Linux Shell Scripting Tutorial – Module 5: Functions in Bash

Module 5: Functions in Bash

Functions are reusable blocks of code that group commands together.
Instead of rewriting the same code multiple times, you can put it inside a function
and call it whenever needed. Functions improve script structure and reduce errors.

✅ Defining a Function

Basic syntax:

function_name () {
  commands
}
# OR
function function_name {
  commands
}
  

✅ Example: Simple Function

#!/bin/bash
hello_world() {
  echo "Hello, World from function!"
}

# Call the function
hello_world
  

Output:

Hello, World from function!
  

✅ Functions with Arguments

• $1, $2, … → Function parameters
• $# → Number of arguments
• $@ → All arguments

#!/bin/bash
greet() {
  echo "Hello, $1! Welcome to $2."
}

greet "Alice" "Linux Scripting"
  

Output:

Hello, Alice! Welcome to Linux Scripting.
  

✅ Returning Values

Functions can return status codes (0 for success, non-zero for error).
For actual values, use echo and command substitution.

#!/bin/bash
add_numbers() {
  result=$(( $1 + $2 ))
  echo $result
}

sum=$(add_numbers 5 10)
echo "Sum is: $sum"
  

Output:

Sum is: 15
  

✅ Local Variables

Use local inside a function to avoid overwriting global variables.

#!/bin/bash
my_func() {
  local msg="Hello"
  echo $msg
}

msg="Hi"
my_func
echo $msg
  

Output:

Hello
Hi
  

✅ Nested Functions

#!/bin/bash
outer() {
  echo "Outer function"
  inner
}

inner() {
  echo "Inner function"
}

outer
  

✅ Summary

• Functions group reusable code into blocks.
• Arguments are passed using $1, $2, etc.
• Use echo with command substitution to return values.
• Use local for variables inside functions.
• Functions can call other functions, making scripts modular.

✅ By the end of this module, you can create, use, and manage functions in Bash,
making your scripts cleaner and more powerful.

Module 5: Bash Function Assignments

These assignments will help you practice using functions, arguments, local variables, and nested functions in Bash. Try solving them practically to reinforce your learning.

✅ Assignment 1: Greeting Multiple Users

Write a function that greets multiple users by reading names from command-line arguments. Use $@ to process all arguments and print a greeting for each user.

Example Output:

Hello, Alice!
Hello, Bob!
Hello, Charlie!

✅ Assignment 2: Calculator Function

Write a function calculator that takes three arguments: two numbers and an operator (+, -, *, /) and returns the result using echo. Use command substitution to capture the result.

Example Call:

result=$(calculator 10 + 5)
echo "Result: $result"

Expected Output:

Result: 15

✅ Assignment 3: Check File Type

Write a function check_file that takes a filename as an argument and prints whether it is a regular file, directory, or something else using conditional statements.

Example Call:

check_file myfile.txt

Expected Output:

myfile.txt is a regular file

✅ Assignment 4: Library Book Tracker

Write a simple library system using functions:

• add_book: Adds a book to a file books.txt
• list_books: Lists all books in books.txt
• search_book: Searches for a book by name

Example:

add_book "The Alchemist"
add_book "Linux Essentials"
list_books
search_book "Alchemist"

Expected Output:

Books in library:
1. The Alchemist
2. Linux Essentials

Book found: The Alchemist

✅ Assignment 5: Factorial Calculator

Write a recursive function factorial that calculates the factorial of a number. Use nested function calls and return the result using echo.

Example Call:

result=$(factorial 5)
echo "Factorial of 5 is $result"

Expected Output:

Factorial of 5 is 120