Linux Server Cheat Sheet

File Operations

ls -lah               # List files with sizes
du -sh *              # Directory sizes
df -h                 # Disk usage
find . -name "*.log"  # Find files
find / -size +100M    # Find large files
tree -L 2             # Directory tree (limit 2 levels)
rsync -avz src/ dst/  # Sync directories

Permissions

chmod 644 file        # rw-r--r-- (files)
chmod 755 dir         # rwxr-xr-x (directories)
chown user:group file # Change owner
chmod +x script.sh    # Make executable
umask 022             # Default: 755 dirs, 644 files

Processes

ps aux                # All processes
ps aux | grep nginx   # Find specific process
top / htop            # Interactive process viewer
kill -9 PID           # Force kill
kill -15 PID          # Graceful kill (SIGTERM)
pkill -f "name"       # Kill by name
nohup cmd &           # Run immune to hangups

System Info

uname -a              # Kernel info
cat /etc/os-release   # OS version
free -h               # RAM usage
uptime                # How long running
lscpu                 # CPU info
nvidia-smi            # GPU status
lspci                 # PCI devices
lsblk                 # Block devices

SSH

ssh user@host                  # Connect
ssh -p 2222 user@host          # Custom port
ssh -i key.pem user@host       # Key-based auth
ssh -L 8080:localhost:80 host  # Port forward
ssh -J jump@host user@target   # Jump host
scp file user@host:/path/      # Copy file
ssh-keygen -t ed25519          # Generate key

Networking

curl -I https://example.com    # HTTP headers
curl -s ifconfig.me            # Public IP
ping -c 4 host                 # Test connectivity
traceroute host                # Route tracing
ss -tulpn                      # Listening ports
netstat -tulpn                 # (older) Listening ports
nc -zv host 80                 # Test port

Logs

journalctl -u nginx -f             # Service logs (follow)
journalctl -u nginx --since "1h"   # Last hour
tail -f /var/log/nginx/access.log  # Follow file
grep "error" /var/log/syslog       # Search logs
dmesg | tail                       # Kernel messages

Cron

crontab -e                # Edit crontab
crontab -l                # List cron jobs
# Format: minute hour day month weekday command
0 3   * /opt/backup.sh           # Daily 3am
/15    * /opt/check.sh         # Every 15 min
@daily /opt/cleanup.sh             # Daily
@reboot /opt/start-service.sh      # On boot

Screen / Tmux

screen -S session_name    # Start named session
screen -ls                # List sessions
screen -r session_name    # Reattach
Ctrl+A, D                 # Detach
tmux new -s name          # New session
tmux ls                   # List sessions
tmux attach -t name       # Reattach
Ctrl+B, d                 # Detach