Bash shell script to find sum of digits

Given a number Num, find the sum of digits of the number.
Examples: 

Input : 
444
Output :
sum of digits of 444 is : 12

Input :
34
Output :
sum of digits of 34 is : 7

Approach:

1. Divide the number into single digits
2. Find the sum of digits .

Bash

# !/bin/bash

# Program to find sum
# of digits

# Static input of the
# number
Num=123
g=$Num

# store the sum of
# digits
s=0

# use while loop to
# calculate the sum
# of all digits
while [ $Num -gt 0 ]
do
    # get Remainder
    k=$(( $Num % 10 ))

    # get next digit
    Num=$(( $Num / 10 ))

    # calculate sum of
    # digit
    s=$(( $s + $k ))
done
echo  "sum of digits of $g is : $s"

Output:

sum of digits of 123 is : 6