Bash Script to Convert Octal to Decimal Number

Use Bash's built-in arithmetic expansion with $((8#octal_number)) to convert an octal number to decimal, for example: decimal=$((8#123)).

Examples

Input10

Output8

Input123

Output83

Input0

Output0

How to Think About It

To convert octal to decimal in Bash, treat the input as a base-8 number and use Bash's arithmetic expansion with the base prefix 8#. This tells Bash to interpret the number as octal and convert it to decimal automatically.

Algorithm

1

Get the octal number input as a string.

2

Use Bash arithmetic expansion with <code>8#</code> prefix to interpret the input as octal.

3

Store the result as a decimal number.

4

Print the decimal number.

Code

bash

#!/bin/bash
read -p "Enter an octal number: " octal
# Convert octal to decimal using base 8 prefix
decimal=$((8#$octal))
echo "Decimal value: $decimal"

Output

Enter an octal number: 123
Decimal value: 83

Dry Run

Let's trace the input '123' through the code

1

Read input

octal='123'

2

Convert using arithmetic expansion

decimal=$((8#123)) which equals 83

3

Print result

Output: 'Decimal value: 83'

Octal Input	Decimal Output
123	83

Why This Works

Step 1: Using base prefix

The 8# prefix tells Bash to treat the following number as base 8 (octal).

Step 2: Arithmetic expansion

Bash evaluates $((8#number)) and converts it to decimal automatically.

Step 3: Output the decimal

The converted decimal value is stored and printed to the user.

Alternative Approaches

Using printf

bash

read -p "Enter octal: " octal
decimal=$(printf "%d" "$octal")
echo "Decimal: $decimal"

Uses printf to convert octal string to decimal; simpler but less flexible for invalid input.

Using bc calculator

bash

read -p "Enter octal: " octal
decimal=$(echo "ibase=8; $octal" | bc)
echo "Decimal: $decimal"

Uses bc for conversion; useful if you want to handle very large numbers or more complex math.

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

Time Complexity

Conversion is done in constant time since Bash arithmetic expansion handles it internally without loops.

Space Complexity

Only a few variables are used to store input and output, so space is constant.

Which Approach is Fastest?

Using Bash arithmetic expansion is fastest and simplest; printf and bc add overhead but offer alternatives.

Approach	Time	Space	Best For
Bash arithmetic expansion	O(1)	O(1)	Simple, fast conversion
printf	O(1)	O(1)	Quick conversion with built-in tool
bc calculator	O(1)	O(1)	Handling large numbers or complex math