class

OptionParser

Inherits Reference / Object

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)
  • Subcommands
  • Automatic help message generation

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

NOTE: To use OptionParser, you must explicitly import it with require "option_parser"

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") do
    puts parser
    exit
  end
  parser.invalid_option do |flag|
    STDERR.puts "ERROR: #{flag} is not a valid option."
    STDERR.puts parser
    exit(1)
  end
  parser.missing_option do |flag|
    STDERR.puts "ERROR: #{flag} is missing an argument."
    STDERR.puts parser
    exit(1)
  end
end

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

Subcommands

OptionParser also supports subcommands.

Short example:

require "option_parser"

verbose = false
salute = false
welcome = false
name = "World"
parser = OptionParser.new do |parser|
  parser.banner = "Usage: example [subcommand] [arguments]"
  parser.on("salute", "Salute a name") do
    salute = true
    parser.banner = "Usage: example salute [arguments]"
    parser.on("-t NAME", "--to=NAME", "Specify the name to salute") { |_name| name = _name }
  end
  parser.on("welcome", "Print a greeting message") do
    welcome = true
    parser.banner = "Usage: example welcome"
  end
  parser.on("-v", "--verbose", "Enabled verbose output") { verbose = true }
  parser.on("-h", "--help", "Show this help") do
    puts parser
    exit
  end
end

parser.parse

if salute
  STDERR.puts "Saluting #{name}" if verbose
  puts "Hello #{name}"
elsif welcome
  STDERR.puts "Welcoming #{name}" if verbose
  puts "Welcome!"
else
  puts parser
  exit(1)
end

Constructors

new(*, gnu_optional_args : Bool = false)

Creates a new parser.

Refer to #gnu_optional_args? for the behaviour of the named parameter.

Source
new(*, gnu_optional_args : Bool = false, &)

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

Refer to #gnu_optional_args? for the behaviour of the named parameter.

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parse(args = ARGV, *, gnu_optional_args : Bool = false, &) : self

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

Refer to #gnu_optional_args? for the behaviour of the named parameter.

Source

Instance methods

before_each

Sets a handler which runs before each argument is parsed. This callback is not passed flag arguments. For example, --foo=foo_arg --bar bar_arg would pass --foo=foo_arg and --bar to the callback only.

You typically use this to implement advanced option parsing behaviour such as treating all options after a filename differently (along with #stop).

Source
gnu_optional_args=(gnu_optional_args : Bool)

Returns whether the GNU convention is followed for optional arguments.

If true, any optional argument must follow the preceding flag in the same token immediately, without any space inbetween:

require "option_parser"

OptionParser.parse(%w(-a1 -a 2 -a --b=3 --b 4), gnu_optional_args: true) do |parser|
  parser.on("-a", "--b [x]", "optional") { |x| p x }
  parser.unknown_args { |args, _| puts "Remaining: #{args}" }
end

Prints:

"1"
""
""
"3"
""
Remaining: ["2", "4"]

Without gnu_optional_args: true, prints the following instead:

"1"
"2"
"--b=3"
"4"
Remaining: []
Source
gnu_optional_args?

Returns whether the GNU convention is followed for optional arguments.

If true, any optional argument must follow the preceding flag in the same token immediately, without any space inbetween:

require "option_parser"

OptionParser.parse(%w(-a1 -a 2 -a --b=3 --b 4), gnu_optional_args: true) do |parser|
  parser.on("-a", "--b [x]", "optional") { |x| p x }
  parser.unknown_args { |args, _| puts "Remaining: #{args}" }
end

Prints:

"1"
""
""
"3"
""
Remaining: ["2", "4"]

Without gnu_optional_args: true, prints the following instead:

"1"
"2"
"--b=3"
"4"
Remaining: []
Source
invalid_option

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 behaviour is to raise InvalidOption.

Source
missing_option

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 behaviour is to raise MissingOption.

Source
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. This method does not support subcommands.

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

Establishes a handler for a flag or subcommand.

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.

Subcommands are any flag passed which does not start with a dash. They cannot take arguments. When a subcommand is parsed, all subcommands are removed from the OptionParser, simulating a "tree" of subcommands. All flags remain valid. For a longer example, see the examples at the top of the page.

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)

Examples of valid subcommands:

  • foo, run
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parse(args = ARGV) : Nil

Parses the passed args (defaults to ARGV), running the handlers associated to each option.

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separator(message = "") : Nil

Adds a separator, with an optional header message, that will be used to print the help. The separator is placed between the flags registered (#on) before, and the flags registered after the call.

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

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stop

Stops the current parse and returns immediately, leaving the remaining flags unparsed. This is treated identically to -- being inserted behind the current parsed flag.

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summary_indent

Indentation for summary.

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summary_indent=(summary_indent : String)

Indentation for summary.

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summary_width

Width for option list portion of summary.

Source
summary_width=(width : Int32)

Width for option list portion of summary.

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to_s(io : IO) : Nil

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

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unknown_args

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). The default behaviour is to do nothing. The arguments can also be extracted from the args array passed to #parse after parsing.

Source

Nested types