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Overview

Linux is a lightweight, open-source operating system based on UNIX, first released by Linus Torvalds in 1991. Highly modular, secure, and customizable, it is widely used in servers, desktops, and embedded systems — and is the default environment for most CTF challenges and penetration testing. Linux comes in many distributions (distros) varying in package management, default tools, and user interfaces. It is generally considered more secure than other operating systems due to its permission model, active community, and rapid patch cycles.

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Terminology

Term	Definition
Shell / Terminal	Interface for entering commands and interacting with the OS
Root	Superuser account with full system privileges; prompt shown as #
Regular User	Standard account with limited privileges; prompt shown as $
Distro (Distribution)	A packaged version of Linux with its own tools, UI, and package manager
File System Hierarchy	Standardized directory structure starting at / (root)
PID (Process ID)	Unique number assigned to each running process
PPID (Parent Process ID)	PID of the process that spawned a given process
Daemon	Background service process managed by the OS
systemd	Modern init system used to manage services and boot processes
System User	UID 1–999; created for system services
Local User	UID 1000+; regular human user accounts
Absolute Path	Full path from root / to a file
Relative Path	Path relative to the current working directory
PATH Variable	Lists directories the OS searches when looking for commands
Inode	Data structure storing file metadata (permissions, ownership, size, timestamps)
STDIN	Standard input — data stream 0
STDOUT	Standard output — data stream 1
STDERR	Standard error — data stream 2
Pager	Tool for reading large files one screen at a time (e.g., more, less)
SUID	Set User ID — runs a file with the permissions of its owner
SGID	Set Group ID — runs a file with the permissions of its owning group
Sticky Bit	Prevents users from deleting files in a directory they don’t own

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Core Concepts

File System Structure

Linux uses a hierarchical file system rooted at /.

Directory	Purpose
/	Root of the entire file system
/etc	System configuration files
/var	Variable data — logs, databases, spool files
/root	Home directory for the root user
/tmp	Temporary files; cleared on reboot
/home	Home directories for regular users
/bin	Essential user binaries
/usr	User programs and utilities
/proc	Virtual filesystem; running processes listed by PID
/etc/passwd	Contains user and system account info; readable by all users
/etc/shadow	Stores hashed passwords and expiration info; root access only
/etc/profile	Stores default settings for user sessions

/etc/passwd fields (colon-separated): Username : Password : UID : GID : User Info : Home Directory : Shell /etc/shadow fields (colon-separated): Username : Hashed Password : Last Change : Min Days : Max Days : Warn : Inactive : Expire

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System Information Commands

Essential commands for gathering basic system information.

Command	Description
whoami	Display current username
id	Return current user identity (UID, GID, groups)
hostname	Print or set the name of the current host
uname -a	Print all OS info: kernel name, hostname, kernel release, version, hardware, OS
pwd	Print the current working directory
ifconfig	View or assign network interface addresses (deprecated; replaced by ip)
ip addr	Show or manipulate routing, network devices, and interfaces
netstat	Show active network connections and ports
ss	Investigate sockets (modern netstat replacement)
ps	Show running processes
ps -aux	Show all processes with detailed output
who	Display logged-in users
env	Print all environment variables — useful for enumeration
lsblk	List block devices (disks and partitions)
lsusb	List USB devices
lsof	List all open files and which processes are using them
lspci	List PCI devices
ssh user@ip	Connect to a remote system via Secure Shell

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Navigation

Command	Description
ls	List files in current directory
ls -l	Long listing with permissions, owner, size, date
ls -la	Long listing including hidden files (dotfiles)
ls -lt	Long listing sorted by modification time
cd <dir>	Change directory
cd ..	Go up one directory level
pwd	Print working directory
tree .	Display directory structure as a graphical tree

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Working with Files and Directories

Command	Description
touch <name>	Create an empty file
mkdir <name>	Create a directory
mkdir -p /path/to/dir	Create directory and all parent directories
mv <src> <dst>	Move or rename a file or directory
cp <file> <dest>	Copy a file
rm <file>	Remove a file
rm -r <dir>	Remove a directory and its contents recursively
cat <file>	Display file contents; also used to write or pipe text
file <name>	Identify file type

Editors: nano is simpler and more common. vim is more powerful — includes Normal, Insert, Visual, Command, Replace, and Ex modes. Run vimtutor to learn vim interactively.

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Finding Files and Directories

Command	Description
which <tool>	Returns the path to a binary or link
locate <name>	Fast search using a local database — run sudo updatedb to refresh
find <path> <options>	Powerful file search with filters

find option reference:

Option	Description
-type f	Match files only
-type d	Match directories only
-name "*.conf"	Match by filename pattern
-user root	Match files owned by a specific user
-size +20k	Match files larger than 20KB
-newermt 2020-03-03	Match files newer than the specified date
-exec ls -al {} \;	Execute a command on each result
2>/dev/null	Suppress permission denied errors (redirect STDERR to null)

Example: find / -type f -name *.conf -user root -size +20k -newermt 2020-03-03 -exec ls -al {} \; 2>/dev/null

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File Descriptors and Redirections

Every process has three default data streams:

Stream	Number	Description
STDIN	0	Standard input
STDOUT	1	Standard output
STDERR	2	Standard error

Operator	Description	Example
> or 1>	Redirect STDOUT to a file (overwrites)	ls > out.txt
>>	Append STDOUT to a file	ls >> out.txt
<	Use a file as STDIN	cat < file.txt
<<	Heredoc — feed multi-line input until a delimiter	cat << EOF > file.txt
2>/dev/null	Redirect STDERR to null (suppress errors)	find / -name x 2>/dev/null
|	Pipe STDOUT of one command as STDIN to the next	`find /etc

EOF is a Linux function that marks the end of input in a heredoc — type content, then type EOF on its own line to finish.

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Filter Contents

Tools for reading, searching, and processing output.

Command	Description
more <file>	Read file one page at a time (forward only)
less <file>	Read file with forward and backward navigation
head <file>	Print first 10 lines of a file
tail <file>	Print last 10 lines of a file
sort <file>	Sort lines alphabetically by default
grep "pattern" <file>	Search for lines matching a pattern
grep -v "pattern"	Exclude lines matching a pattern (inverse grep)
cut -d":" -f1	Split on delimiter : and return the first field
tr ":" " "	Replace all : characters with spaces
column -t	Display piped input in aligned tabular form
awk '{print $1, $NF}'	Print first and last fields of each line
sed 's/old/new/g'	Replace all occurrences of old with new in a stream
wc -l	Count lines in input

Example pipeline: find /etc/ -name *.conf 2>/dev/null | grep system — find .conf files, suppress errors, filter for “system” apt list --installed | grep -o installed | wc -l — count installed packages

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Regular Expressions

Regular expressions (regex) allow precise searching, filtering, and text manipulation. Available in grep, sed, awk, and many other tools.

Operator	Description
()	Group parts of a regex
[]	Character class — e.g., [a-z], [0-9]
{}	Quantifier — specifies how many times the previous pattern repeats
.	Match any single character
*	Match zero or more of the preceding element
^	Match start of line
$	Match end of line

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Users and Permissions

User Types

User Type	UID Range	Description
Root	0	Full system access
System Users	1–999	Created for system services and daemons
Local Users	1000+	Regular human user accounts

Permissions are assigned to three entities: Owner | Group | Others

Permission Breakdown

- rwx rw- r--   1 root root 1641 May 4 23:42 /etc/passwd
  |   |   |     |  |    |
  |   |   |     |  |    └── Group
  |   |   |     |  └─────── User (Owner)
  |   |   └───────────────── Others permissions
  |   └───────────────────── Group permissions
  └───────────────────────── Owner permissions
File type: - = file, d = directory, l = symlink

Permission	Octal	File Effect	Directory Effect
Read (r)	4	View file contents	List directory contents
Write (w)	2	Modify file	Create, delete, rename files inside
Execute (x)	1	Run as program	Navigate into directory

Octal Notation

Binary	Octal	Symbolic
111	7	rwx
101	5	r-x
100	4	r—
110	6	rw-
000	0	---

Permission Commands

Command	Description
chmod 755 <file>	Set permissions using octal notation
chmod u+x <file>	Add execute for owner using symbolic notation
chmod a-w <file>	Remove write for all (a=all, o=others, g=group, u=user)
chown user:group <file>	Change ownership of a file or directory
ls -l	View permissions on files

Special Permissions

Permission	Symbol	Description
SUID	s in owner execute field	Program runs with owner’s privileges — risk if executable launches a shell
SGID	s in group execute field	Program runs with group’s privileges
Sticky Bit (T)	T in others execute field	Others have NO execute; cannot see or run files in directory
Sticky Bit (t)	t in others execute field	Others have execute; only owner/root can delete files they don’t own

SUID/SGID binaries can be a privilege escalation vector — check GTFOBins (https://gtfobins.github.io/↗) if a flagged binary can launch a shell.

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Operators

Operator	Description
&	Run command in background immediately
&&	Chain commands; second runs only if first succeeds
;	Run commands sequentially regardless of success
|	Pipe STDOUT of one command as STDIN to next
>	Redirect output to file (overwrites)
>>	Append output to file
[CTRL+Z]	Suspend current process (send SIGTSTP)
bg	Resume suspended process in background
fg <job#>	Bring background job to foreground
jobs	List stopped/background jobs

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Related Concepts

• Operating System Fundamentals
• Linux Shells
• Linux System Management
• Linux File System
• Linux Networking
• Linux Hardening
• Boot and File Systems
• Protocols

Related Techniques

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References / Images

• Linux File System Hierarchy Standard (FHS)
• SSH and SCP usage documentation
• https://explainshell.com/↗ — break down any shell command
• https://gtfobins.github.io/↗ — SUID/SGID privilege escalation reference