Kaprekar's routine. Undoubtedly not the best way to do it.

kaprekar.rb

#!/usr/bin/env ruby

# See: http://en.wikipedia.org/wiki/Kaprekar_constant

$MAX_DIGITS = 4
$KAPREKARS_CONSTANT = 6174

class Fixnum
  def sort(padding=0)
    ordered = Array.new
    self.to_s.each_char do |c|
      ordered << c
    end
    if padding > 0
      (padding - self.to_s.size).times do
        ordered << '0'
      end
    end
    ordered.sort
  end
  protected :sort

  def sort_ascending(padding)
    self.sort(padding).join.to_i
  end

  def sort_descending(padding)
    self.sort(padding).reverse.join.to_i
  end

  def is_repdigits?
    product_of_first = self.to_s.chars.next.to_i * $MAX_DIGITS
    sum_of_all = 0
    self.to_s.each_char do |c|
      sum_of_all += c.to_i
    end
    product_of_first == sum_of_all
  end
end

def kaprekar(number)
  return number if number == $KAPREKARS_CONSTANT
  asc = number.sort_ascending $MAX_DIGITS
  desc = number.sort_descending $MAX_DIGITS
  if asc > desc
    greater = asc
    lesser = desc
  else
    greater = desc
    lesser = asc
  end
  result = greater - lesser
  puts "#{greater.to_s} - #{lesser.to_s} = #{result.to_s}"
  return kaprekar(result)
end

number = ARGV[0]
if number.size > $MAX_DIGITS or number.size < 1 or number.to_i == 0
  puts "Number must be a nonzero four-digit number!"
  exit
end
if number.to_i.is_repdigits?
  puts "Number cannot be a repdigit, like 1111 or 3333."
  exit
end
kaprekar number.to_i