParser#on` is identical to method `OptionParser#define`, except that it returns the parser object `self`.

- Method `OptionParser#on_head` is identical to method `OptionParser#define_head`, except that it returns the parser object `self`.

- Method `OptionParser#on_tail` is identical to method `OptionParser#define_tail`, except that it returns the parser object `self`.

Though you may never need to call it directly, here’s the core method for defining an option:

- Method `OptionParser#make_switch` accepts an array of parameters and a block. See Parameters for New Options. This method is unlike others here in that it:

  - Accepts an array of parameters; others accept a sequence of parameter arguments.

  - Returns an array containing the created option object, option names, and other values; others return either the created option object or the parser object `self`.

### Parsing

`OptionParser` has six instance methods for parsing.

Three have names ending with a “bang” (`!`):

- parse!

- order!

- permute!

Each of these methods:

- Accepts an optional array of string arguments `argv`; if not given, `argv` defaults to the value of `OptionParser#default_argv`, whose initial value is ARGV.

- Accepts an optional keyword argument `into` (see Keyword Argument into).

- Returns `argv`, possibly with some elements removed.

The three other methods have names not ending with a “bang”:

- parse

- order

- permute

Each of these methods:

- Accepts an array of string arguments or zero or more string arguments.

- Accepts an optional keyword argument `into` and its value into. (see Keyword Argument into).

- Returns `argv`, possibly with some elements removed.

#### Method `parse!`

Method `parse!`:

- Accepts an optional array of string arguments `argv`; if not given, `argv` defaults to the value of `OptionParser#default_argv`, whose initial value is ARGV.

- Accepts an optional keyword argument `into` (see Keyword Argument into).

- Returns `argv`, possibly with some elements removed.

The method processes the elements in `argv` beginning at `argv[0]`, and ending, by default, at the end.

Otherwise processing ends and the method returns when:

- The terminator argument `--` is found; the terminator argument is removed before the return.

- Environment variable `POSIXLY_CORRECT` is defined and a non-option argument is found; the non-option argument is not removed. Note that the value of that variable does not matter, as only its existence is checked.

`File` `parse_bang.rb`:

    require 'optparse'
    parser = OptionParser.new
    parser.on('--xxx') do |value|
      p ['--xxx', value]
    end
    parser.on('--yyy YYY') do |value|
      p ['--yyy', value]
    end
    parser.on('--zzz [ZZZ]') do |value|
      p ['--zzz', value]
    end
    ret = parser.parse!
    puts "Returned: #{ret} (#{ret.class})"

Help:

    $ ruby parse_bang.rb --help
    Usage: parse_bang [options]
            --xxx
            --yyy YYY
            --zzz [ZZZ]

Default behavior:

    $ ruby parse_bang.rb input_file.txt output_file.txt --xxx --yyy FOO --zzz BAR
    ["--xxx", true]
    ["--yyy", "FOO"]
    ["--zzz", "BAR"]
    Returned: ["input_file.txt", "output_file.txt"] (Array)

Processing ended by terminator argument:

    $ ruby parse_bang.rb input_file.txt output_file.txt --xxx --yyy FOO -- --zzz BAR
    ["--xxx", true]
    ["--yyy", "FOO"]
    Returned: ["input_file.txt", "output_file.txt", "--zzz", "BAR"] (Array)

Processing ended by non-option found when `POSIXLY_CORRECT` is defined:

    $ POSIXLY_CORRECT=true ruby parse_bang.rb --xxx input_file.txt output_file.txt -yyy FOO
    ["--xxx", true]
    Returned: ["input_file.txt", "output_file.txt", "-yyy", "FOO"] (Array)

#### Method `parse`

Method `parse`:

- Accepts an array of string arguments or zero or more string arguments.

- Accepts an optional keyword argument `into` and its value into. (see Keyword Argument into).

- Returns `argv`, possibly with some elements removed.

If given an array `ary`, the method forms array `argv` as `ary.dup`. If given zero or more string arguments, those arguments are formed into array `argv`.

The method calls

    parse!(argv, into: into)

Note that environment variable `POSIXLY_CORRECT` and the terminator argument `--` are honored.

`File` `parse.rb`:

    require 'optparse'
    parser = OptionParser.new
    parser.on('--xxx') do |value|
      p ['--xxx', value]
    end
    parser.on('--yyy YYY') do |value|
      p ['--yyy', value]
    end
    parser.on('--zzz [ZZZ]') do |value|
      p ['--zzz', value]
    end
    ret = parser.parse(ARGV)
    puts "Returned: #{ret} (#{ret.class})"

Help:

    $ ruby parse.rb --help
    Usage: parse [options]
            --xxx
            --yyy YYY
            --zzz [ZZZ]

Default behavior:

    $ ruby parse.rb input_file.txt output_file.txt --xxx --yyy FOO --zzz BAR
    ["--xxx", true]
    ["--yyy", "FOO"]
    ["--zzz", "BAR"]
    Returned: ["input_file.txt", "output_file.txt"] (Array)

Processing ended by terminator argument:

    $ ruby parse.rb input_file.txt output_file.txt --xxx --yyy FOO -- --zzz BAR
    ["--xxx", true]
    ["--yyy", "FOO"]
    Returned: ["input_file.txt", "output_file.txt", "--zzz", "BAR"] (Array)

Processing ended by non-option found when `POSIXLY_CORRECT` is defined:

    $ POSIXLY_CORRECT=true ruby parse.rb --xxx input_file.txt output_file.txt -yyy FOO
    ["--xxx", true]
    Returned: ["input_file.txt", "output_file.txt", "-yyy", "FOO"] (Array)

#### Method `order!`

Calling method `OptionParser#order!` gives exactly the same result as calling method `OptionParser#parse!` with environment variable `POSIXLY_CORRECT` defined.

#### Method `order`

Calling method `OptionParser#order` gives exactly the same result as calling method `OptionParser#parse` with environment variable `POSIXLY_CORRECT` defined.

#### Method `permute!`

Calling method `OptionParser#permute!` gives exactly the same result as calling method `OptionParser#parse!` with environment variable `POSIXLY_CORRECT` not defined.

#### Method `permute`

Calling method `OptionParser#permute` gives exactly the same result as calling method `OptionParser#parse` with environment variable `POSIXLY_CORRECT` not defined.