Send a command on or off through the serial monitor to turn ON or OFF the LED.
String readString;
void setup() {
  Serial.begin(115200);
  pinMode(LED_BUILTIN, OUTPUT);
}
void loop() {
  while (Serial.available()) {
    delay(3);
    char c = Serial.read();
    readString += c;
  }
  if (readString.length() > 0) {
    Serial.println(readString);
    if (readString == "on") {
      ledON();
    }
    if (readString == "off") {
      ledOFF();
    }
    readString = "";
  }
}
void ledON() {
  Serial.println("LED ON");
  digitalWrite(LED_BUILTIN, LOW);
}
void ledOFF() {
  Serial.println("LED OFF");
  digitalWrite(LED_BUILTIN, HIGH);
}BOARD?=arduino:avr:uno
PORT?=/dev/cu.usbmodem14*
.PHONY: default lint all flash clean
default: lint all flash clean
lint:
	cpplint --extensions=ino --filter=-legal/copyright *.ino
all:
	arduino-cli compile --fqbn $(BOARD) ./
flash:
	arduino-cli upload -p $(PORT) --fqbn $(BOARD) ./
clean:
	rm -f .*.hex
	rm -f .*.elf