gistfile1.rb

ip="1.2.3.4"
netmask="4.3.2.1"
gateway="8.8.8.8"

interfaces = IO.readlines('interfaces').map do |line|
        if line.start_with?("iface eth0")
                "iface eth0 inet static\n" 
                "\taddress #{ip}\n" 
                "\tnetmask #{netmask}\n"
                "\tgateway  #{gateway}\n"
        else
                line
        end
end
File.open('interfaces.new', 'w') do |file|
  file.puts interfaces
end