Bash Script to Find Sum of Digits of a Number

Use a Bash script that reads a number and sums its digits by looping through each digit with while and modulus operations, like: while [ $num -gt 0 ]; do sum=$((sum + num % 10)); num=$((num / 10)); done.

Examples

Input123

Output6

Input0

Output0

Input99999

Output45

How to Think About It

To find the sum of digits, think of the number as a collection of single digits. Extract each digit by taking the remainder when divided by 10, add it to a sum, then remove that digit by dividing the number by 10. Repeat until the number is zero.

Algorithm

1

Get the input number.

2

Initialize sum to zero.

3

While the number is greater than zero:

4

Extract the last digit using modulus 10.

5

Add the digit to sum.

6

Remove the last digit by dividing the number by 10.

7

Return the sum.

Code

bash

#!/bin/bash
read -p "Enter a number: " num
sum=0
while [ $num -gt 0 ]; do
  digit=$((num % 10))
  sum=$((sum + digit))
  num=$((num / 10))
done
echo "$sum"

Output

6

Dry Run

Let's trace input 123 through the code

1

Initial values

num=123, sum=0

2

First iteration

digit=3, sum=3, num=12

3

Second iteration

digit=2, sum=5, num=1

4

Third iteration

digit=1, sum=6, num=0

num	digit	sum
123	3	3
12	2	5
1	1	6

Why This Works

Step 1: Extract digit

Using num % 10 gets the last digit of the number.

Step 2: Add to sum

Add the extracted digit to the running total stored in sum.

Step 3: Remove digit

Divide the number by 10 using integer division to remove the last digit.

Alternative Approaches

Using string manipulation

bash

#!/bin/bash
read -p "Enter a number: " num
sum=0
for (( i=0; i<${#num}; i++ )); do
  digit=${num:i:1}
  sum=$((sum + digit))
done
echo "$sum"

This method treats the number as a string and sums digits by indexing, which is simpler but less arithmetic-focused.

Using awk command

bash

read -p "Enter a number: " num
echo "$num" | awk '{sum=0; for(i=1;i<=length($0);i++) sum+=substr($0,i,1); print sum}'

This uses awk to sum digits, useful for quick one-liners but depends on external tool availability.

Complexity: O(n) time, O(1) space

Time Complexity

The loop runs once per digit, so time grows linearly with the number of digits, O(n).

Space Complexity

Only a few variables are used regardless of input size, so space is constant, O(1).

Which Approach is Fastest?

Arithmetic looping is fast and memory efficient; string methods are easy but slightly slower due to string handling.

Approach	Time	Space	Best For
Arithmetic loop	O(n)	O(1)	Efficient numeric processing
String manipulation	O(n)	O(1)	Simple code, easy to read
awk command	O(n)	O(1)	Quick one-liners, external tool dependent