Bash Scripting Fundamentals: Your First Automation Scripts

#!/bin/bash
# This is a comment - Bash ignores this line
echo "Hello, World!"
echo "Current date: $(date)"
echo "Logged in as: $(whoami)"

#!/bin/bash
# This is a single-line comment
# Comments are for humans, not computers

echo "This runs"  # You can comment after a command as well

: '
This is a multi-line comment
It can span multiple lines
Useful for longer explanations
'

# Create the script file
nano myscript.sh

# Make it executable
chmod +x myscript.sh

# Run it
./myscript.sh

# Run it from anywhere (add to PATH later)
# Alternative: run with bash command (doesn't need +x)
bash myscript.sh

#!/bin/bash
# Variables - NO spaces around = !!!
name="Alex"                  # String
age=30                        # Number
current_date=$(date)          # Command output
files_count=$(ls | wc -l)    # Another command

# Using variables - use $
echo "My name is $name"
echo "I am $age years old"
echo "Today is $current_date"
echo "There are $files_count files here"

# Variable naming rules
# Valid: MY_VAR, myVar, _my_var, var123
# Invalid: 123var (can't start with number), my-var (no hyphens)

#!/bin/bash
# Asking for user input

echo "What is your name?"
read username
echo "Hello, $username!"

# Prompt with the same line
read -p "Enter your age: " age
echo "You are $age years old"

# Silent input (for passwords)
read -sp "Enter password: " password
echo "Password received"

# Default value if user presses enter
read -p "Enter directory to backup [default: /home]: " backup_dir
backup_dir=${backup_dir:-/home}
echo "Backing up $backup_dir"

#!/bin/bash
# Exit codes and error handling

echo "This command will succeed"
ls /tmp
echo "Exit code: $?"  # Should be 0

echo "This command will fail"
ls /nonexistent_directory
echo "Exit code: $?"  # Should be non-zero

# Set custom exit code
if [ -f "/etc/passwd" ]; then
    echo "File exists"
    exit 0  # Success
else
    echo "File not found"
    exit 1  # Failure
fi

mkdir ~/bash-practice
cd ~/bash-practice
touch first_script.sh second_script.sh
chmod +x *.sh

#!/bin/bash
# system_info.sh - Display system information

echo "====================================="
echo "     SYSTEM INFORMATION REPORT"
echo "====================================="
echo ""

# User info
echo "User Information:"
echo "  Username: $(whoami)"
echo "  Home directory: $HOME"
echo "  Current shell: $SHELL"
echo ""

# System info
echo "System Information:"
echo "  Hostname: $(hostname)"
echo "  OS: $(uname -o)"
echo "  Kernel: $(uname -r)"
echo "  Architecture: $(uname -m)"
echo "  Uptime: $(uptime -p)"
echo ""

# Disk usage
echo "Disk Usage:"
df -h | head -4
echo ""

# Memory usage
echo "Memory Usage:"
free -h
echo ""

# Current processes
echo "Top 5 CPU-consuming processes:"
ps aux --sort=-%cpu | head -6
echo ""

echo "Report generated on: $(date)"

#!/bin/bash
# backup.sh - Create a timestamped backup

# Color codes for output
RED='\033[0;31m'
GREEN='\033[0;32m'
YELLOW='\033[1;33m'
NC='\033[0m' # No Color

echo -e "${GREEN}=== Backup Utility ===${NC}"

# Get source directory
read -p "Enter directory to backup: " source_dir

# Check if source exists
if [ ! -d "$source_dir" ]; then
    echo -e "${RED}Error: Directory '$source_dir' does not exist!${NC}"
    exit 1
fi

# Get backup destination (default: current directory)
read -p "Enter backup destination [default: current directory]: " dest_dir
dest_dir=${dest_dir:-.}

# Create backup filename with timestamp
timestamp=$(date +"%Y%m%d_%H%M%S")
backup_name="backup_$(basename "$source_dir")_$timestamp.tar.gz"

# Perform backup
echo -e "${YELLOW}Backing up $source_dir to $dest_dir/$backup_name...${NC}"
tar -czf "$dest_dir/$backup_name" "$source_dir"

# Check if backup succeeded
if [ $? -eq 0 ]; then
    echo -e "${GREEN}✓ Backup successful!${NC}"
    echo "Backup saved as: $dest_dir/$backup_name"
    ls -lh "$dest_dir/$backup_name"
else
    echo -e "${RED}✗ Backup failed!${NC}"
    exit 1
fi