sent commends by bluetooth to sd card as one line of string

 void loop() {

  if (Bluetooth.available() > 0) {
           dataIn = Bluetooth.readString(); 
   //the problem is here
           while(dataIn !=""){
                   dataSd += dataIn;
                   dataSd += "," ;
           }

 if (dataIn.startsWith("s1")) {-----------}
 if (dataIn.startsWith("save"){ save to sd card }

// what I want is to sent command by bluetooth to variable dataSd as a line 
of string seperated by commas 
like ("left,right,up,down,left,stop,right") and  when i send command save, it 
will save to sd card, but this code didnt work