String commands for Arduino sketch kind of working but not really

Question 1

So I modified my sketch with the following code, and now it seems to processing the serial commands the way I want it too.

 // flow_A LED
 int led = 4;

 // relay_A
 const int RELAY_A = A0;

 // variables from sketch example
 String inputString = ""; // a string to hold incoming data
 boolean stringComplete = false; // whether the string is complete

 // variables from SO thread
 boolean LED_state = false;
 boolean vavle_open = false;

// flowmeter shit
unsigned long totalCount = 0;
unsigned long previousCount = 0;
int pin = 2;
unsigned long duration;

// storage variable for the timer
unsigned long previousMillis=0;
int interval=1000; //in milliseconds

// counters for each flowmeter
unsigned long countA = 0;

void setup() {
   // initialize serial
   Serial.begin(9600); // open serial port, sets data rate to 115200bps
   // Serial.println("Power on test - println");
   // line below is for iPhone testing
   // Serial.write("Power on test - write");
   inputString.reserve(200);

   pinMode(RELAY_A, OUTPUT);

   // flowmeter shit
   pinMode(pin, INPUT);

}

void open_valve() {

  digitalWrite(RELAY_A, HIGH); // turn RELAY_A on
  // Serial.println("Valve Open");
  Serial.write("{valve_open}");
}

void close_valve() {
  digitalWrite(RELAY_A, LOW); // turn RELAY_A off
  // Serial.println("Vavle Closed");
  Serial.write("{valve_close}");
}

void flow_A_blink() {

  digitalWrite(led, HIGH); // turn the LED on (HIGH is the voltage level)
  delay(1000);              // wait for one second
  digitalWrite(led, LOW);   // turn the LED off by making the voltage LOW
  delay(1000);              // wait for a second
}

void flow_A_blink_stop() {

  digitalWrite(led, LOW);
}

void getFlow() {

  duration = pulseIn(pin, HIGH);
  Serial.print(duration);
  Serial.println("");
  delay(200);
}

/*
 * Main program loop, runs over and over repeatedly
 */

void loop() {

 //print the string when a newline arrives:
 if(stringComplete) {
//    Serial.println(inputString);

    if(inputString.equals("{open_valve}\n")) {
//       Serial.println("opening valve.");
       open_valve();
    }

    if(inputString.equals("{open_valve}")) {
//       Serial.println("opening valve.");
       open_valve();
    }

    if(inputString.equals("{close_valve}\n")) {
//      Serial.println("close vavle.");
      close_valve();
    }

    if(inputString.equals("{close_valve}")) {
//      Serial.println("close vavle.");
      close_valve();
    }

  // clear the string:
  inputString = "";
  stringComplete = false;
  }
}

/*
 SerialEvent occurs whenever a new data comes in the
 hardware serial RX.  This routine is run between each
 time loop() runs, so using delay inside loop can delay
 response.  Multiple bytes of data may be available.
*/

void serialEvent() {
  while(Serial.available()) {
    // get the new byte:
    char inChar = (char)Serial.read();
    // add it to the inputString:
    inputString += inChar;
    // if the incoming character is a newline, set a flag
    // so the main loop can do something about it:
    if (inChar == '\n') {
      stringComplete = true;
    }
    // Serial.println(inputString.length());
  }
}

Question 2

Your code should not compile as pasted here. The checkForCorrectCommand function does not have a return value for both the match and unmatch. Your code shows you intend to empty the inputString buffer for both the match and non matched case. If the string match is not true, you want to leave the input buffer unchanged so that the following test cases can run.

bool checkForCorrectCommand(String cmd) {  

  if(inputString == cmd) { 
    // for match case, the string is consumed from the buffer
    inputString = "";
    stringComplete = false;
    return true;
  } 
  else {
    // for the non-match case, leave the buffer for further Rx or further tests
    return false;
  }

Question 3

The note from jdr5ca is quite correct. The checkForCorrectCommand routine needed an else clause with a separate return statement. The solution devised by Chris is good. It is clearly better to only process the contents of inputString if it is complete and discard it (the contents of inputString) after checking for the valid commands. I would like to offer a minor change to serialEvent.

The serialEvent routine should not keep adding characters to a string that is already complete. Instead it should leave them in the buffer to help form the next command. See the code below.

void serialEvent() {
  if (stringComplete)
    return;
  while(Serial.available()) {
    // get the new byte:
    char inChar = (char)Serial.read();
    // add it to the inputString:
    inputString += inChar;
    // if the incoming character is a newline, set a flag
    // so the main loop can do something about it:
    if (inChar == '\n') {
      stringComplete = true;
    }
  }
}