Spit out any lines which are in the first file and not in the second

inone.rb

#!/usr/local/bin/ruby -w

# Assuming the two files are sorted, this will walk through them in parallel comparing
# and spitting out any lines which are in the first one and not in the second.
# its equivalend to the unix command:  diff A.txt B.txt|grep '^<'|awk '{print $2}'
fa = File.open(ARGV[0])
fb = File.open(ARGV[1])

linea = nil
lineb = nil
loop do
  if linea==lineb
    linea = fa.gets || break
    lineb = fb.gets || ""
  elsif linea<lineb
    puts linea
    linea = fa.gets || break
  elsif  linea>lineb
    lineb = fb.gets || ""
  end
end