Read Arduino serial port and print the line

arduino_serial_echo.ino

String readString;
int state = LOW;

void setup() {
  Serial.begin(9600);
  pinMode(LED_BUILTIN, OUTPUT);
  Serial.println("Starting");
}

void loop() {

  while (Serial.available()) {
    if (state == LOW) {
      state = HIGH;
      digitalWrite(LED_BUILTIN, HIGH);
    }
    delay(2);  // wait to allow buffer to fill 
    if (Serial.available() > 0) {
      char c = Serial.read();  // gets one byte from serial buffer
      readString += c;  // makes the str readString
    } 
  }

  if (readString.length() > 0) {
    Serial.println(readString);  // see what was received
    readString="";
    if (state == HIGH) {
      state = LOW;
      digitalWrite(LED_BUILTIN, LOW);
    }
  } 
}