The simplest version I can think of, works on bash and sh:

n=2
while [ $n -le 1000 ]; do
  d=2
  flag=1
  while [ $flag -eq 1 -a $((d*d)) -le $n ]; do
    if [ $((n % d)) -eq 0 ]; then
      flag=0
    else
      d=$((d+1))
    fi
  done
  if [ $flag -eq 1 ]; then
    echo $n
  fi
  n=$(( n + 1 ))
done