OptionParse with no arguments show banner

Question

I'm using OptionParser with Ruby.

I other languages such as C, Python, etc., there are similar command-line parameters parsers and they often provide a way to show help message when no parameters are provided or parameters are wrong.

options = {}
OptionParser.new do |opts|
  opts.banner = "Usage: calc.rb [options]"

  opts.on("-l", "--length L", Integer, "Length") { |l| options[:length] = l }
  opts.on("-w", "--width W", Integer, "Width") { |w| options[:width] = w }

  opts.on_tail("-h", "--help", "Show this message") do
    puts opts
    exit
  end
end.parse!

Questions:

1. Is there a way to set that by default show help message if no parameters were passed (ruby calc.rb)?
2. What about if a required parameter is not given or is invalid? Suppose length is a REQUIRED parameter and user do not pass it or pass something wrong like -l FOO?

Answer 1

Just add the -h key to the ARGV, when it is empty, so you may to do something like this:

require 'optparse'

ARGV << '-h' if ARGV.empty?

options = {}
OptionParser.new do |opts|
  opts.banner = "Usage: calc.rb [options]"

  opts.on("-l", "--length L", Integer, "Length") { |l| options[:length] = l }
  opts.on("-w", "--width W", Integer, "Width") { |w| options[:width] = w }

  opts.on_tail("-h", "--help", "Show this message") do
    puts opts
    exit
  end
end.parse!

Answer 2

My script requires exactly 2 args, so I used this code to check ARGV.length after parsing:

if ARGV.length != 2 then
  puts optparse.help
  exit 1
end

Answer 3

1. You can either check ARGV before parsing (as the answer above):
   ARGV << '-h' if ARGV.empty?

   Or check your options hash after parsing:

   if @options.empty?
     puts optparse.help
     puts 'At least 1 argument should be supplied!'
   end
   

2. This is what i do to ensure mandatory args with OptionParse (couldn't find any builtin feature for this):

   begin
     optparse.parse!
     mandatory = [:length, :width]                                         # Enforce the presence of
     missing = mandatory.select{ |param| @options[param].nil? }            # mandatory switches: :length, :width
     if not missing.empty?                                                 #
           puts "Missing options: #{missing.join(', ')}"                   #
           puts optparse.help                                              #
           exit 2                                                          #
     end                                                                   #
   rescue OptionParser::InvalidOption, OptionParser::MissingArgument => error     #
     puts error                                                                   # Friendly output when parsing fails
     puts optparse                                                                #
     exit 2                                                                       #
   end