Bash Script to Subtract Two Numbers Easily

Use arithmetic expansion in Bash like result=$((num1 - num2)) to subtract two numbers and then print the result with echo.

Examples

Inputnum1=10, num2=5

Output5

Inputnum1=100, num2=25

Output75

Inputnum1=7, num2=10

Output-3

How to Think About It

To subtract two numbers in Bash, first get the two numbers as input or variables, then use the arithmetic expansion syntax $(( )) with the minus operator - to calculate the difference, and finally print the result.

Algorithm

1

Get the first number and store it in a variable.

2

Get the second number and store it in another variable.

3

Calculate the difference using arithmetic expansion with the minus operator.

4

Print the result to the screen.

Code

bash

#!/bin/bash

num1=15
num2=8
result=$((num1 - num2))
echo "$result"

Output

7

Dry Run

Let's trace subtracting 15 and 8 through the code

1

Assign num1

num1=15

2

Assign num2

num2=8

3

Calculate result

result=$((15 - 8)) = 7

4

Print result

echo 7

Variable	Value
num1	15
num2	8
result	7

Why This Works

Step 1: Arithmetic Expansion

The $(( )) syntax tells Bash to calculate the expression inside as a math operation.

Step 2: Subtraction Operator

Inside the arithmetic expansion, the - operator subtracts the second number from the first.

Step 3: Output Result

The echo command prints the calculated difference to the terminal.

Alternative Approaches

Using expr command

bash

#!/bin/bash
num1=20
num2=5
result=$(expr $num1 - $num2)
echo "$result"

Uses external command expr; less efficient but works in older shells.

Using let command

bash

#!/bin/bash
num1=12
num2=7
let result=num1-num2
echo "$result"

Uses built-in let for arithmetic; simpler but less flexible for complex expressions.

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

Time Complexity

Subtraction is a single arithmetic operation, so it runs in constant time.

Space Complexity

Only a few variables are used, so space usage is constant.

Which Approach is Fastest?

Using arithmetic expansion $(( )) is fastest as it is built into Bash, unlike external commands like expr.

Approach	Time	Space	Best For
Arithmetic Expansion $(( ))	O(1)	O(1)	Fast, modern Bash scripts
expr command	O(1)	O(1)	Compatibility with older shells
let command	O(1)	O(1)	Simple arithmetic in Bash