Bash Script to Print Reverse Number Pattern

Use a nested loop in Bash: for ((i=n; i>=1; i--)); do for ((j=1; j<=i; j++)); do echo -n "$j "; done; echo; done to print a reverse number pattern.

Examples

Input3

Output1 2 3 1 2 1

Input5

Output1 2 3 4 5 1 2 3 4 1 2 3 1 2 1

Input1

Output1

How to Think About It

Start from the given number and count down to 1. For each number, print all numbers from 1 up to that number on one line. Repeat this until you reach 1.

Algorithm

1

Get the input number n

2

Start a loop from i = n down to 1

3

For each i, start another loop from j = 1 up to i

4

Print j followed by a space without a newline

5

After inner loop ends, print a newline

6

Repeat until i reaches 1

Code

bash

read -p "Enter a number: " n
for ((i=n; i>=1; i--)); do
  for ((j=1; j<=i; j++)); do
    echo -n "$j "
  done
  echo
 done

Output

Enter a number: 5
1 2 3 4 5 
1 2 3 4 
1 2 3 
1 2 
1 

Dry Run

Let's trace input 3 through the code

1

Outer loop starts with i=3

Print numbers 1 to 3: 1 2 3

2

Outer loop i=2

Print numbers 1 to 2: 1 2

3

Outer loop i=1

Print number 1: 1

i	Printed line
3	1 2 3
2	1 2
1	1

Why This Works

Step 1: Outer loop controls lines

The outer loop for ((i=n; i>=1; i--)) counts down from the input number to 1, deciding how many numbers to print on each line.

Step 2: Inner loop prints numbers

The inner loop for ((j=1; j<=i; j++)) prints numbers from 1 up to the current line number i.

Step 3: Print newline after each line

After printing numbers for one line, echo prints a newline to move to the next line.

Alternative Approaches

Using while loops

bash

read -p "Enter a number: " n
i=$n
while [ $i -ge 1 ]; do
  j=1
  while [ $j -le $i ]; do
    echo -n "$j "
    ((j++))
  done
  echo
  ((i--))
done

Uses while loops instead of for loops; slightly longer but same logic.

Using seq command

bash

read -p "Enter a number: " n
for ((i=n; i>=1; i--)); do
  seq -s ' ' 1 $i
 done

Uses the seq command to print numbers, simpler inner loop but depends on external command.

Complexity: O(n^2) time, O(1) space

Time Complexity

The nested loops run approximately n + (n-1) + ... + 1 times, which sums to O(n^2).

Space Complexity

The script uses constant extra space for counters and printing, so O(1).

Which Approach is Fastest?

Using built-in commands like seq can be faster but depends on system calls; pure loops are portable and efficient enough for small n.

Approach	Time	Space	Best For
For loops	O(n^2)	O(1)	Simple, portable scripts
While loops	O(n^2)	O(1)	Alternative control flow
seq command	O(n^2)	O(1)	Simpler code, depends on external command