Bash Script to Print Hollow Square Pattern

Use nested loops in Bash: outer loop for rows and inner loop for columns, print '*' for borders and space for inside, e.g., for ((i=1;i<=n;i++)); do for ((j=1;j<=n;j++)); do if [[ $i -eq 1 || $i -eq n || $j -eq 1 || $j -eq n ]]; then echo -n '*'; else echo -n ' '; fi; done; echo; done.

Examples

Input3

Output*** * * ***

Input5

Output***** * * * * * * *****

Input1

Output*

How to Think About It

To print a hollow square, think of a grid with rows and columns. Print stars on the first and last rows and columns to form the border. For all other positions inside, print spaces to keep it hollow.

Algorithm

1

Get the size of the square from the user.

2

Loop through each row from 1 to size.

3

Inside each row, loop through each column from 1 to size.

4

If the current position is on the border (first or last row or column), print '*'.

5

Otherwise, print a space.

6

After each row, print a newline.

Code

bash

#!/bin/bash
read -p "Enter size of square: " n
for ((i=1; i<=n; i++)); do
  for ((j=1; j<=n; j++)); do
    if [[ $i -eq 1 || $i -eq n || $j -eq 1 || $j -eq n ]]; then
      echo -n '*'
    else
      echo -n ' '
    fi
  done
  echo
 done

Output

Enter size of square: 5
*****
*   *
*   *
*   *
*****

Dry Run

Let's trace the code for input size 3 through the loops.

1

First row (i=1)

j=1: border, print '*'; j=2: border, print '*'; j=3: border, print '*'

2

Second row (i=2)

j=1: border, print '*'; j=2: inside, print ' '; j=3: border, print '*'

3

Third row (i=3)

j=1: border, print '*'; j=2: border, print '*'; j=3: border, print '*'

Row i	Column j	Print
1	1	*
1	2	*
1	3	*
2	1	*
2	2
2	3	*
3	1	*
3	2
3	3	*

Why This Works

Step 1: Outer and inner loops

The outer loop runs through each row, and the inner loop runs through each column in that row.

Step 2: Border check

We check if the current position is on the first or last row or column using if conditions.

Step 3: Print star or space

If on border, print *; otherwise, print a space to create the hollow effect.

Alternative Approaches

Using single loop with arithmetic

bash

#!/bin/bash
read -p "Enter size: " n
for ((i=1; i<=n*n; i++)); do
  row=$(((i-1)/n+1))
  col=$(((i-1)%n+1))
  if [[ $row -eq 1 || $row -eq n || $col -eq 1 || $col -eq n ]]; then
    echo -n '*'
  else
    echo -n ' '
  fi
  if (( i % n == 0 )); then echo; fi
done

Uses one loop and arithmetic to calculate row and column; slightly less readable but compact.

Using printf for formatting

bash

#!/bin/bash
read -p "Enter size: " n
for ((i=1; i<=n; i++)); do
  line=""
  for ((j=1; j<=n; j++)); do
    if [[ $i -eq 1 || $i -eq n || $j -eq 1 || $j -eq n ]]; then
      line+="*"
    else
      line+=" "
    fi
  done
  printf "%s\n" "$line"
done

Builds each line as a string then prints it, which can be easier to modify or extend.

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

Time Complexity

The script uses two nested loops each running n times, so it performs n*n iterations, resulting in O(n^2) time.

Space Complexity

The script uses a fixed amount of extra memory regardless of input size, so space complexity is O(1).

Which Approach is Fastest?

All approaches have similar O(n^2) time, but single loop with arithmetic may be slightly faster due to fewer loop overheads, while building lines as strings improves readability.

Approach	Time	Space	Best For
Nested loops	O(n^2)	O(1)	Simplicity and clarity
Single loop with arithmetic	O(n^2)	O(1)	Compact code, slight speed gain
Building line string	O(n^2)	O(n)	Readability and easy modification