John Gill Commentary 1 Samuel 17:19

"Now Saul, and they, and all the men of Israel, were in the vale of Elah, fighting with the Philistines." — 1 Samuel 17:19 (ASV)

Now Saul, and they
That is, the sons of Jesse, and brethren of David:

and all the men of Israel ;
the soldiers in the army:

were in the valley Elah ;
or "by" it, near unto it; for they were set in array on the mountain on the side of it:

fighting with the Philistines ;
not actually engaged in battle, but drawn up for it; prepared and in readiness to engage whenever it was necessary, or they were obliged to it; and perhaps there might be now and then some skirmishes in the outer parts of the camp.