> A quick and dirty way might be:

Oh well if that's allowed, then how about:

File.open("file.rb") do |r|
  File.open("file.html","w") do |w|
    w.puts "<pre>"
    r.each_line do |line|
      line.gsub!(/&/,'&amp;')
      line.gsub!(/</,'&lt;')
      line.gsub!(/>/,'&gt;')
      w.puts line
    end
    w.puts "</pre>"
  end
end

:-)