Functions are essential for writing organized, maintainable Bash scripts. They allow you to group commands into reusable blocks, making your scripts cleaner and more modular. In this section, we'll explore how to define, use, and manage functions in Bash.

Function Basics

A Bash function is a block of reusable code designed to perform a particular task. Here's the basic syntax:

function_name() {
    # Commands
}

Alternatively, you can use the function keyword:

function function_name {
    # Commands
}

Both styles are functionally equivalent, but the first form is more portable across different shells.

Here's a simple example of a function that says hello:

say_hello() {
    echo "Hello, world!"
}

# Call the function
say_hello

Functions with Parameters

Functions can accept parameters, which are accessed using positional parameters similar to how scripts access command-line arguments:

greet() {
    echo "Hello, $1!"
}

# Call with an argument
greet "John"  # Output: Hello, John!

You can access multiple parameters:

print_info() {
    echo "Name: $1"
    echo "Age: $2"
    echo "Occupation: $3"
}

# Call with multiple arguments
print_info "Jane" "30" "Engineer"

Inside a function, you can use the special parameters:

$1, $2, $3, ... for individual parameters
$@ for all parameters as separate strings
$* for all parameters as a single string
$# for the number of parameters

Example using these special parameters:

function process_args() {
    echo "Number of arguments: $#"
    echo "All arguments: $@"

    echo "Processing each argument:"
    for arg in "$@"; do
        echo "- $arg"
    done
}

# Call the function with multiple arguments
process_args apple banana cherry

Return Values

Unlike functions in many programming languages, Bash functions don't return values with a return statement in the traditional sense. Instead, they can:

Return an exit status (a number between 0 and 255)
Output values that can be captured
Set global variables

Using exit status:

is_even() {
    if (( $1 % 2 == 0 )); then
        return 0  # Success (true in Bash)
    else
        return 1  # Failure (false in Bash)
    fi
}

# Call and check the result
if is_even 42; then
    echo "42 is even"
else
    echo "42 is odd"
fi

Using command output:

get_square() {
    echo $(( $1 * $1 ))
}

# Capture the output
result=$(get_square 5)
echo "The square of 5 is $result"

Using global variables:

calculate_area() {
    # Set a global variable
    area=$(( $1 * $2 ))
}

# Call the function
calculate_area 4 5
echo "The area is $area"

Local Variables

By default, variables defined in a function are global and can be accessed from anywhere in the script. To make a variable local to a function, use the local keyword:

global_function() {
    variable="This is a global variable"
}

local_function() {
    local variable="This is a local variable"
    echo "Inside function: $variable"
}

# Call the functions
local_function
echo "After local_function: $variable"  # Variable is not set

global_function
echo "After global_function: $variable"  # Shows "This is a global variable"

Using local variables is a good practice to avoid unintended side effects and name conflicts.

Function Libraries

For larger projects, you might want to organize related functions into library files that can be sourced into multiple scripts:

# File: math_functions.sh
add() {
    echo $(( $1 + $2 ))
}

subtract() {
    echo $(( $1 - $2 ))
}

multiply() {
    echo $(( $1 * $2 ))
}

divide() {
    if [ $2 -eq 0 ]; then
        echo "Error: Division by zero"
        return 1
    fi
    echo $(( $1 / $2 ))
}

You can then source this file in your main script:

#!/bin/bash

# Load the function library
source math_functions.sh
# or
. math_functions.sh  # Shorthand form

# Use the functions
sum=$(add 5 3)
product=$(multiply 4 7)

echo "Sum: $sum"
echo "Product: $product"

Recursive Functions

Bash supports recursive functions (functions that call themselves), though they're not commonly used due to performance limitations:

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

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

Be careful with recursion in Bash as it can quickly lead to performance issues with large inputs.

Function Scope

Functions can access variables from the parent scope, but by default, variables defined in functions are global unless declared with local:

# Global variable
name="Global"

demonstrate_scope() {
    # Access global variable
    echo "Inside function, name is: $name"

    # Change global variable
    name="Changed inside function"

    # Create local variable
    local age=30
    echo "Inside function, age is: $age"
}

# Call the function
demonstrate_scope
echo "After function, name is: $name"
echo "After function, age is: $age"  # This will be empty

Function Overloading and Default Arguments

Bash doesn't support function overloading in the traditional sense (having multiple functions with the same name but different parameters). However, you can simulate default arguments:

greet() {
    local name=${1:-"Guest"}  # Default to "Guest" if $1 is not provided
    echo "Hello, $name!"
}

# Different calls
greet "John"    # Output: Hello, John!
greet           # Output: Hello, Guest!

Anonymous Functions and Function Variables

Bash doesn't really have anonymous functions, but you can assign a function to a variable and then execute it:

# Assign a function to a variable (Bash 4.3+)
greet='() { echo "Hello, $1!"; }'

# Create the function
eval "$greet"

# Call the function
greet "World"

This is somewhat advanced and not commonly used in everyday scripts.

Practical Function Examples

Let's look at some practical examples of functions that you might find useful:

Logging Function

# Colors for log levels
readonly LOG_RED='\033[0;31m'
readonly LOG_YELLOW='\033[0;33m'
readonly LOG_GREEN='\033[0;32m'
readonly LOG_BLUE='\033[0;34m'
readonly LOG_NONE='\033[0m'

# Log a message with a timestamp and log level
log() {
    local level="$1"
    local message="$2"
    local timestamp=$(date +"%Y-%m-%d %H:%M:%S")

    case "$level" in
        "ERROR")
            echo -e "${LOG_RED}[ERROR]${LOG_NONE} $timestamp - $message"
            ;;
        "WARN")
            echo -e "${LOG_YELLOW}[WARN]${LOG_NONE} $timestamp - $message"
            ;;
        "INFO")
            echo -e "${LOG_GREEN}[INFO]${LOG_NONE} $timestamp - $message"
            ;;
        "DEBUG")
            echo -e "${LOG_BLUE}[DEBUG]${LOG_NONE} $timestamp - $message"
            ;;
        *)
            echo "$timestamp - $message"
            ;;
    esac
}

# Usage
log "INFO" "Script started"
log "DEBUG" "Processing argument: $1"
log "WARN" "Config file not found, using defaults"
log "ERROR" "Failed to connect to server"

Confirm Action Function

# Ask for confirmation before proceeding
confirm() {
    local prompt="$1"
    local default="${2:-Y}"

    local prompt_options
    if [ "$default" = "Y" ]; then
        prompt_options="[Y/n]"
        default="Y"
    else
        prompt_options="[y/N]"
        default="N"
    fi

    while true; do
        read -p "$prompt $prompt_options: " choice
        choice=${choice:-$default}

        case "$choice" in
            [Yy]*)
                return 0
                ;;
            [Nn]*)
                return 1
                ;;
            *)
                echo "Please answer yes or no."
                ;;
        esac
    done
}

# Usage
if confirm "Are you sure you want to delete this file?"; then
    echo "Deleting file..."
    # rm file.txt
else
    echo "Operation cancelled"
fi

Retry Function

# Retry a command multiple times before giving up
retry() {
    local cmd="$1"
    local max_attempts=${2:-3}
    local delay=${3:-5}
    local attempt=1

    until $cmd; do
        if (( attempt == max_attempts )); then
            log "ERROR" "Command failed after $max_attempts attempts: $cmd"
            return 1
        fi

        log "WARN" "Command failed, attempt $attempt/$max_attempts. Retrying in $delay seconds..."
        ((attempt++))
        sleep $delay
    done

    return 0
}

# Usage
retry "ping -c 1 google.com" 5 2

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Backup Function

# Create a timestamped backup of a file or directory
backup() {
    local source="$1"
    local timestamp=$(date +"%Y%m%d_%H%M%S")

    if [ ! -e "$source" ]; then
        log "ERROR" "Source does not exist: $source"
        return 1
    fi

    local target="${source}_${timestamp}.bak"

    if [ -d "$source" ]; then
        # Directory backup
        cp -r "$source" "$target"
    else
        # File backup
        cp "$source" "$target"
    fi

    if [ $? -eq 0 ]; then
        log "INFO" "Created backup: $target"
        return 0
    else
        log "ERROR" "Failed to create backup of $source"
        return 1
    fi
}

# Usage
backup "/path/to/important/file.txt"

Function Best Practices

To write effective and maintainable functions:

Keep functions focused: Each function should do one thing and do it well.
Use descriptive names: Function names should clearly indicate what they do.
Document your functions: Add comments explaining the purpose, parameters, and return values.
Use local variables: Prevent unintended side effects by localizing your variables.
Check inputs: Validate parameters to catch errors early.
Return appropriate exit codes: Use return values to indicate success or failure.
Handle errors gracefully: Don't assume operations will succeed.
Organize related functions: Group related functions together or in library files.
Follow a consistent style: Establish and follow naming and formatting conventions.
Test your functions: Verify that functions work as expected with different inputs.

Functions are a powerful tool for organizing your Bash scripts. By breaking your code into well-defined functions, you can create more readable, maintainable, and reusable scripts. In the next section, we'll explore script permissions and execution, which are crucial for ensuring your scripts can run properly in different environments.

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