OptionParser -

# class OptionParser

- OptionParser
- Reference
- Object

## Overview

`OptionParser` is a class for command-line options processing. It supports:

- Short and long modifier style options (example: `-h`, `--help`)
- Passing arguments to the flags (example: `-f filename.txt`)
- Automatic help message generation

Run `crystal` for an example of a CLI built with `OptionParser`.

Short example:

    require "option_parser"

    upcase = false
    destination = "World"

    OptionParser.parse! do |parser|
      parser.banner = "Usage: salute [arguments]"
      parser.on("-u", "--upcase", "Upcases the salute") { upcase = true }
      parser.on("-t NAME", "--to=NAME", "Specifies the name to salute") { |name| destination = name }
      parser.on("-h", "--help", "Show this help") { puts parser }
      parser.invalid_option do |flag|
        STDERR.puts "ERROR: #{flag} is not a valid option."
        STDERR.puts parser
        exit(1)
      end
    end

    destination = destination.upcase if upcase
    puts "Hello #{destination}!"

## Defined in:

## Constructors

- .new

  Creates a new parser.

- .new(&block)

  Creates a new parser, with its configuration specified in the block.

- .parse(args, &block) : self

  Creates a new parser, with its configuration specified in the block, and uses it to parse the passed args.

- .parse!(&block) : self

  Creates a new parser, with its configuration specified in the block, and uses it to parse the arguments passed to the program.

## Instance Method Summary

- #banner=(banner : String?)

  Establishes the initial message for the help printout.

- #invalid_option(&invalid_option : String -> )

  Sets a handler for option arguments that didn't match any of the setup options.

- #missing_option(&missing_option : String -> )

  Sets a handler for when a option that expects an argument wasn't given any.

- #on(short_flag : String, long_flag : String, description : String, &block : String -> )

  Establishes a handler for a pair of short and long flags.

- #on(flag : String, description : String, &block : String -> )

  Establishes a handler for a flag.

- #parse(args)

  Parses the passed args, running the handlers associated to each option.

- #parse!

  Parses the passed the arguments passed to the program, running the handlers associated to each option.

- #separator(message = "")

  Adds a separator, with an optional header message, that will be used to print the help.

- #to_s(io : IO) : Nil

  Returns all the setup options, formatted in a help message.

- #unknown_args(&unknown_args : Array(String), Array(String) -> )

  Sets a handler for regular arguments that didn't match any of the setup options.

### Instance methods inherited from class `Reference`

==(other : self)
==(other : JSON::Any)
==(other : YAML::Any)
==(other) ==, dup dup, hash(hasher) hash, inspect(io : IO) : Nil inspect, object_id : UInt64 object_id, pretty_print(pp) : Nil pretty_print, same?(other : Reference)
same?(other : Nil) same?, to_s(io : IO) : Nil to_s

### Constructor methods inherited from class `Reference`

new new

### Instance methods inherited from class `Object`

!=(other) !=, !~(other) !~, ==(other) ==, ===(other : JSON::Any)
===(other : YAML::Any)
===(other) ===, =~(other) =~, class class, dup dup, hash(hasher)
hash hash, inspect(io : IO) : Nil
inspect : String inspect, itself itself, not_nil! not_nil!, pretty_inspect(width = 79, newline = "\n", indent = 0) : String pretty_inspect, pretty_print(pp : PrettyPrint) : Nil pretty_print, tap(&block) tap, to_json(io : IO)
to_json to_json, to_pretty_json(indent : String = " ")
to_pretty_json(io : IO, indent : String = " ") to_pretty_json, to_s : String
to_s(io : IO) : Nil to_s, to_yaml(io : IO)
to_yaml to_yaml, try(&block) try, unsafe_as(type : T.class) forall T unsafe_as

### Constructor methods inherited from class `Object`

from_json(string_or_io, root : String) : self
from_json(string_or_io) : self from_json, from_yaml(string_or_io : String | IO) : self from_yaml

## Constructor Detail

def self.new #

Creates a new parser.

def self.new(&block) #

Creates a new parser, with its configuration specified in the block.

def self.parse(args, &block) : self #

Creates a new parser, with its configuration specified in the block, and uses it to parse the passed args.

def self.parse!(&block) : self #

Creates a new parser, with its configuration specified in the block, and uses it to parse the arguments passed to the program.

## Instance Method Detail

def banner=(banner : String?) #

Establishes the initial message for the help printout. Typically, you want to write here the name of your program, and a one-line template of its invocation.

Example:

    require "option_parser"

    parser = OptionParser.new
    parser.banner = "Usage: crystal [command] [switches] [program file] [--] [arguments]"

def invalid_option(&invalid_option : String -> ) #

Sets a handler for option arguments that didn't match any of the setup options.

You typically use this to display a help message. The default raises `InvalidOption`.

def missing_option(&missing_option : String -> ) #

Sets a handler for when a option that expects an argument wasn't given any.

You typically use this to display a help message. The default raises `MissingOption`.

def on(short_flag : String, long_flag : String, description : String, &block : String -> ) #

Establishes a handler for a pair of short and long flags.

See the other definition of `#on` for examples.

def on(flag : String, description : String, &block : String -> ) #

Establishes a handler for a flag.

Flags must start with a dash or double dash. They can also have an optional argument, which will get passed to the block. Each flag has a description, which will be used for the help message.

Examples of valid flags:

- `-a`, `-B`
- `--something-longer`
- `-f FILE`, `--file FILE`, `--file=FILE` (these will yield the passed value to the block as a string)

def parse(args) #

Parses the passed args, running the handlers associated to each option.

def parse! #

Parses the passed the arguments passed to the program, running the handlers associated to each option.

def separator(message = "") #

Adds a separator, with an optional header message, that will be used to print the help.

This way, you can group the different options in an easier to read way.

def to_s(io : IO) : Nil #

Returns all the setup options, formatted in a help message.

def unknown_args(&unknown_args : Array(String), Array(String) -> ) #

Sets a handler for regular arguments that didn't match any of the setup options.

You typically use this to get the main arguments (not modifiers) that your program expects (for example, filenames)