Program in UNIX to check entered number is prime or not.

Write a shell script to check entered number is prime or not.
echo Enter a number to check it is prime or not.
read no
ct=2
if [ $no -le 1 ]
then
echo Invalid number!!!!!
else
while [ $ct -le `expr $no / 2` ]
do
if [ `expr $no % $ct` -eq 0 ]
then
echo $no is not prime.
exit
fi
ct=`expr $ct + 1`
done
echo $no is prime.
fi