== 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+:

  :include: ruby/argv.rb

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}[#label-To+Begin+With]
- {Defining Options}[#label-Defining+Options]
- {Option Names}[#label-Option+Names]
  - {Short Option Names}[#label-Short+Option+Names]
  - {Long Option Names}[#label-Long+Option+Names]
  - {Mixing Option Names}[#label-Mixing+Option+Names]
  - {Option Name Abbreviations}[#label-Option+Name+Abbreviations]
- {Option Arguments}[#label-Option+Arguments]
  - {Option with No Argument}[#label-Option+with+No+Argument]
  - {Option with Required Argument}[#label-Option+with+Required+Argument]
  - {Option with Optional Argument}[#label-Option+with+Optional+Argument]
  - {Argument Abbreviations}[#label-Argument+Abbreviations]
- {Argument Values}[#label-Argument+Values]
  - {Explicit Argument Values}[#label-Explicit+Argument+Values]
    - {Explicit Values in Array}[#label-Explicit+Values+in+Array]
    - {Explicit Values in Hash}[#label-Explicit+Values+in+Hash]
  - {Argument Value Patterns}[#label-Argument+Value+Patterns]
- {Keyword Argument into}[#label-Keyword+Argument+into]
  - {Collecting Options}[#label-Collecting+Options]
  - {Checking for Missing Options}[#label-Checking+for+Missing+Options]
  - {Default Values for Options}[#label-Default+Values+for+Options]
- {Argument Converters}[#label-Argument+Converters]
- {Help}[#label-Help]
- {Top List and Base List}[#label-Top+List+and+Base+List]
- {Methods for Defining Options}[#label-Methods+for+Defining+Options]
- {Parsing}[#label-Parsing]
  - {Method parse!}[#label-Method+parse-21]
  - {Method parse}[#label-Method+parse]
  - {Method order!}[#label-Method+order-21]
  - {Method order}[#label-Method+order]
  - {Method permute!}[#label-Method+permute-21]
  - {Method permute}[#label-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, <tt>-x</tt>,
<tt>-y</tt>, and <tt>-z</tt>, each with a descriptive string,
and each with a block.

  :include: ruby/basic.rb

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 (<tt>-</tt>).
- Long (multi-character) name, beginning with two hyphens (<tt>--</tt>).

==== 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, <tt>-x</tt>,
and an option with two short names (aliases, in effect) <tt>-y</tt> and <tt>-z</tt>.

  :include: ruby/short_names.rb

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, <tt>--xxx</tt>,
and an option with two long names (aliases, in effect) <tt>--y1%</tt> and <tt>--z2#</tt>.

  :include: ruby/long_names.rb

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.

  :include: ruby/long_with_negation.rb

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.

  :include: ruby/mixed_names.rb

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.

  :include: ruby/name_abbrev.rb

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+.

  :include: ruby/no_abbreviation.rb

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.

  :include: ruby/required_argument.rb

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.

  :include: ruby/optional_argument.rb

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.

  :include: ruby/argument_abbreviation.rb

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.

  :include: ruby/explicit_array_values.rb

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.

  :include: ruby/explicit_hash_values.rb

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.

  :include: ruby/matched_values.rb

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.

  :include: ruby/collected_options.rb

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 <tt>--yyy</tt>
was overwritten.

==== Checking for Missing Options

Use the collected options to check for missing options.

  :include: ruby/missing_options.rb

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.

  :include: ruby/default_values.rb

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.

  :include: ruby/date.rb

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}[./argument_converters.rdoc]
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:

  :include: ruby/help.rb

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:
<tt>Usage: #{program_name} [options]</tt>;
you can change the program name.

  :include: ruby/help_program_name.rb

Execution:

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

You can also change the entire banner.

  :include: ruby/help_banner.rb

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.

  :include: ruby/help_format.rb

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 <em>top list</em>, given by OptionParser#top.
- The <em>base list</em>, 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}[optparse/option_params.rdoc].
  This method is unlike others here in that it:
  - Accepts an <em>array of parameters</em>;
    others accept a <em>sequence of parameter arguments</em>.
  - 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" (<tt>!</tt>):

- 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}[#label-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}[#label-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}[#label-Keyword+Argument+into]).
- Returns +argv+, possibly with some elements removed.

The method processes the elements in +argv+ beginning at <tt>argv[0]</tt>,
and ending, by default, at the end.

Otherwise processing ends and the method returns when:

- The terminator argument <tt>--</tt> 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+:

  :include: ruby/parse_bang.rb

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}[#label-Keyword+Argument+into]).
- Returns +argv+, possibly with some elements removed.

If given an array +ary+, the method forms array +argv+ as <tt>ary.dup</tt>.
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 <tt>--</tt> are honored.

File +parse.rb+:

  :include: ruby/parse.rb

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.
