Linux Administrator Cheat Sheet

👤 User Management

adduser username              # Add a new user
usermod -aG group username    # Add user to group
passwd username               # Change user password
id username                   # Show user UID, GID, groups
whoami                        # Show current user

🔐 Permissions & Ownership

chmod 755 file               # Set permissions (rwxr-xr-x)
chown user:group file        # Change owner and group
ls -l                        # List files with permissions
umask                        # Default permission mask

📁 File & Directory Commands

ls, cd, pwd, mkdir, rm, mv, cp, touch
find /path -name filename    # Search for files
df -h                        # Disk space usage
du -sh folder/               # Folder size

📦 Package Management (APT)

sudo apt update              # Refresh package list
sudo apt upgrade             # Upgrade packages
sudo apt install pkg         # Install package
sudo apt remove pkg          # Remove package
apt search pkg               # Search for package

⚙️ Service & Process Management

systemctl status service     # Check service status
systemctl start service      # Start service
systemctl stop service       # Stop service
systemctl restart service    # Restart service
ps aux                       # List running processes
kill PID                     # Kill process by ID

🌐 Networking

ip a                         # Show IP addresses
ping host                    # Test connectivity
traceroute host              # Trace route to host
netstat -tuln                # Show open ports
ss -tuln                     # Show socket stats

📜 Logs & Monitoring

tail -f /var/log/syslog      # Live system log
journalctl -xe               # Systemd logs
htop                         # Interactive process viewer
dmesg                        # Boot and kernel messages

🔒 Security

sudo ufw enable              # Enable firewall
sudo ufw allow 22/tcp        # Allow SSH
fail2ban-client status       # Check Fail2Ban status

⏰ Scheduled Tasks

crontab -e                   # Edit user cron jobs
crontab -l                   # List user cron jobs

💾 Backup & Transfer

rsync -av source/ dest/      # Sync files
scp file user@host:/path     # Secure copy

🧾 Scripting

#!/bin/bash                  # Bash script header
echo "Hello World"           # Print text