Linux Cheatsheet

What is Linux?

Linux is an open-source operating system kernel that powers servers, embedded systems, and increasingly desktop computers. It’s essential for:

Server Administration: Most web servers run Linux
Cloud Computing: AWS, Azure, Google Cloud use Linux
Cybersecurity: Penetration testing, security analysis
Development: Many development tools work best on Linux
Career Growth: Linux skills are highly valued in tech

Getting Started

Popular Distributions

Ubuntu: User-friendly, great for beginners
CentOS/RHEL: Enterprise-focused, server environments
Debian: Stable, widely used in servers
Kali Linux: Security-focused, penetration testing
Arch Linux: Advanced users, rolling release

Getting Linux

Virtual Machine: VirtualBox, VMware (recommended for beginners)
Dual Boot: Install alongside Windows
WSL: Windows Subsystem for Linux
Cloud: AWS EC2, Google Cloud, Azure

Essential Commands

# Navigation
pwd                    # Print working directory
ls                     # List files
ls -la                 # List all files with details
cd /path/to/directory  # Change directory
cd ~                   # Go to home directory
cd ..                  # Go up one directory
cd -                   # Go to previous directory

# File Operations
touch filename         # Create empty file
mkdir directory       # Create directory
mkdir -p path/to/dir  # Create nested directories
rm filename           # Delete file
rm -rf directory      # Delete directory recursively
cp source dest        # Copy file
mv source dest        # Move/rename file

File Permissions

# View permissions
ls -la                 # Shows permissions (rwxrwxrwx)

# Change permissions
chmod 755 filename     # rwxr-xr-x (owner: rwx, group: r-x, other: r-x)
chmod +x filename      # Add execute permission
chmod -x filename      # Remove execute permission

# Change ownership
chown user:group file  # Change owner and group
chown user file        # Change owner only

Text Processing

# View files
cat filename           # Display entire file
less filename          # View file page by page
head -n 10 filename    # Show first 10 lines
tail -n 10 filename    # Show last 10 lines
tail -f filename       # Follow file changes (logs)

# Search in files
grep "pattern" file    # Search for pattern in file
grep -r "pattern" dir  # Search recursively in directory
grep -i "pattern" file # Case-insensitive search

Process Management

# View processes
ps                     # Show running processes
ps aux                 # Show all processes with details
top                    # Real-time process monitor
htop                   # Enhanced process monitor (install with: apt install htop)

# Process control
kill PID               # Kill process by ID
kill -9 PID            # Force kill process
killall process_name   # Kill all processes with name
nohup command &        # Run command in background

System Information

# System info
uname -a               # System information
whoami                 # Current user
id                     # User and group IDs
df -h                  # Disk space usage
free -h                # Memory usage
uptime                 # System uptime

Package Management

Ubuntu/Debian (apt)

# Update package lists
sudo apt update

# Install package
sudo apt install package_name

# Remove package
sudo apt remove package_name

# Search packages
apt search keyword

# List installed packages
dpkg -l

# Update system
sudo apt upgrade

CentOS/RHEL (yum/dnf)

# Install package
sudo yum install package_name
sudo dnf install package_name

# Remove package
sudo yum remove package_name

# Update system
sudo yum update

Network Commands

# Network information
ip addr                # Show network interfaces
ip route              # Show routing table
netstat -tulpn        # Show network connections
ss -tulpn             # Modern alternative to netstat

# Connectivity
ping hostname          # Test connectivity
traceroute hostname    # Trace network path
nslookup domain       # DNS lookup
dig domain            # Advanced DNS lookup

File Compression

# Create archives
tar -czf archive.tar.gz directory/    # Create gzipped tar
tar -xzf archive.tar.gz               # Extract gzipped tar
zip -r archive.zip directory/        # Create zip archive
unzip archive.zip                     # Extract zip archive

# Common tar options
tar -czf backup.tar.gz /path/to/backup
tar -xzf backup.tar.gz
tar -tf archive.tar.gz               # List contents without extracting

Text Editors

Nano (Beginner-friendly)

nano filename          # Open file in nano
# Ctrl+X to exit
# Ctrl+O to save
# Ctrl+W to search

Vim (Advanced)

vim filename           # Open file in vim

# Vim modes
i                      # Insert mode
Esc                    # Command mode
:w                     # Save
:q                     # Quit
:wq                    # Save and quit
:q!                    # Quit without saving

Basic Vim Commands

# Navigation
h, j, k, l            # Left, down, up, right
w                     # Next word
b                     # Previous word
0                     # Beginning of line
$                     # End of line
gg                    # Beginning of file
G                     # End of file

# Editing
i                     # Insert before cursor
a                     # Insert after cursor
o                     # New line below
O                     # New line above
x                     # Delete character
dd                    # Delete line
yy                    # Copy line
p                     # Paste
u                     # Undo

Shell Scripting Basics

Creating Scripts

#!/bin/bash           # Shebang line
# This is a comment

# Variables
NAME="John"
echo "Hello, $NAME"

# User input
read -p "Enter your name: " USERNAME
echo "Hello, $USERNAME"

# Conditional statements
if [ $AGE -gt 18 ]; then
    echo "Adult"
else
    echo "Minor"
fi

# Loops
for i in {1..5}; do
    echo "Number: $i"
done

# Functions
function greet() {
    echo "Hello, $1"
}
greet "World"

Making Scripts Executable

chmod +x script.sh    # Make executable
./script.sh           # Run script

System Administration

User Management

# Add user
sudo useradd -m username
sudo passwd username

# Delete user
sudo userdel username

# Add to group
sudo usermod -aG groupname username

# Switch user
su username
sudo -u username command

Service Management

# Systemd (modern Linux)
sudo systemctl start service_name
sudo systemctl stop service_name
sudo systemctl restart service_name
sudo systemctl enable service_name
sudo systemctl status service_name

# Service (older systems)
sudo service service_name start
sudo service service_name stop
sudo service service_name restart

Log Files

# Common log locations
/var/log/syslog        # System log
/var/log/auth.log     # Authentication log
/var/log/apache2/     # Web server logs
/var/log/nginx/       # Nginx logs

# View logs
tail -f /var/log/syslog
journalctl -f         # Systemd journal

Security Basics

Firewall (UFW)

# Enable firewall
sudo ufw enable

# Allow/deny ports
sudo ufw allow 22     # SSH
sudo ufw allow 80     # HTTP
sudo ufw allow 443    # HTTPS
sudo ufw deny 23      # Telnet

# Check status
sudo ufw status

SSH

# Connect to remote server
ssh username@hostname
ssh -p 2222 username@hostname

# Copy files
scp file.txt username@hostname:/path/
scp -r directory/ username@hostname:/path/

# Generate SSH key
ssh-keygen -t rsa -b 4096
ssh-copy-id username@hostname

Useful Aliases

# Add to ~/.bashrc or ~/.zshrc
alias ll='ls -la'
alias la='ls -A'
alias l='ls -CF'
alias ..='cd ..'
alias ...='cd ../..'
alias grep='grep --color=auto'
alias fgrep='fgrep --color=auto'
alias egrep='egrep --color=auto'

Learning Resources

Online Practice

Linux Journey - Interactive Linux tutorial
OverTheWire Bandit - Linux security wargame
Linux Academy - Comprehensive Linux training
Red Hat Learning - Enterprise Linux training

Books

“Linux Command Line and Shell Scripting Bible” - Comprehensive reference
“The Linux Command Line” by William Shotts - Free online book
“Linux Administration Handbook” - System administration guide

Practice Environments

VirtualBox with Ubuntu
AWS EC2 free tier
Docker containers
WSL on Windows

Common Troubleshooting

Permission Denied

sudo chmod +x filename    # Make executable
sudo chown user:group file  # Change ownership

Command Not Found

which command          # Find command location
whereis command        # Find command and man pages
sudo apt install package  # Install missing package

Disk Space Issues

df -h                  # Check disk usage
du -sh directory/      # Check directory size
sudo apt autoremove    # Remove unused packages
sudo apt autoclean     # Clean package cache

Remember: Linux is about practice. Start with basic commands, build a home lab, and gradually work your way up to system administration and scripting. The command line is your friend - embrace it!