how to move forward forever

hi i need help with a problem i cant find a script to make my char move forward forever (also a code for moving left and right would be good

Without physic :

var dir : Vector3;
var speed : float;
function Update(){
    transform.position += dir.normalized * speed;
}

With physic :

var dir : Vector3;
var speed : float;
function FixedUpdate(){
    rigidbody.velocity = dir.normalized * speed;
}

or

var dir : Vector3;
var speed : float;
function Start(){
    ConstantForce cf = gameObject.AddComponent( ConstantForce );
    cf.relativeForce = dir.normalized * speed;
}

or

var dir : Vector3;
var speed : float;
function Start(){
    rigidbody.velocity = dir.normalized * speed;
    rigidbody.useGravity = false;
    rigidbody.drag = 0;
}

Warning : air code, there might be mistakes.