Prime Number Check by Command-Line Argument

#!/bin/bash
if [ $1 -le 0 ];
        then
        echo "Invalid input. Enter a positive number."
        exit 1
fi
for((i=2; i<$1/2; i++)){
        if [ `expr $1 % $i` -eq 0 ];
                then
                echo "$1 is not prime."
                exit 1
        fi
}
echo "$1 is prime."