dpi2mm.rb

#!/usr/bin/env ruby
require 'docopt'

doc = <<DOCOPT
Usage:
  #{File.basename __FILE__} <dpi> <px> [--verbose]

Options:
  -v --verbose  Show numbers after the decimal point.
  -h --help     Show this screen.
DOCOPT

begin
  opt = Docopt::docopt(doc)
  dpi = opt['<dpi>'].to_f
  px = opt['<px>'].to_f
  tmp = opt['--verbose'] ? "%f mm" : "%d mm"
  puts tmp % [25.4 * px / dpi]

rescue Docopt::Exit => e
  puts e.message
end