An attacking player is in the penalty are with just the goal keeper in front of them the attacker is tripped from behind by a defender What action would you take as a referee?

There is usually a bit of jostling during the set up for a corner kick, but if it's a bit too rough, the referee can verbally warn the players. If there is a blatant shove that the referee deems is a foul, the referee should blow his whistle and either caution (yellow card) the offending player or, if it is serious enough and violent enough, send off (red card) the offending player. (Naturally if it's a second caution, a send off will be issued.) After that issue is dealt with, the corner kick will be taken. The ball was not in play at the time of the foul. It will be put back in play the way it was going to be before the violation of the Laws took place. The violation of Law 12, the pushing, was dealt with in a manner consistent with the opinion of the referee. The corner is taken. Law 9 dealing with the ball being in or out of play (it was out of play) subordinates Law 12 as far as what happens regarding the resumption of play in this situation.