Trivial example of traversing a large file via command line.

largefile.rb

#!/usr/bin/env ruby

filename = ARGV[0]
usage = "USAGE: largefile.rb <filename>"

unless filename
  STDERR.write(usage + "\n")
  exit(1)
end

fd = IO.sysopen(filename, "r") 
file = IO.new(fd, "r")
while (!file.eof?) do
  line = file.gets
  STDOUT.write(line)
end
file.close