Linux Commands Reference Guide

Search, filter by categories, and quickly copy Linux commands.

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📁 File & Directory Management

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Command	Description	Example
cat	Concatenate and display files	cat file.txt
cd	Change directory	cd /var/www
cp	Copy files and directories	cp -r source/ dest/
df	Display disk space usage	df -h
du	Disk usage of files/directories	du -sh *
file	Determine file type	file unknown.dat
find	Search for files and directories	find / -name *.conf
head	Display first lines of file	head -n 20 file.txt
less	View file content page by page	less /var/log/syslog
locate	Find files by name quickly	locate passwd
ls	List directory contents	ls -lah
mkdir	Make directories	mkdir -p /path/to/dir
more	Display file one screen at a time	more file.txt
mv	Move/rename files	mv old new
pwd	Print working directory	pwd
rm	Remove files or directories	rm -rf directory/
rmdir	Remove empty directories	rmdir dirname
tail	Display the last lines of file	tail -f /var/log/auth.log

📝 Text Processing

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Command	Description	Example
fgrep	Fast grep for fixed strings	fgrep 'string' file.txt
grep	Search text patterns in files	grep -r 'error' /var/log/
join	Join lines of two files	join file1 file2
sed	Stream editor for text manipulation	sed 's/old/new/g' file.txt
sort	Sort lines of text files	sort -n numbers.txt
split	Split the file into pieces	split -l 1000 bigfile.txt
tr	Translate or delete characters	tr '[:lower:]' '[:upper:]'
uniq	Report or omit repeated lines	uniq -c sorted.txt
wc	Count words, lines and characters	wc -l file.txt

🖥️ System Information & Monitoring

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Command	Description	Example
cal	Display calendar	cal 2024
date	Display/set system date/time	date '+%Y-%m-%d %H:%M:%S'
dmesg	Print kernel ring buffer messages	dmesg \| tail
free	Display memory usage	free -h
hostname	Show/set system hostname	hostname
htop	Interactive process viewer	htop
id	Print user and group IDs	id username
iostat	Report CPU and I/O statistics	iostat -x 5

⚙️ Process Management

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Command	Description	Example
bg	Put jobs in background	bg %1
fg	Bring jobs to foreground	fg %1
jobs	List active jobs	jobs
kill	Send a signal to a process	kill -9 1234
killall	Kill processes by name	killall firefox
nice	Run with modified priority	nice -n 10 command
nohup	Run immune to hangups	nohup script.sh &
pgrep	Look up processes by name	pgrep nginx
pkill	Signal processes by name	pkill -u username
renice	Alter priority of running process	renice -5 -p 1234
screen	Screen manager with terminals	screen -S session1
tmux	Terminal multiplexer	tmux new -s mysession

🌐 Network Commands

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Command	Description	Example
arp	Manipulate ARP cache	arp -a
curl	Transfer data from/to server	curl -O https://file.zip
dig	DNS lookup utility	dig google.com ANY
ftp	File transfer protocol client	ftp ftp.example.com
host	DNS lookup utility	host google.com
ifconfig	Configure network interface	ifconfig eth0
ip	Show/manipulate routing, devices	ip addr show
iptables	IPv4 packet filtering admin	iptables -L
nc	Netcat – networking utility	nc -zv host 1-1000
netstat	Network statistics	netstat -tulpn
nslookup	Query DNS servers	nslookup google.com
ping	Send ICMP ECHO_REQUEST	ping -c 4 google.com
route	Show/manipulate IP routing	route -n
rsync	Remote file sync	rsync -avz src/ dst/
scp	Secure copy over SSH	scp file user@host:/path/
ss	Socket statistics	ss -tulwn
ssh	Secure shell client	ssh user@192.168.1.1

📦 Compression & Archiving

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Command	Description	Example
7z	7-Zip archiver	7z a archive.7z folder/
bunzip2	Decompress bzip2 files	bunzip2 file.txt.bz2
bzip2	Block-sorting file compressor	bzip2 file.txt
gunzip	Decompress gzip files	gunzip file.txt.gz
gzip	Compress files	gzip file.txt
rar	Create RAR archives	rar a archive.rar file
tar	Archive utility	tar -czvf archive.tar.gz dir/
unrar	Extract RAR archives	unrar x archive.rar
unzip	Extract compressed files	unzip archive.zip
zip	Package and compress files	zip -r archive.zip folder/

👤 User & Group Management

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Command	Description	Example
groupadd	Create a new group	groupadd developers
groupdel	Delete group	groupdel groupname
groups	Show user's groups	groups username
passwd	Change user password	passwd username
su	Switch user	su - username
sudo	Execute as superuser	sudo command
useradd	Create a new user	useradd -m username
userdel	Delete user account	userdel -r username
usermod	Modify user account	usermod -aG sudo username
visudo	Edit the sudoers file	visudo

💾 Disk & Filesystem Management

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Command	Description	Example
blkid	Locate/print block device attributes	blkid
dd	Convert and copy file	dd if=/dev/sda of=backup.img
fdisk	Partition table manipulator	fdisk -l
fsck	Check and repair filesystem	fsck /dev/sda1
lsof	List open files	lsof -i :80
mkfs	Build a Linux filesystem	mkfs.ext4 /dev/sdb1
mount	Mount filesystem	mount /dev/sdb1 /mnt
parted	Partition manipulation program	parted /dev/sda print
sync	Synchronise cached writes	sync
umount	Unmount filesystem	umount /mnt

⏱️ System Control & Scheduling

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Command	Description	Example
at	Schedule one-time tasks	at 10:00 AM
crontab	Schedule periodic tasks	crontab -e
dmesg	Print kernel messages	dmesg \| grep error
halt	Halt the system	halt
journalctl	Query systemd journal	journalctl -u nginx
logger	Make entries in system log	logger 'Test message'
poweroff	Power off the system	poweroff
reboot	Reboot the system	reboot
service	Run System V init script	service apache2 restart
shutdown	Shutdown or restart system	shutdown -h now
systemctl	Control systemd services	systemctl status nginx

🔐 Security & Penetration Testing

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Command	Description	Example
aircrack-ng	WiFi security auditing	aircrack-ng -w wlist capture.cap
gpg	Encryption and signing tool	gpg -c file.txt
hashcat	Advanced password recovery	hashcat -m 0 hash.txt wlist.txt

⚠️ Security Reminders

Always obtain proper authorization before security testing.
Use sudo carefully — understand what commands do before running.
Backup data before destructive commands (rm, dd).
Test in safe environments before production use.
Penetration testing tools are for authorized testing ONLY.