Program in UNIX to calculate the sum of digits

If a three-digit positive number is input through the keyboard, write a program to calculate the sum of digits.

echo "Enter a three digit number : "
read num

if [ $num -gt 999 -o $num -lt 0 ]
then
echo "Invalid number"
exit
fi

temp=$num
sum=0

while [ $num -ne 0 ]
do
r=`expr $num % 10`
sum=`expr $sum + $r`
num=`expr $num / 10`
done
echo "Sum of digits of $temp is $sum"