else if is ignored if not Serial.println anything before

Question

I encountered to me inexplicable behavior on an arduino uno. I'm serial reading a string from the serial monitor/python serial. This string is the conditional on several if/else if statements. The crazy thing is, some strings just get ignored, but if I print out the read string after reading it, the ignored statements then get executed and the script works the way it's supposed to. This is the code:

// power outlet 1
# define OUT_1_1 2 // relay #4
# define OUT_1_2 3 // relay #3

// power outlet 2
# define OUT_2_1 4 // relay #2
# define OUT_2_2 5 // relay #1


void setup() {
  pinMode(LED_BUILTIN, OUTPUT);

  // power outlet 1
  pinMode(OUT_1_1, OUTPUT);
  pinMode(OUT_1_2, OUTPUT);

  // power outlet 2
  pinMode(OUT_2_1, OUTPUT);
  pinMode(OUT_2_2, OUTPUT);

  // init outputs to HIGH
  digitalWrite(OUT_1_1, HIGH);
  digitalWrite(OUT_1_2, HIGH);
  digitalWrite(OUT_2_1, HIGH);
  digitalWrite(OUT_2_2, HIGH);
  
  Serial.begin(9600);
}

void loop() {
  String str = "";
    
  // checking for bytes in the buffer
  if (Serial.available() > 0) {
    str = Serial.readStringUntil("\n");
  }
  // Serial.println(str); // if this line is uncommented serial input "2_ON\n" and "2_OFF\n" is recognized and outled 2 is successfully switched


  // identity
  if (str == "IDN\n") {
    Serial.println("Serial Power Outlet");
  }

  // controlling power outlet 1
  else if (str == "1_ON\n") {
    digitalWrite(OUT_1_1, LOW);
    digitalWrite(OUT_1_2, LOW);
  }
  else if (str == "1_OFF\n") {
    digitalWrite(OUT_1_1, HIGH);
    digitalWrite(OUT_1_2, HIGH);
  }
  
  // controlling power outlet 2
  else if (str == "2_ON\n") {
    digitalWrite(OUT_2_1, LOW);
    digitalWrite(OUT_2_2, LOW);
  }
  else if (str == "2_OFF\n") {
    digitalWrite(OUT_2_1, HIGH);
    digitalWrite(OUT_2_2, HIGH);
  }
  delay(1000);
}

If I serial input 2_ON\n or 2_OFF\n the according else if blocks are just ignored (all other inputs work correctly).

But after I've uncommented // Serial.println(str); these blocks are executed the way they are supposed to and all works just fine.

Where is my mistake, what am I getting wrong here? These blocks also get executed if I Serial.println any random output. Why do I have to Serial.println something first so my script works according to plan? What am I missing?

EDIT: added all of the code.

Answer 1

Even though you read until \n, that just controls the ending of your buffered string. It does not control it's start. There may be some garbage before the command that you have not cut out. In other words, I suggest you check if the string endsWith as opposed to == the command string.

As for what noise could be causing garbage or why activity on the tx side of the serial cleaned it up, i do not know. Heck something might just be weird enough in the wiring that '2' is just the set of bits needed to aggravate it.