gistfile1.rb

#!/Users/moro/opt/ruby192/bin/ruby

magicome = "# coding: #{Encoding.default_external.to_s.downcase}"
ARGV.each do |fname|
  File.open(fname, 'r+:BINARY') do |src|
    content = src.read
    src.rewind
    src.puts magicome
    src.write content
  end
end