unity update only called once code example

Example 1: how to call something once in update

bool isDone = false;

void Update() {
    if (!isDone) {
        // put your code that runs once here
        isDone = true;
    }
}

Example 2: Start() Awake() Update()

Start()
{
	//Gets called on the first frame when the script is run
}
Update()
{
	//Gets called everyframe, for example if you wanted to 
    //increase a number every frame the do it here
}
Awake()
{
	//The same as start but can sometimes be more effiecient in some situations
}