Ruby shell code - Find song files that have a single digit track number and prefix 0 to it

AddLeadingZeroToSongFile.rb

#!/usr/bin/env ruby

# Tested in REE 1.8.7. To run from command line, type -
# ruby AddLeadingZeroToSongFile.rb
`ls -1 *.mp3`.split("\n").each do | songfile | 
  x = /^(\d .*)/.match(songfile)
  `mv "#{x.captures[0]}" "0#{x.captures[0]}"` if x
end