"Shell Script Functions Explained with Real-Time Backup Example"

What is a Function in Shell Scripting?
A function in shell scripting is a named block of reusable code that performs a specific task. Instead of repeating the same commands multiple times, you can write them once in a function and call the function whenever needed.
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✅ Key Features of Functions:
• Reusable: Write once, use multiple times.
• Organized: Breaks your script into logical blocks.
• Maintainable: Easier to update and debug.
• Flexible: Can accept input arguments (like $1, $2) and return output using echo.
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📌 Function Syntax
function_name() {
commands
}

OR

function function_name {
commands
}

Calling the function
function_name


EX1: Taking backup


#!/bin/bash

Function to perform backup
backup_directory() {
src_dir="$1"
backup_base="$2"
timestamp=$(date +%F_%T)
backup_dir="${backup_base}/backup_${timestamp}"

Create backup directory
mkdir -p "$backup_dir"

Copy source to backup
cp -r "$src_dir" "$backup_dir"

echo "Backup of '$src_dir' completed at '$backup_dir'"
}

Call the function with source and destination
backup_directory /etc /tmp


Line-by-Line Explanation
1. #!/bin/bash – Shebang to use Bash shell.
2. backup_directory() – Declares a function named backup_directory.
3. src_dir="$1" – First argument: source directory (e.g., /etc).
4. backup_base="$2" – Second argument: base destination path (e.g., /tmp).
5. timestamp=$(date +%F_%T) – Gets current date and time (e.g., 2025-05-07_15:22:30).
6. backup_dir="${backup_base}/backup_${timestamp}" – Constructs full backup path.
7. mkdir -p "$backup_dir" – Creates the backup directory.
8. cp -r "$src_dir" "$backup_dir" – Recursively copies source to backup.
9. echo ... – Prints a confirmation message.
10. backup_directory /etc /tmp – Calls the function with source /etc and destination /tmp