Cheat Sheets — GeekU.us

Linux Essentials // file & directory

Command	What it does
ls -lah	List all files with sizes, hidden included
pwd	Print current working directory
cd /path	Change to absolute path
cd ~	Go to home directory
mkdir -p dir/sub	Create nested directories at once
rm -rf dir/	Delete directory and all contents — no prompt
cp -r src/ dst/	Copy directory recursively
mv old new	Move or rename file/directory
find / -name "*.conf"	Find files by name from root
chmod 755 file	rwxr-xr-x — owner write, others execute
chown user:group file	Change file ownership
ln -s target link	Create a symbolic link

Linux Essentials // processes & system

Command	What it does
ps aux	List all running processes
top / htop	Live process monitor (htop is friendlier)
kill -9 PID	Force-kill a process by PID
pkill nginx	Kill all processes matching name
systemctl status svc	Check service status
systemctl restart svc	Restart a systemd service
systemctl enable svc	Start service automatically on boot
journalctl -u nginx -f	Follow live logs for a service
df -h	Disk usage by mount point, human-readable
du -sh dir/	Total size of a directory
free -h	Memory usage summary
uptime	How long the system has been running
uname -a	Kernel version and architecture
whoami	Current user
sudo !!	Re-run last command with sudo

Tip: Pipe ps aux into grep nginx to filter — ps aux | grep nginx

Linux Essentials // text & search

Command	What it does
cat file	Print file contents to terminal
less file	Page through file (q to quit)
tail -n 50 file	Show last 50 lines
tail -f file	Follow file as it grows (logs)
grep -r "term" dir/	Search recursively for term in directory
grep -i "term" file	Case-insensitive search
sed -i 's/old/new/g' f	Find and replace in file (in-place)
awk '{print }' file	Print first column of each line
wc -l file	Count lines in file
sort file \| uniq -c	Count unique lines
diff file1 file2	Show differences between two files

SSH // connecting & keys

Command	What it does
ssh user@host	Connect to remote host as user
ssh -p 2222 user@host	Connect on non-default port
ssh -i ~/.ssh/key user@host	Use specific private key
ssh-keygen -t ed25519	Generate modern Ed25519 keypair
ssh-copy-id user@host	Push your public key to remote authorized_keys
ssh-keygen -R host	Remove stale host fingerprint
cat ~/.ssh/id_ed25519.pub	Print your public key to copy
chmod 700 ~/.ssh	Fix SSH directory permissions
chmod 600 ~/.ssh/id_*	Fix private key permissions

Prefer Ed25519 over RSA — shorter keys, faster, more secure. Use -t ed25519 when generating.

SSH // tunnels & config

Command	What it does
ssh -L 8080:localhost:80 user@host	Forward remote port 80 to local 8080
ssh -R 9090:localhost:3000 user@host	Expose local port 3000 to remote 9090
ssh -D 1080 user@host	SOCKS5 proxy through SSH
ssh -N -f user@host ...	Background tunnel, no shell
ssh -v user@host	Verbose — debug connection issues

~/.ssh/config example:

Host myserver

  HostName 203.0.113.10

  User ubuntu

  IdentityFile ~/.ssh/id_ed25519

  Port 22

Then just: ssh myserver

Networking // diagnostics

Command	What it does
ip a	Show all interfaces and IP addresses
ip r	Show routing table
ping -c 4 host	Send 4 ICMP packets to host
traceroute host	Trace packet path to destination
nslookup domain	Query DNS for a domain
dig domain	Detailed DNS lookup
dig +short domain	Just the IP answer
curl -I https://site	Fetch HTTP headers only
wget -O- url	Fetch URL contents to stdout
ss -tulnp	Show listening ports and which process owns them
netstat -tulnp	Same as ss — older systems
tcpdump -i eth0 port 80	Capture HTTP traffic on eth0
nmap -sV host	Scan host for open ports and service versions

Security // ufw & hardening

Command	What it does
ufw status verbose	Show firewall status and all rules
ufw default deny incoming	Block all inbound by default
ufw default allow outgoing	Allow all outbound by default
ufw allow 22/tcp	Allow SSH
ufw allow 80,443/tcp	Allow HTTP and HTTPS
ufw deny from IP	Block specific IP address
ufw enable	Enable firewall (rules take effect)
ufw delete allow 80/tcp	Remove a rule
fail2ban-client status	Check fail2ban jail status
last	Show recent login history
lastb	Show failed login attempts
passwd	Change current user password
chage -l user	Show password expiry for user

Hardening order: default deny → allow only what you need → enable fail2ban → disable password auth in sshd_config → reboot and verify.

Security // certbot & ssl

Command	What it does
certbot --nginx -d domain.com	Issue and install Let's Encrypt cert for Nginx
certbot renew --dry-run	Test renewal without actually renewing
certbot certificates	List all managed certs and expiry dates
openssl s_client -connect host:443	Inspect TLS handshake details
openssl x509 -in cert.pem -text	Read certificate details from a PEM file