15. Cheat Sheet

15.1 File Operations Quick Reference

Operation	Command
List files	ls -lah
Change directory	cd /path
Show current directory	pwd
Create directory	mkdir -p dir/sub
Create file	touch file.txt
Copy	cp -r src/ dst/
Move/Rename	mv old new
Delete	rm -rf dir/
View file	cat / less / head -20 / tail -20
Real-time view	tail -f logfile
Search files	find / -name "*.log"
Search text	grep -rn "keyword" /path/
Create link	ln -s target linkname
View file type	file filename
Count lines	wc -l file

15.2 Permissions & Users Quick Reference

Operation	Command
Change permissions	chmod 755 file
Change owner	chown user:group file
Add user	sudo adduser username
Delete user	sudo userdel -r username
Add to group	sudo usermod -aG group user
View user info	id username
Switch user	su - username
Run with elevated privileges	sudo command

15.3 Process Management Quick Reference

Operation	Command
View all processes	ps aux
Real-time monitoring	top / htop
Kill process by name	killall name
Force kill process	kill -9 PID
Run in background	nohup cmd &
Service management	systemctl start/stop/status
View logs	journalctl -u service
View ports	ss -tulnp

15.4 Network Quick Reference

Operation	Command
View IP	ip addr / ifconfig
View routing	ip route
Test connectivity	ping host
DNS lookup	dig domain
HTTP request	curl url
Download file	wget url
View firewall	sudo ufw status
Trace route	traceroute host

15.5 Disk Quick Reference

Operation	Command
View disk space	df -hT
View directory size	du -sh dir/
View partitions	lsblk / fdisk -l
Mount	mount /dev/sdb1 /mnt
Unmount	umount /mnt
View memory	free -h
View inodes	df -i

15.6 Package Management Quick Reference

Operation	APT (Debian/Ubuntu)	DNF (Fedora/RHEL)	Pacman (Arch)
Update index	apt update	dnf check-update	pacman -Sy
Upgrade all	apt upgrade	dnf upgrade	pacman -Syu
Install	apt install pkg	dnf install pkg	pacman -S pkg
Uninstall	apt remove pkg	dnf remove pkg	pacman -Rns pkg
Search	apt search keyword	dnf search keyword	pacman -Ss keyword
Clean cache	apt clean	dnf clean all	pacman -Sc

15.7 Keyboard Shortcuts Quick Reference

Shortcut	Function
Ctrl+C	Terminate current command
Ctrl+Z	Suspend current command (resume in background with bg)
Ctrl+D	Exit current Shell / send EOF
Ctrl+L	Clear screen (equivalent to clear)
Ctrl+A	Move to beginning of line
Ctrl+E	Move to end of line
Ctrl+R	Reverse search command history
Ctrl+W	Delete previous word
Ctrl+U	Delete to beginning of line
Ctrl+K	Delete to end of line
Tab	Auto-complete command/path
!!	Repeat last command (commonly used with sudo !!)
!$	Last argument of the previous command

15.8 Useful Tips

# sudo !! — re-run the last command with sudo
$ apt update
Permission denied
$ sudo !!              # automatically becomes sudo apt update

# Command substitution — embed the output of one command inside another
echo "Today's date is $(date +%Y-%m-%d)"

# Brace expansion — batch operations
mkdir project_{01..10}
cp file.txt{,.bak}       # equivalent to cp file.txt file.txt.bak
mv file.{txt,md}         # equivalent to mv file.txt file.md

# Command history
history                   # view history
!42                      # execute the 42nd history command
history | grep "docker"   # search history

# Multiple command execution
cmd1 && cmd2             # cmd2 runs only if cmd1 succeeds
cmd1 || cmd2             # cmd2 runs only if cmd1 fails
cmd1 ; cmd2              # run sequentially regardless of result

# xargs — convert input into command arguments
find . -name "*.tmp" | xargs rm
cat urls.txt | xargs -I {} curl -O {}

# watch — periodically execute a command and refresh output
watch -n 2 'df -h'       # refresh disk usage every 2 seconds
watch -n 1 'ps aux | wc -l'  # monitor process count changes

# alias — set command aliases
alias ll='ls -lah'
alias ports='ss -tulnp'
alias meminfo='free -h && echo "" && ps aux --sort=-%mem | head -10'
# To make permanent, add to ~/.bashrc

📝 Note: 📝 Learning advice: Practice more — the command line is like riding a bike, no amount of tutorials can replace hands-on practice. Make good use of Tab: Tab auto-completion is your best friend. Make good use of man: if unsure about usage, type man command. Don't be afraid of mistakes: you can experiment freely in a virtual machine. Back up important data: rm has no recycle bin.