1 Samuel 17:20

And David rose up early in the morning, and left the sheep with a keeper, and took, and went, as Jesse had commanded him; and he came to the trench, as the host was going forth to the fight, and shouted for the battle.

Ver. 20. And left the sheep with a keeper.] A commendable care: neither was there cause that Eliab should so check and chide him for the contrary. 1Sa 17:28

And shouted for the battle.] This was the old way of beginning the fight: so to show their courage, and to fright the enemy.