Bash Script to Reverse a Number with Output and Explanation

Use read number; echo "$number" | rev in Bash to reverse a number by treating it as a string and reversing its characters.

Examples

Input12345

Output54321

Input1000

Output0001

Input7

Output7

How to Think About It

To reverse a number in Bash, think of the number as a string of characters. Then reverse the order of these characters to get the reversed number. This works because reversing digits is like reversing letters in a word.

Algorithm

1

Get the input number as a string.

2

Reverse the string characters.

3

Print the reversed string as the reversed number.

Code

bash

read -p "Enter a number: " number
reversed=$(echo "$number" | rev)
echo "Reversed number: $reversed"

Output

Enter a number: 12345
Reversed number: 54321

Dry Run

Let's trace input 12345 through the code

1

Read input

number="12345"

2

Reverse string

echo "12345" | rev outputs "54321"

3

Print result

Print "Reversed number: 54321"

Input Number	Reversed Output
12345	54321

Why This Works

Step 1: Input as string

The number is read as a string so we can manipulate its characters easily with Bash tools.

Step 2: Use rev command

The rev command reverses the order of characters in the string, effectively reversing the digits.

Step 3: Output reversed number

The reversed string is printed as the reversed number, showing the digits in reverse order.

Alternative Approaches

Using a loop to reverse digits

bash

read -p "Enter a number: " num
rev_num=0
while [ $num -gt 0 ]; do
  digit=$(( num % 10 ))
  rev_num=$(( rev_num * 10 + digit ))
  num=$(( num / 10 ))
done
echo "Reversed number: $rev_num"

This method treats the number as an integer and reverses digits mathematically, avoiding string commands but losing leading zeros.

Using parameter expansion

bash

read -p "Enter a number: " num
rev_num=""
for (( i=${#num}-1; i>=0; i-- )); do
  rev_num+="${num:i:1}"
done
echo "Reversed number: $rev_num"

This uses Bash string slicing to reverse characters manually, useful if <code>rev</code> is not available.

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

Time Complexity

Reversing the string takes time proportional to the number of digits, so O(n).

Space Complexity

The reversed string requires extra space proportional to the input size, so O(n).

Which Approach is Fastest?

Using rev is fastest and simplest. Loop-based reversal is slower but works without external commands.

Approach	Time	Space	Best For
Using rev command	O(n)	O(n)	Simple and fast string reversal
Loop with math operations	O(n)	O(1)	Numeric reversal without leading zeros
Bash parameter expansion loop	O(n)	O(n)	Pure Bash string reversal without external tools