Big numbers not recognised as integers by bash?

Question

So I am trying to write a script that prints the sum of every digit in a number, to the power of another number.

So for example, if the input is: script 123 2 than the script calculates 1^2 + 2^2 + 3^2 and outputs 14.

My script:

Num=$1
s=0
while [ $Num -gt 0 ]
do
   k=$(( $Num % 10 ))

   Num=$(( $Num / 10 ))

   s=$(( $s + ($k**$2)))
done
echo $s

This works for smaller numbers, but numbers like "3706907995955475988644381" cause the following error: line 5: [: 3706907995955475988644381: integer expression expected

How can I make this script work with big numbers?

Answer 1

You can handle the input as a string, calculate its length and use substring parameter expansion to extract single digits.

Num=$1
Exp=$2
s=0
# string length
i=${#Num}
while [ $i -gt 0 ]
do
   # index is 0-based, decrement before using
   : $((i--))
   # 1 character at index i
   k=${Num:$i:1}

   s=$(( $s + ($k**$Exp)))
done
echo $s

Note: The code lacks error handling. You might get unexpected results if the input contains non-digit characters, and the calculation might overflow for very long numbers.