Linux / UNIX PATH, ls Command and File Access Permissions (Security)

Linux / UNIX PATH, ls Command and File Access Permissions (Security)

Introduction

Linux and UNIX are powerful open-source operating systems widely used in servers, cloud platforms, development environments, and academic institutions.
To work efficiently in Linux/UNIX, it is very important to understand PATH concepts, the ls command, and File Access Permissions.

This article explains:

● PATH in Linux/UNIX

● ls command with options

● Linux File Access Permissions (Security)

● chmod and umask commands

All topics are explained in a simple, detailed, and beginner-friendly manner using clear definitions, syntax, and examples.

PATH in Linux / UNIX

Definition:
A path is the location or address of a file or directory in the Linux/UNIX file system.
It tells the operating system where a file or directory exists.

Types of Paths

There are two types of paths in Linux/UNIX:

Absolute Path

Definition:
An absolute path specifies the complete location of a file or directory starting from the root directory (/).

If a path starts with /, it is an absolute path.

Syntax: /starting/from/root

Examples:

/home/user/Documents/file.txt

/etc/passwd

Key Points:

● Always starts with /

● Gives the full location

● Works from anywhere in the system

Relative Path

Definition:
A relative path specifies the location of a file or directory relative to the current working directory.

If a path does not start with /, it is usually a relative path.

Syntax:

directoryname/file

Examples:

Documents/file.txt

../Downloads

Home Directory Symbol (~)

Definition:
The symbol ~ represents the home directory of the current user.

Examples:

~/Documents

~/Downloads/file.txt

Special Path Symbols

Symbol	Meaning
/	Root directory
.	Current directory
..	Parent directory
~	Home directory

ls Command (List Command)

Definition:
The ls command is used to list files and directories.
It displays the contents of the current directory or a specified directory.

Syntax

ls [options] [file/directory]

Basic Usage Examples

ls – Lists files and directories in the current directory.

ls /home/user – Lists contents of a specific directory.

ls Command Options

ls -a – Displays all files and directories, including hidden files (files starting with .).

ls -A- Displays all files and directories except: Current directory (.) & Parent directory (..)

ls -B- Displays all files and directories except backup files (files ending with ~).

ls -l-Displays files and directories in long listing format .

Understanding ls -l Output

drwxr-xr-x 4 user user 4096 Dec 20 07:35 dir1

Field	Description
d	Directory (- for file)
rwxr-xr-x	Permissions
4	Number of links
user	Owner
user	Group
4096	Size (bytes)
Dec 20	Last modified date
07:35	Last modified time
dir1	Name

ls -m-Displays files and directories separated by commas.

ls -n-Displays user ID (UID) and group ID (GID) instead of username and group name.

ls -o-Displays long listing format without the group name.

ls -Q-Displays file and directory names within double quotes.

ls -i-Displays the inode number of each file and directory.

ls -r-Displays files and directories in reverse order.

ls -s-Displays the size of files and directories (in blocks).

ls -sh-Displays file and directory sizes in a human-readable format (KB, MB, GB).

ls –full-time-Displays complete date and time (timestamp) of files and directories.

Combining Multiple Options

Multiple options can be combined in one command.

Example:

ls -alh ➡ Shows hidden files, long listing format, and human-readable sizes.

Importance of PATH and ls

Helps navigate the Linux file system

Essential for beginners and interns

Frequently asked in exams and interviews

Used daily by Linux administrators and developers

 

Linux / UNIX File Access Permissions (Security)

Introduction

Linux/UNIX is a multi-user operating system.
To protect files and directories from unauthorized access, Linux uses File Access Permissions.

Linux Security Levels

Linux security works at different levels:

Level	Description
Hardware Level	Physical security of hardware
Operating System Level	Kernel and OS protection
Login Level	User authentication (username & password)
File Level	File and directory permissions

File Access Permissions belong to the File Level, which controls file security.

File Access Permissions define who can access a file or directory and what actions they can perform (read, write, execute).

Every file and directory has permissions for three user categories.

User Categories

Category	Symbol	Meaning
Owner	u	User who owns the file
Group	g	Group associated with the file
Others	o	All remaining users
All	a	Owner + Group + Others

Permission Types

Permission	Symbol	Meaning
Read	r	Read file content
Write	w	Modify or delete content
Execute	x	Run file as a program

Permissions for Directories

Permission	Symbol	Meaning
Read	r	List directory contents
Write	w	Create/delete files
Execute	x	Enter/access directory

Important Concept:

Without execute (x) permission on a directory, you cannot access files inside it, even if read permission is available.

Understanding Permission Structure (ls -l)

Example: drwxr-xr–

Part	Meaning
d	Directory (- means file)
rwx	Owner permissions
r-x	Group permissions
r–	Others permissions

File Types in Permission Field

Symbol	Type
–	Regular file
d	Directory
l	Link file
b	Block special file
c	Character special file

Permission Positions

Linux permissions contain 10 fields:

Position	Meaning
1st	File type
2–4	Owner permissions
5–7	Group permissions
8–10	Others permissions

Numeric (Octal) Permissions

Permission	Value
Read (r)	4
Write (w)	2
Execute (x)	1

Common Values

Number	Permission
7	rwx
6	rw-
5	r-x
4	r–
0	—

chmod Command

chmod (change mode) is used to change permissions of existing files and directories.

chmod – Numeric Mode

Syntax: chmod XYZ file_or_directory

X → Owner

Y → Group

Z → Others

Example 1: Full Access to Owner chmod 700 script.sh Result: -rwx——

Example 2: Common Program Permission chmod 755 program.sh Owner → rwx Group → r-x Others → r-x

Example 3: Read-Only File chmod 444 file.txt

Example 4: Directory Permission chmod 750 project Only owner and group can access.

chmod – Symbolic Mode

Symbol	Meaning
u	Owner
g	Group
o	Others
a	All
+	Add
–	Remove
=	Assign exactly

Symbolic Examples

Add execute permission to owner: chmod u+x file.sh

Remove write permission from others: chmod o-w file.txt

Set exact permissions: chmod u=rwx,g=rx,o=r file.txt

Equivalent numeric: chmod 754 file.txt

chmod on Directories

chmod 644 mydir

Cannot enter directory (no execute permission)

Correct: chmod 755 mydir

Recursive Permission Change

chmod -R 755 project

Applies to all files and subdirectories.

umask

umask controls the default permissions assigned to newly created files and directories.

It does not affect existing files.

Default Permissions

Type	Default
File	666 (rw-rw-rw-)
Directory	777 (rwxrwxrwx)

Formula: Final Permission = Default Permission − umask

Check Current umask

umask

Example 1: Default umask (022) umask 022 Type – Result File   – 644 (rw-r–r–) Directory – 755 (rwxr-xr-x)

Example 2: umask 000 umask 000 Everyone gets full access (not secure).

Example 3: umask 077 (High Security) umask 077 | File | 600 | | Directory | 700 | Used in secure environments.

Example 4: Real-World Example umask 027 touch demo.txt mkdir testdir Results: File → 640 (rw-r—–) Directory → 750 (rwxr-x—) Note umask values cannot exceed 7 Files never get execute permission by default Directories get execute permission by default

Conclusion

This article provided a complete and beginner-friendly explanation of:

● PATH

● ls command

● File Access Permissions

● chmod and umask

 

 

Author    : Lavanya Nandairi

LinkedIn : https://www.linkedin.com/in/lavanya-nandagiri-817194375/

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