Send command via serial monitor

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