Commentary of Charles Spurgeon, Adam Clarke & Matthew Henry
When Saul and all Israel heard those words of the Philistine, they were dismayed, and greatly afraid. 
Now David was the son of that Ephrathite of Bethlehemjudah, whose name was Jesse; and he had eight sons: and the man went among men for an old man in the days of Saul.
And the three eldest sons of Jesse went and followed Saul to the battle: and the names of his three sons that went to the battle were Eliab the firstborn, and next unto him Abinadab, and the third Shammah. And David was the youngest: and the three eldest followed Saul.
But David went and returned from Saul to feed his father's sheep at Bethlehem. And the Philistine drew near morning and evening, and presented himself forty days.
And Jesse said unto David his son, Take now for thy brethren an ephah of this parched corn, and these ten loaves, and run to the camp to thy brethren; And carry these ten cheeses unto the captain of their thousand, and look how thy brethren fare, and take their pledge.
Vs.11-18
The present state of his family. His father was old. He went among men for an old man,
was taken notice of for his great age, above what was usual at that
time, and therefore was excused from pubic services, and went not in
person to the wars, but sent his sons; he had the honors paid him that
were due his age, his hoary head was a 
crown of glory to him.
David's
three elder brethren, who perhaps envied his place at the court, got
their father to send for him home, and let them go to the camp, where
they hoped to signalize themselves and eclipse him.
The orders his father gave him to go and visit his brethren in the camp.
He did not himself ask leave to go, to satisfy his curiosity, or to
gain experience and make observations; but his father sent him on a mean
and homely errand, on which any of his servants might have gone.
Perhaps David, like Joseph, had formerly brought to his father their
evil report, and now he sends him to enquire concerning their manners.
See the care the pious parents about their children when they are abroad
from them, especially in places of temptation; they are solicitous how
they conduct themselves, and particularly what company they keep.
Let
children think of this, and conduct themselves accordingly, remembering
that, when they are from under their parents' eye, they are still under
God's eye.
