Recursive functions in Bash Scripting - Practice Problems & Coding Challenges

Challenge - 5 Problems

💻 Command Output

intermediate

Output of a simple recursive factorial function

What is the output of this Bash script when run?

Bash Scripting

#!/bin/bash
factorial() {
  if [ "$1" -le 1 ]; then
    echo 1
  else
    local prev=$(factorial $(( $1 - 1 )))
    echo $(( $1 * prev ))
  fi
}
factorial 5

A 120

B 24

C Error: command not found

D 1

💡 Hint

Think about how factorial is defined and how recursion multiplies numbers down to 1.

The function calls itself with decreasing values until it reaches 1, then multiplies back up: 5*4*3*2*1 = 120.

💻 Command Output

intermediate

Output of recursive Fibonacci function

What does this Bash script print when executed?

Bash Scripting

#!/bin/bash
fib() {
  if [ "$1" -le 1 ]; then
    echo $1
  else
    local a=$(fib $(( $1 - 1 )))
    local b=$(fib $(( $1 - 2 )))
    echo $(( a + b ))
  fi
}
fib 6

A Error: maximum recursion depth exceeded

B 13

C 5

D 8

💡 Hint

Remember the Fibonacci sequence starts 0,1,1,2,3,5,8...

fib(6) = fib(5) + fib(4) = 5 + 3 = 8.

📝 Syntax

advanced

Identify the syntax error in this recursive function

Which option correctly identifies the syntax error in this Bash recursive function?

Bash Scripting

#!/bin/bash
countdown() {
  if [ $1 -le 0 ]
    echo "Done!"
  else
    echo "$1"
    countdown $(( $1 - 1 ))
  fi
}
countdown 3

A Using echo inside recursion is not allowed

B Incorrect use of $(( )) for arithmetic

C Missing 'then' after the if condition

D Missing closing brace '}' for the function

💡 Hint

Check the if statement syntax carefully.

In Bash, an if statement must have 'then' after the condition. It's missing here.

🚀 Application

advanced

What is the output of this recursive directory listing script?

Given this Bash script that recursively lists files, what will it output when run in a directory with files 'a.txt' and subdirectory 'sub' containing 'b.txt'?

Bash Scripting

#!/bin/bash
list_files() {
  for item in "$1"/*; do
    if [ -d "$item" ]; then
      list_files "$item"
    else
      echo "$item"
    fi
  done
}
list_files "."

A

./a.txt
./sub/b.txt

B

a.txt
b.txt

C

./sub
./a.txt
./sub/b.txt

D Error: infinite recursion

💡 Hint

The script prints file paths, not directories.

The script prints only files, recursively descending into directories. It prints './a.txt' and './sub/b.txt'.

🔧 Debug

expert

Why does this recursive function cause a stack overflow?

Examine this Bash recursive function. Why does it cause a stack overflow when called with 3?

Bash Scripting

#!/bin/bash
infinite() {
  echo "$1"
  infinite $1
}
infinite 3

A The arithmetic expansion is missing in the recursive call

B The function calls itself with the same argument, causing infinite recursion

C Bash does not support recursion

D The function lacks a base case to stop recursion

💡 Hint

Think about what stops recursion normally.

The function calls itself with the same argument endlessly, no base case to stop it, causing stack overflow.

Practice

1. What is the main purpose of a base case in a recursive Bash function?

easy

A. To increase the input value for the next call

B. To call the function again with the same input

C. To print the current value of a variable

D. To stop the recursion and prevent infinite loops

Solution

1. Step 1: Understand recursion basics

   A recursive function calls itself repeatedly with smaller inputs.

2. Step 2: Role of base case

   The base case stops the recursion to avoid infinite loops and eventual crash.

3. Final Answer:

   To stop the recursion and prevent infinite loops -> Option D

4. Quick Check:

   Base case = stop recursion [OK]

Hint: Base case always stops recursion to avoid endless calls [OK]

Common Mistakes:

• Forgetting the base case causes infinite recursion
• Using base case to increase input instead of stopping
• Confusing base case with recursive step

2. Which of the following is the correct syntax to define a recursive function named countdown in Bash?

easy

A. countdown() { while [ $1 -gt 0 ]; do echo $1; (( $1-- )); done; echo Done; }

B. def countdown(n): if n <= 0: print('Done') else: print(n) countdown(n-1)

C. function countdown() { if [[ $1 -le 0 ]]; then echo Done; else echo $1; countdown $(( $1 - 1 )); fi }

D. function countdown { echo $1; countdown $(( $1 - 1 )); }

Solution

1. Step 1: Identify Bash function syntax

   Bash functions use the syntax: function name() { commands; } or name() { commands; }.

2. Step 2: Check recursive call and base case

   function countdown() { if [[ $1 -le 0 ]]; then echo Done; else echo $1; countdown $(( $1 - 1 )); fi } correctly defines a function with a base case (if $1 <= 0) and recursive call countdown $(( $1 - 1 )).

3. Final Answer:

   function countdown() { if [[ $1 -le 0 ]]; then echo Done; else echo $1; countdown $(( $1 - 1 )); fi } -> Option C

4. Quick Check:

   Bash recursion syntax = function countdown() { if [[ $1 -le 0 ]]; then echo Done; else echo $1; countdown $(( $1 - 1 )); fi } [OK]

Hint: Look for Bash syntax with base case and recursive call [OK]

Common Mistakes:

• Using Python syntax in Bash function
• Missing base case causing infinite recursion
• Incorrect function declaration without parentheses

3. What will be the output of this Bash script?

factorial() {
  if [[ $1 -le 1 ]]; then
    echo 1
  else
    local prev=$(factorial $(( $1 - 1 )))
    echo $(( $1 * prev ))
  fi
}
factorial 3

medium

A. 1

B. 6

C. 3

D. 9

Solution

1. Step 1: Understand factorial recursion

   factorial(3) calls factorial(2), which calls factorial(1). factorial(1) returns 1.

2. Step 2: Calculate factorial values

   factorial(2) = 2 * 1 = 2; factorial(3) = 3 * 2 = 6.

3. Final Answer:

   6 -> Option B

4. Quick Check:

   3! = 6 [OK]

Hint: Factorial n = n * factorial(n-1), factorial(1) = 1 [OK]

Common Mistakes:

• Confusing multiplication order
• Forgetting base case returns 1
• Assuming output is input value

4. Identify the error in this recursive Bash function that calculates the sum of numbers from 1 to n:

sum() {
  if [[ $1 -eq 0 ]]; then
    echo 0
  else
    echo $(( $1 + sum $(( $1 - 1 )) ))
  fi
}
sum 3

medium

A. Using echo inside arithmetic expression causes error

B. Missing base case for $1 == 1

C. Function name 'sum' is a reserved keyword

D. Incorrect decrement syntax $(( $1 - 1 ))

Solution

1. Step 1: Analyze recursive call inside arithmetic

   The function calls sum recursively inside $(( ... )), but sum outputs text via echo, not a number directly.

2. Step 2: Understand Bash arithmetic limitations

   Bash arithmetic expects numbers, but echo outputs string with newline, causing error.

3. Final Answer:

   Using echo inside arithmetic expression causes error -> Option A

4. Quick Check:

   Echo inside $(( )) = error [OK]

Hint: Avoid echo inside $(( )); use command substitution instead [OK]

Common Mistakes:

• Assuming echo returns a number usable in arithmetic
• Ignoring base case correctness
• Thinking function name 'sum' is reserved

5. You want to write a recursive Bash function list_files that prints all files in a directory and its subdirectories. Which approach correctly handles recursion and avoids infinite loops?

hard

A. Check if the argument is a directory; if yes, list files and call list_files on each subdirectory; else print the file name

B. Call list_files on the directory without checking type, then print the directory name

C. Use a loop to list files only in the current directory without recursion

D. Call list_files on all files and directories without base case

Solution

1. Step 1: Understand recursive directory traversal

   To list files recursively, check if the argument is a directory; if so, process contents recursively.

2. Step 2: Avoid infinite loops by checking file type

   Only recurse into directories; print file names directly to avoid infinite recursion.

3. Final Answer:

   Check if the argument is a directory; if yes, list files and call list_files on each subdirectory; else print the file name -> Option A

4. Quick Check:

   Directory check + recursive call = correct recursion [OK]

Hint: Always check if input is directory before recursive call [OK]

Common Mistakes:

• Recursing without checking if input is directory
• Missing base case causing infinite recursion
• Not printing files when input is a file