const uint8_t PIN = 4;

void setup() {
    // $ stty -F /dev/ttyUSB0 | grep speed
    Serial.begin(115200);
    pinMode(PIN, OUTPUT);
}

void loop() {
    // https://www.arduino.cc/reference/en/language/functions/communication/serial/read/
    if (Serial.available() > 0) {
        // $ printf '\0' > /dev/ttyUSB0
        // $ printf '\x01' > /dev/ttyUSB0
        digitalWrite(PIN, (Serial.read() > 0) ? HIGH : LOW);
    }
}