Basic Linux Commands

Linux Commands

Linux command‑line tools are the backbone of reconnaissance, exploitation, and post‑exploitation on Unix‑like systems.

Understanding Linux commands provides direct access to system internals, automation, and control. Many security tools and scripts rely on command-line operations for efficiency and precision. In ethical hacking, the ability to navigate filesystems, manage processes, inspect network configurations, and manipulate data through the terminal creates a strong foundation for tasks such as enumeration, privilege escalation, and persistence.

Proficiency with core commands accelerates workflow, reduces reliance on graphical interfaces, and improves situational awareness during engagements. Whether working locally or through remote shells, command-line skills increase control and adaptability across diverse environments.

Common Commands

Navigation & File Management

Command	Description	Usage
pwd	Print working directory	pwd
cd	Change directory	cd [DIRECTORY]
ls	List directory contents	ls [OPTIONS] [FILE...]
tree	Recursively list directories as tree	tree [DIRECTORY]
mkdir	Create directory	mkdir [OPTIONS] DIRECTORY
rmdir	Remove empty directory	rmdir DIRECTORY
cp	Copy files or directories	cp SOURCE DEST
mv	Move or rename files/directories	mv SOURCE DEST
rm	Delete files or directories	rm [OPTIONS] FILE...
touch	Create empty file or update timestamp	touch FILENAME
ln	Create hard or symbolic links	ln [OPTIONS] TARGET LINK_NAME
stat	Display detailed file information	stat FILE
file	Detect file type	file FILE
basename	Strip directory and suffix from filename	basename PATH
dirname	Extract directory part of file path	dirname PATH
find	Search for files and directories	find [PATH] [OPTIONS] [EXPRESSION]
locate	Find files using database (faster than find)	locate FILENAME
chmod	Change file permissions	chmod [OPTIONS] MODE FILE
chown	Change file ownership (user)	chown [OPTIONS] USER FILE
chgrp	Change group ownership	chgrp [OPTIONS] GROUP FILE
umask	Set default file permission mask	umask [MASK]
tar	Archive files into tarball	tar -czf ARCHIVE.tar.gz FILES
gzip	Compress files using Gzip	gzip FILE
gunzip	Decompress Gzip files	gunzip FILE.gz
zip	Compress files into ZIP archive	zip ARCHIVE.zip FILES
unzip	Extract ZIP archive	unzip ARCHIVE.zip
dd	Copy and convert data (low-level)	dd if=SOURCE of=DEST bs=BLOCKSIZE
mount	Mount a filesystem	mount [DEVICE] [MOUNTPOINT]
umount	Unmount a filesystem	umount [MOUNTPOINT]
namei	Follow a file path, showing each component	namei PATH
readlink	Resolve symbolic link to actual path	readlink FILE
realpath	Print the resolved absolute file path	realpath FILE
mktemp	Create a temporary file or directory	mktemp [TEMPLATE]

Viewing & Text Processing

Command	Description	Usage
cat	Concatenate & display files	cat [OPTIONS] FILE...
less	Paginate large files	less FILE
more	View file one page at a time	more FILE
head	Show first lines of a file	head [OPTIONS] FILE
tail	Show last lines / follow file	tail [OPTIONS] FILE
grep	Search for patterns	grep [OPTIONS] PATTERN FILE
awk	Pattern scanning & processing language	awk 'pattern { action }' FILE
sed	Stream editor for filtering & transforming text	sed [OPTIONS] 'SCRIPT' FILE
cut	Remove sections from each line	cut [OPTIONS] FILE
tr	Translate or delete characters	tr SET1 SET2
sort	Sort lines of text files	sort [OPTIONS] FILE
uniq	Filter out repeated lines	uniq [OPTIONS] FILE
wc	Count lines, words, bytes	wc [OPTIONS] FILE
nl	Number lines of a file	nl FILE
diff	Show differences between files	diff FILE1 FILE2
tee	Read from stdin and write to file and stdout	tee FILE

System & User Info

Command	Description	Usage
whoami	Show current user	whoami
id	Display user/group IDs	id [USERNAME]
df	Disk usage overview	df [OPTIONS]
du	Show disk usage for files/directories	du [OPTIONS] [PATH]
uptime	Show how long the system has been running	uptime
uname	System information (kernel, OS, architecture)	uname -a
hostname	Show or set system hostname	hostname
date	Display or set system date/time	date
cal	Display a calendar	cal
env	Show environment variables	env
set	Display shell variables and functions	set
history	List previously executed commands	history
ps	Display running processes	ps aux
top	Real-time system process monitor	top
kill	Terminate process by PID	kill [-9] PID
pkill	Kill processes by name	pkill PROCESS_NAME
pgrep	Search for processes by name	pgrep PROCESS_NAME
w	Show who is logged in and what they are doing	w
who	Show who is logged in	who
last	Show login history	last
free	Show memory usage	free -h

User & Group Management

Command	Description	Usage
useradd	Add new user	useradd [OPTIONS] USERNAME
usermod	Modify existing user	usermod [OPTIONS] USERNAME
userdel	Remove user	userdel [OPTIONS] USERNAME
groupadd	Create new group	groupadd GROUP
groupdel	Delete group	groupdel GROUP
passwd	Set/modify user password	passwd USERNAME

Security & Permissions

Command	Description	Usage
sudo	Execute command as another user (root by default)	sudo COMMAND
su	Switch user context	su [USER]
setfacl	Set POSIX ACL on files/directories	setfacl -m u:USER:rwx FILE
getfacl	View POSIX ACL	getfacl FILE

Process & Job Control

Command	Description	Usage
jobs	List background/paused jobs	jobs -l
bg	Resume job in background	bg %1
fg	Bring job to foreground	fg %1
nohup	Run command immune to hangup	nohup COMMAND &
disown	Remove job from shell’s job table	disown %1
nice	Start task with priority	nice -n 10 COMMAND
renice	Change priority of running PID	renice +5 PID

System Monitoring & Performance

Command	Description	Usage
htop	Interactive process viewer	htop
vmstat	Report virtual memory stats	vmstat 1
iostat	CPU & I/O statistics	iostat -xz 1
iotop	Real‑time I/O usage	iotop
dstat	Versatile resource stats	dstat -tcm --top-io

Disk & Filesystem Management

Command	Description	Usage
lsblk	List block devices	lsblk -f
blkid	Locate/print block device attributes	blkid DEVICE
fdisk	Partition table editor (MBR)	fdisk /dev/sda
parted	Partition tool (GPT/MBR)	parted /dev/sda
mkfs	Create filesystem on device	mkfs.ext4 /dev/sda1
fsck	Check/repair filesystem	fsck -f /dev/sda1
tune2fs	Adjust ext2/3/4 filesystem params	tune2fs -l /dev/sda1

Networking

Command	Description	Usage
ping	Test network connectivity (ICMP)	ping [OPTIONS] DESTINATION
curl	Retrieve data from URLs / APIs	curl [OPTIONS] URL
ssh	Secure remote shell & tunnelling	ssh [OPTIONS] USER@HOST

Package & Software Management

Command	Description	Usage
apt	Debian/Ubuntu package tool	apt update && apt upgrade
dnf	Fedora/RHEL package manager	dnf install PACKAGE
yum	Legacy RHEL/CentOS package tool	yum remove PACKAGE
pacman	Arch Linux package manager	pacman -Syu
dpkg	Debian low‑level package tool	dpkg -i PACKAGE.deb
rpm	RPM low‑level package tool	rpm -q PACKAGE
snap	Universal package system	snap install APP
flatpak	Sandboxed app manager	flatpak install REMOTE APP

Scripting & Automation

Command	Description	Usage
bash	GNU Bourne‑Again Shell	#!/usr/bin/env bash
sh	POSIX shell	sh SCRIPT.sh
crontab	Schedule recurring jobs	crontab -e
at	Schedule one‑time tasks	at 02:00
alias	Create command shortcuts	alias ll='ls -alh'
export	Set environment variables	export PATH=$PATH:/custom/bin

Help & Documentation

Command	Description	Usage
man	Read manual pages	man COMMAND
echo	Display text / variables	echo [STRING]

|}

Examples

Print Working Directory

pwd
# Output:
# /home/user

↑ Commands

Change Directory

cd /var/www
# Output:
# (no output)

↑ Commands

Substitute Text in String

echo "Hello world" | sed 's/world/Linux/'
# Output:
# Hello Linux

↑ Commands

List All Files (long + hidden)

ls -la
# Output:
# total 12
# drwxr-xr-x  3 user user 4096 May 16 22:15 .
# drwxr-xr-x 18 user user 4096 May 16 21:00 ..
# -rw-r--r--  1 user user    0 May 16 22:15 index.html

↑ Commands

Create Directory

mkdir backups
# Output:
# (no output)

↑ Commands

Remove Empty Directory

rmdir backups
# Output:
# (no output)

↑ Commands

Copy File

cp secrets.txt /tmp/secrets.bak
# Output:
# (no output)

↑ Commands

Move & Rename

mv old.log archive/old.log
# Output:
# (no output)

↑ Commands

Force-Delete Directory

rm -rf /tmp/testdir
# Output:
# (no output)

↑ Commands

Show File Contents

cat /etc/passwd | grep ":/bin/bash"
# Output:
# root:x:0:0:root:/root:/bin/bash
# user:x:1000:1000:User,,,:/home/user:/bin/bash

↑ Commands

View Large Log

less /var/log/auth.log
# Output:
# (opens file in pager; press q to quit)

↑ Commands

First 10 Lines

head -n 10 notes.txt
# Output:
# Line 1
# Line 2
# Line 3
# Line 4
# Line 5
# Line 6
# Line 7
# Line 8
# Line 9
# Line 10

↑ Commands

Follow File Growth

tail -f /var/log/nginx/access.log
# Output:
# 192.168.1.5 - - [16/May/2025:22:30:01 +0200] "GET / HTTP/1.1" 200 1234
# 192.168.1.5 - - [16/May/2025:22:30:02 +0200] "GET /favicon.ico HTTP/1.1" 404 564

↑ Commands

Search Pattern

grep -R "passwd" /etc
# Output:
# /etc/login.defs:PASS_MAX_DAYS   99999
# /etc/pam.d/common-password:password requisite pam_pwquality.so retry=3

↑ Commands

Current User

whoami
# Output:
# user

↑ Commands

User & Group IDs

id
# Output:
# uid=1000(user) gid=1000(user) groups=1000(user),27(sudo)

↑ Commands

Human-Readable Disk Usage

df -h
# Output:
# Filesystem      Size  Used Avail Use% Mounted on
# /dev/sda1        30G   12G   17G  42% /

↑ Commands

Show Last 20 Commands

history | tail -n 20
# Output:
#  981  ls
#  982  cd /var/www
#  983  vim index.html
# ...
# 1000  exit

↑ Commands

Ping Host 4 Times

ping -c 4 example.com
# Output:
# PING example.com (93.184.216.34) 56(84) bytes of data.
# 64 bytes from 93.184.216.34: icmp_seq=1 ttl=56 time=12.3 ms
# 64 bytes from 93.184.216.34: icmp_seq=2 ttl=56 time=12.1 ms
# 64 bytes from 93.184.216.34: icmp_seq=3 ttl=56 time=12.0 ms
# 64 bytes from 93.184.216.34: icmp_seq=4 ttl=56 time=12.2 ms
# --- example.com ping statistics ---
# 4 packets transmitted, 4 received, 0% packet loss, time 3005ms
# rtt min/avg/max/mdev = 12.012/12.173/12.325/0.123 ms

↑ Commands

Download Web Page

curl -o index.html http://example.com
# Output:
#   % Total    % Received % Xferd  Average Speed   Time    Time     Time  Current
#                                  Dload  Upload   Total   Spent    Left  Speed
# 100  1270  100  1270    0     0   2500      0 --:--:-- --:--:-- --:--:--  2500

↑ Commands

SSH on Custom Port

ssh -p 2222 user@target
# Output:
# The authenticity of host 'target (192.168.1.50)' can't be established.
# ED25519 key fingerprint is SHA256:abc123...
# Are you sure you want to continue connecting (yes/no/[fingerprint])?

↑ Commands

Read Manual Page

man ssh
# Output:
# (opens manual page; press q to quit)

↑ Commands

Write to File

echo "hacked" > /tmp/proof.txt
# Output:
# (no output)

↑ Commands

See Also

• Bash Scripting Basics
• Privilege Escalation
• Netcat