1. Introduction to Bash Scripting

What is Bash?

• Bash (Bourne Again SHell) is a command-line interpreter that allows you to run commands in Unix-like operating systems.
• Scripts are files containing a series of commands that the shell can execute.

2. Basic Concepts

Shebang (#!)

• The shebang (#!) at the top of a script specifies the interpreter that should be used to run the script.

#!/bin/bash
echo "Hello, World!"

Creating and Running a Script

1. Create a Script File: Use a text editor to create a new file, e.g., hello.sh.

nano hello.sh

2.Add Executable Permissions: Make the script executable

chmod +x hello.sh

3. Edit the Script File: Add commands to the script

#!/bin/bash
echo "Hello, World!"

4. Run the Script: Execute the script.

./hello.sh

3. Variables and Data Types

Declaring Variables

• Variables are used to store data.

name="John"
echo "Hello, $name"

Arithmetic Operations

• Perform arithmetic using $((...)).

num1=5
num2=3
sum=$((num1 + num2))
echo "Sum: $sum"

4. Control Structures

Conditional Statements

• if statement:

#!/bin/bash
num=10
if [ $num -gt 5 ]; then
  echo "The number is greater than 5"
fi

if-else statement:

#!/bin/bash
num=3
if [ $num -gt 5 ]; then
  echo "The number is greater than 5"
else
  echo "The number is not greater than 5"
fi

Loops

• for loop:

#!/bin/bash
for i in 1 2 3 4 5; do
  echo "Number: $i"
done

while loop:

#!/bin/bash
count=1
while [ $count -le 5 ]; do
  echo "Count: $count"
  ((count++))
done

5. Functions

Defining and Calling Functions

• Define a function:

#!/bin/bash
greet() {
  echo "Hello, $1"
}
greet "World"

Passing Arguments to Functions

• Functions can take arguments

#!/bin/bash
add() {
  sum=$(( $1 + $2 ))
  echo "Sum: $sum"
}
add 3 4

6. Advanced Concepts

Arrays

• Arrays store multiple values

#!/bin/bash
fruits=("Apple" "Banana" "Cherry")
echo "First fruit: ${fruits[0]}"
echo "All fruits: ${fruits[@]}"

File Operations

• Reading from a file:

#!/bin/bash
while IFS= read -r line; do
  echo "$line"
done < "file.txt"

Writing to a file:

#!/bin/bash
echo "Hello, File!" > output.txt

Error Handling

• Using exit to exit a script with a status

#!/bin/bash
if [ -z "$1" ]; then
  echo "No argument provided"
  exit 1
fi
echo "Argument: $1"

Debugging

• Enable debugging mode:

#!/bin/bash
set -x
echo "Debug mode enabled"
set +x
echo "Debug mode disabled"

7. Scripting Best Practices

Commenting

• Use comments to explain your code

#!/bin/bash
# This script prints a greeting message
echo "Hello, World!"

Code Organization

• Use functions and organize code into logical sections

#!/bin/bash
greet() {
  echo "Hello, $1"
}
greet "Alice"
greet "Bob"

Security Considerations

• Validate user inputs to prevent security issues

#!/bin/bash
read -p "Enter a number: " num
if ! [[ "$num" =~ ^[0-9]+$ ]]; then
  echo "Invalid input"
  exit 1
fi
echo "You entered: $num"

Example Scripts

1. Basic Script

#!/bin/bash
echo "Hello, World!"

2.Advanced Script: Backup Script

#!/bin/bash
backup_files="/home/user/docs"
dest="/home/user/backup"
date=$(date +%Y-%m-%d)
hostname=$(hostname -s)
archive_file="$hostname-$date.tgz"

echo "Backing up $backup_files to $dest/$archive_file"
tar czf $dest/$archive_file $backup_files

Some of Important and useful script

1. System Monitoring Script

CPU and Memory Usage

#!/bin/bash

echo "CPU and Memory Usage:"
echo "====================="
echo "CPU Load:"
uptime
echo ""
echo "Memory Usage:"
free -h
echo ""

Disk Usage Monitoring

#!/bin/bash

echo "Disk Usage:"
echo "==========="
df -h
echo ""

2. File Management Scripts

Backup Script

#!/bin/bash

# Variables
backup_source="/home/user/docs"
backup_dest="/home/user/backup"
date=$(date +%Y-%m-%d)
backup_file="$backup_dest/backup-$date.tar.gz"

# Create a backup
echo "Creating backup of $backup_source at $backup_file"
tar -czvf $backup_file $backup_source
echo "Backup completed."

Cleanup Script

#!/bin/bash

# Directory to clean up
cleanup_dir="/tmp"

# Find and delete files older than 7 days
echo "Cleaning up files older than 7 days in $cleanup_dir"
find $cleanup_dir -type f -mtime +7 -exec rm -f {} \;
echo "Cleanup completed."

3. User Management Scripts

Add User Script

#!/bin/bash

# Check if user is root
if [ $(id -u) -ne 0 ]; then
  echo "You must be root to add a user."
  exit 1
fi

# Read username
read -p "Enter username: " username

# Add user
useradd -m $username

# Set password
passwd $username

echo "User $username has been added to the system."

Delete User Script

#!/bin/bash

# Check if user is root
if [ $(id -u) -ne 0 ]; then
  echo "You must be root to delete a user."
  exit 1
fi

# Read username
read -p "Enter username to delete: " username

# Delete user
userdel -r $username

echo "User $username has been deleted from the system."

4. Network Management Scripts

Check Network Connectivity

#!/bin/bash

# Check connectivity to Google
echo "Checking connectivity to Google..."
ping -c 4 google.com

# Check connectivity to a local server
local_server="192.168.1.1"
echo "Checking connectivity to local server ($local_server)..."
ping -c 4 $local_server

Network Interface Details

#!/bin/bash

# List network interfaces and their IP addresses
echo "Network Interfaces and IP Addresses:"
ip -brief address

5. System Update and Maintenance Scripts

Update System Script

#!/bin/bash

# Update package lists
echo "Updating package lists..."
sudo apt update

# Upgrade all packages
echo "Upgrading all packages..."
sudo apt upgrade -y

echo "System update completed."

Check for Security Updates

#!/bin/bash

# List available security updates
echo "Checking for security updates..."
sudo apt list --upgradable | grep -i security

6. Automation and Scheduling Scripts

Cron Job Setup Script

#!/bin/bash

# Add a cron job to run a script every day at midnight
cron_job="0 0 * * * /path/to/your/script.sh"

# Check if the cron job already exists
(crontab -l | grep -q "$cron_job") && echo "Cron job already exists." || (crontab -l; echo "$cron_job") | crontab -

echo "Cron job setup completed."

7. Advanced Scripts

Process Monitoring Script

#!/bin/bash

# Monitor a specific process by name
process_name="apache2"
if pgrep $process_name > /dev/null
then
    echo "$process_name is running"
else
    echo "$process_name is not running"
fi

Log Rotation Script

#!/bin/bash

# Log directory
log_dir="/var/log/myapp"

# Rotate logs older than 7 days
find $log_dir -type f -mtime +7 -exec gzip {} \;

echo "Log rotation completed."

8. Backup Database Script

MySQL Database Backup

#!/bin/bash

# Variables
db_user="root"
db_password="yourpassword"
db_name="mydatabase"
backup_dir="/home/user/db_backup"
date=$(date +%Y-%m-%d)
backup_file="$backup_dir/$db_name-$date.sql"

# Create a backup
echo "Backing up database $db_name to $backup_file"
mysqldump -u $db_user -p$db_password $db_name > $backup_file
echo "Backup completed."

Comparison Operators

Numeric Comparisons

• -eq: equal to
• -ne: not equal to
• -lt: less than
• -le: less than or equal to
• -gt: greater than
• -ge: greater than or equal to

String Comparisons

• =: equal to
• !=: not equal to
• <: less than (in ASCII alphabetical order)
• >: greater than (in ASCII alphabetical order)
• -z: string is null (has zero length)
• -n: string is not null (has non-zero length)

Combining Conditions

You can also combine conditions using logical operators:

• &&: logical AND
• ||: logical OR