2.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)");
}

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

  while(Serial.available() > 0) {
    if(Serial.find("turn on")) {
      Serial.println("turn on");
      ledStatus = 1;
    }
    else if(Serial.find("turn off")) {
      Serial.println("turn off");
      ledStatus = 0;
    }
    else {
      Serial.println("usage : write (turn on | turn off)");
    }
  }
}