1.c

int pinLED = 13;
int ledStatus = 0;

void setup() {
  // put your setup code here, to run once:
  Serial.begin(115200);
  pinMode(pinLED, OUTPUT);

  Serial.println("usage : write (turn on | turn off)");
}

String Serialread()
{
  String buf ="";
  char c;

  while(Serial.available() > 0) {
    c = Serial.read();
    buf.concat(c);
    delay(10);
  }

  return buf;
}

void loop() {
  // put your main code here, to run repeatedly:
  String str = "";
  digitalWrite(pinLED, ledStatus);

  str = Serialread();
  if(str == "") {
    
  }
  else if(str == "turn off") {
    Serial.println(str);
    ledStatus = 0;
  }
  else if(str == "turn on") {
    Serial.println(str);
    ledStatus = 1;
  }
  else {
    Serial.println("usage : write (turn on | turn off)");
  }

}