Linux Commands

This article is a practical reference for essential Linux commands, particularly useful for users transitioning from Windows. Linux's command-line interface (CLI) is its greatest strength: most system administration, development, and automation tasks can be accomplished efficiently through the terminal. While the graphical desktop has improved greatly, the CLI remains the preferred tool for developers, system administrators, and power users.

Commands are organized by category with brief explanations and practical examples. For pipes, redirection, and combining commands, see the dedicated section below.

File System Navigation

Linux uses a single unified file system tree rooted at /, unlike Windows with its drive letters (C:, D:).

Command	Purpose	Example
pwd	Print working (current) directory	pwd → /home/alice
cd	Change directory	cd /var/log
cd ..	Move up one directory	cd ..
cd ~	Go to home directory	cd ~ (or just cd)
cd -	Go to previous directory	cd -
ls	List directory contents	ls -la /etc
ls -l	Long format with permissions, sizes	ls -l *.txt
ls -a	Show hidden files (starting with .)	ls -a ~
find	Search for files by name, type, date, etc.	find /home -name "*.pdf" -type f
locate	Fast file search using a pre-built index	locate wikantik.properties
tree	Display directory tree structure	tree -L 2 /var

Key differences from Windows:

• Paths use forward slashes (/), not backslashes (\)
• File names are case-sensitive (File.txt and file.txt are different files)
• Hidden files start with a dot (.bashrc, .ssh)
• There is no recycle bin on the command line — rm deletes immediately

File Operations

Command	Purpose	Example
cp	Copy files or directories	cp file.txt backup/
cp -r	Copy directories recursively	cp -r project/ project_backup/
mv	Move or rename files	mv old_name.txt new_name.txt
rm	Remove files	rm unwanted.txt
rm -r	Remove directories recursively	rm -r old_directory/
rm -i	Remove with confirmation prompt	rm -i important.txt
mkdir	Create a directory	mkdir new_project
mkdir -p	Create nested directories	mkdir -p src/main/java
touch	Create an empty file or update timestamp	touch newfile.txt
chmod	Change file permissions	chmod 755 script.sh
chown	Change file ownership	chown alice:developers file.txt
ln -s	Create a symbolic link	ln -s /usr/bin/python3 /usr/bin/python

Permissions

Linux file permissions use three groups (owner, group, others) and three types (read, write, execute):

-rwxr-xr-- 1 alice developers 4096 Jan 15 10:30 script.sh
│├─┤├─┤├─┤
│ │   │  └── Others: read only (4)
│ │   └───── Group: read + execute (5)
│ └───────── Owner: read + write + execute (7)
└──────────── File type (- = regular file, d = directory, l = link)

Numeric notation: read=4, write=2, execute=1. So chmod 755 means owner=rwx(7), group=r-x(5), others=r-x(5).

Text Processing

Text processing is where Linux truly excels. These commands form a powerful toolkit for analyzing and transforming text data.

Command	Purpose	Example
cat	Display entire file contents	cat config.txt
less	View file with scrolling (q to quit)	less /var/log/syslog
head	Show first N lines (default 10)	head -20 logfile.txt
tail	Show last N lines	tail -50 error.log
tail -f	Follow a file in real time	tail -f /var/log/syslog
grep	Search for patterns in text	grep "ERROR" app.log
grep -r	Search recursively in directories	grep -r "TODO" src/
grep -i	Case-insensitive search	grep -i "warning" output.txt
grep -c	Count matching lines	grep -c "404" access.log
sed	Stream editor for text substitution	sed 's/old/new/g' file.txt
awk	Pattern scanning and processing	awk '{print \$1, \$3}' data.txt
wc	Count lines, words, characters	wc -l file.txt
sort	Sort lines	sort -n numbers.txt
uniq	Remove adjacent duplicates	sort names.txt \| uniq -c
cut	Extract columns from text	cut -d',' -f2 data.csv
tr	Translate or delete characters	echo "HELLO" \| tr 'A-Z' 'a-z'
diff	Compare two files	diff file1.txt file2.txt

Practical Examples

Count unique IP addresses in an access log:

awk '{print \$1}' access.log | sort | uniq -c | sort -rn | head -10

Replace all occurrences of a string in multiple files:

find . -name "*.java" -exec sed -i 's/oldMethod/newMethod/g' {} +

Extract the third column from a CSV, sorted and deduplicated:

cut -d',' -f3 data.csv | sort -u

Process Management

Command	Purpose	Example
ps	Show running processes	ps aux
ps aux	Detailed list of all processes	ps aux \| grep java
top	Real-time process monitor	top
htop	Enhanced process monitor (interactive)	htop
kill	Send signal to a process	kill 1234
kill -9	Force kill a process	kill -9 1234
killall	Kill processes by name	killall firefox
bg	Resume a stopped job in background	bg %1
fg	Bring a background job to foreground	fg %1
jobs	List background jobs	jobs
nohup	Run command immune to hangups	nohup ./server.sh &
&	Run command in background	./long_task.sh &
Ctrl+C	Interrupt (kill) foreground process	—
Ctrl+Z	Suspend foreground process	—

Package Management

Different Linux distributions use different package managers:

Distribution	Package Manager	Install Example	Update System
Ubuntu/Debian	apt	sudo apt install nginx	sudo apt update && sudo apt upgrade
Fedora/RHEL	dnf	sudo dnf install nginx	sudo dnf upgrade
Arch Linux	pacman	sudo pacman -S nginx	sudo pacman -Syu
openSUSE	zypper	sudo zypper install nginx	sudo zypper update

Common patterns:

• Search for packages: apt search keyword / dnf search keyword
• Remove a package: apt remove package / dnf remove package
• List installed packages: apt list --installed / dnf list installed

Disk and System Information

Command	Purpose	Example
df -h	Disk space usage (human-readable)	df -h
du -sh	Directory size summary	du -sh /var/log
du -h --max-depth=1	Size of immediate subdirectories	du -h --max-depth=1 /home
free -h	Memory usage	free -h
uname -a	System information	uname -a
lsblk	List block devices (disks/partitions)	lsblk
lscpu	CPU information	lscpu
uptime	System uptime and load averages	uptime
hostname	Display system hostname	hostname

Networking

Command	Purpose	Example
ip addr	Show network interfaces and IP addresses	ip addr
ip route	Show routing table	ip route
ss -tlnp	Show listening TCP ports	ss -tlnp
curl	Transfer data from/to a URL	curl -O https://example.com/file.tar.gz
wget	Download files	wget https://example.com/file.tar.gz
ping	Test network connectivity	ping -c 4 google.com
dig	DNS lookup	dig example.com
ssh	Secure remote login	ssh user@192.168.1.100
scp	Secure copy over SSH	scp file.txt user@host:/path/
rsync	Efficient file synchronization	rsync -avz src/ user@host:dest/
netstat -tlnp	(Legacy) Show listening ports	netstat -tlnp

Pipes and Redirection

Pipes and redirection are fundamental to the Linux philosophy of combining small, focused tools into powerful pipelines.

Operator	Purpose	Example
\|	Pipe: send output of one command to another	ls -l \| grep ".txt"
>	Redirect output to file (overwrite)	echo "hello" > file.txt
>>	Redirect output to file (append)	echo "line 2" >> file.txt
<	Redirect file as input	sort < unsorted.txt
2>	Redirect error output	command 2> errors.log
2>&1	Redirect errors to same place as output	command > all.log 2>&1
&>	Redirect both stdout and stderr	command &> all.log
tee	Write to file AND display on screen	command \| tee output.log
xargs	Build commands from piped input	find . -name "*.tmp" \| xargs rm

Pipeline Example

Find the 10 largest files in a directory tree:

find /var -type f -exec du -h {} + 2>/dev/null | sort -rh | head -10

Compression and Archiving

Command	Purpose	Example
tar -czf	Create compressed archive (.tar.gz)	tar -czf backup.tar.gz /home/user/
tar -xzf	Extract .tar.gz archive	tar -xzf backup.tar.gz
tar -xjf	Extract .tar.bz2 archive	tar -xjf archive.tar.bz2
gzip	Compress a file	gzip large_file.txt
gunzip	Decompress a .gz file	gunzip large_file.txt.gz
zip	Create ZIP archive	zip -r archive.zip directory/
unzip	Extract ZIP archive	unzip archive.zip

User Management

Command	Purpose	Example
sudo	Run command as superuser	sudo apt update
su	Switch to another user	su - alice
whoami	Show current username	whoami
id	Show user ID and group memberships	id
groups	Show groups current user belongs to	groups
passwd	Change password	passwd
useradd	Create a new user	sudo useradd -m newuser
usermod	Modify a user account	sudo usermod -aG docker alice

Essential Tips for Windows Users

1. Tab completion works everywhere — press Tab to auto-complete file names and commands.
2. Ctrl+R searches your command history — start typing and it finds matching previous commands.
3. man command opens the manual page for any command (man grep, man chmod).
4. command --help provides a quick usage summary.
5. The home directory (~ or /home/username) is the equivalent of C:\Users\Username.
6. sudo is the equivalent of "Run as Administrator."
7. Use alias to create shortcuts: alias ll='ls -la' in your .bashrc.