script to find prime number

#!/bin/bash

echo -n "Enter a number: "

readnum

expr $num + 1  &> /dev/null

if [ $? -ne 0 ]

then

echo "You did not supplied number value"

exit 1

fi

[ $num -lt 2 ] && echo "Values < 2 are not prime numbers" && exit 1

i=2

while [ $i -lt $num ]

do

if [ `expr $num % $i` -eq 0 ]

then

echo "$num is not a prime number"

echo "Since it is divisible by $i"

exit

fi

i=`expr $i + 1`

done

echo "$num is a prime number "

$ ./primecheck

Enter a number: 17

17 is a prime number