Here what I am using in my game:

I found it somewhere:

function Start ()
{
   // Set all animations to loop
   animation.wrapMode = WrapMode.Loop;
   // except shooting
   animation["shoot"].wrapMode = WrapMode.Once;

   // Put idle and walk into lower layers (The default layer is always 0)
   // This will do two things
   // - Since shoot and idle/walk are in different layers they will not affect
   //   each other's playback when calling CrossFade.
   // - Since shoot is in a higher layer, the animation will replace idle/walk
   //   animations when faded in.
   animation["shoot"].layer = 1;

   // Stop animations that are already playing
   //(In case user forgot to disable play automatically)
   animation.Stop();
}

function Update () {
   // Based on the key that is pressed,
   // play the walk animation or the idle animation
   if (Mathf.Abs(Input.GetAxis("Vertical")) > 0.1){
      animation.CrossFade("walk");
      }

       else if (Mathf.Abs(Input.GetAxis("Horizontal")) > 0.1){
      animation.CrossFade("walk");
      }

   else
      animation.CrossFade("idle");

   // Shoot
   if (Input.GetButtonDown ("Jump"))
   // animation.CrossFade("walk");
      animation.Play("jumping");
} 

For your request to play something once: pay attention to the line below

animation["shoot"].wrapMode = WrapMode.Once;

I hope this helps and thanks to the original writer of the script NOT me