Ruby Language OptionParser Mandatory and optional command line options


It's relatively easy to parse the command line by hand if you aren't looking for anything too complex:

# Naive error checking
abort('Usage: ' + $0 + ' site id ...') unless ARGV.length >= 2

# First item (site) is mandatory
site = ARGV.shift

ARGV.each do | id |
  # Do something interesting with each of the ids

But when your options start to get more complicated, you probably will need to use an option parser such as, well, OptionParser:

require 'optparse'

# The actual options will be stored in this hash
options = {}

# Set up the options you are looking for
optparse = do |opts|
  opts.banner = "Usage: #{$0} -s NAME id ..."

  opts.on("-s", "--site NAME", "Site name") do |s|
    options[:site] = s

  opts.on( '-h', '--help', 'Display this screen' ) do
    puts opts

# The parse! method also removes any options it finds from ARGV.

There's also a non-destructive parse, but it's a lot less useful if you plan on using the remainder of what's in ARGV.

The OptionParser class doesn't have a way to enforce mandatory arguments (such as --site in this case). However you can do you own checking after running parse!:

# Slightly more sophisticated error checking
if options[:site].nil? or ARGV.length == 0

For a more generic mandatory option handler, see this answer. In case it isn't clear, all options are optional unless you go out of your way to make them mandatory.