I'm using an Arduino Mega 2560 and writing a really simple piece of code that doesn't work.

String a, letra;
#define led 13

void setup() {
  Serial.begin(9600); // opens serial port, sets data rate to 9600 bps
  pinMode(led, OUTPUT);

void loop() {
  while(Serial.available()) {
    letra= Serial.readStringUntil('\n');

      Serial.println("ABRE 1");
    if(letra == "aa\r\n"){
      Serial.println("ABRE 2");
    if(letra.compareTo("aa\r\n") == 0){
      Serial.println("ABRE 3");

enter image description here

It's returning what I typed, but it's not entering any of the if conditions... Can someone help me with that ?

  • 2
    there can be no \r\n in the string after trim()
    – Juraj
    Sep 2, 2023 at 19:34
  • Additionally, the terminating \n is removed from the input buffer, but excluded from the returned result, even without trim. Oct 5, 2023 at 10:26

1 Answer 1


Trim removes whitespace, and you are comparing for \r and \n to be there. Since they are whitespace naturally the compare fails.

See here for one definition of whitespace for C.

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