Lesson 05 — Linux File Permissions

Lesson Overview

Imagine a company where every employee can read confidential HR records, modify financial reports, or delete production application files.

It would quickly become a security disaster.

Linux prevents this by using File Permissions.

Every file and directory in Linux has permissions that determine:

Who can read it
Who can modify it
Who can execute it

File permissions are one of the most important security mechanisms in Linux and form the foundation of access control across cloud platforms, enterprise servers, Kubernetes clusters, and cybersecurity environments.

Understanding Linux permissions is essential for every Linux Administrator, Cloud Engineer, DevOps Engineer, and Cybersecurity Professional.

Learning Objectives

After completing this lesson, you will be able to:

Explain Linux file ownership.
Understand read, write, and execute permissions.
Interpret symbolic and numeric permissions.
Use chmod, chown, and chgrp.
Understand special permissions.
Apply Linux permission best practices.

Why File Permissions Matter

Without permissions:

Anyone could delete system files.
Sensitive data could be stolen.
Applications could be modified.
Malware could spread easily.

Permissions protect Linux systems by ensuring users can only access what they are authorized to use.

File Ownership

Every file in Linux has an owner.

Example:

report.txt

Owner:

student

The owner usually has the greatest level of control over the file.

Group Ownership

Files also belong to a group.

Example:

report.txt

Owner:

student

Group:

developers

Group permissions allow multiple users to collaborate securely.

Permission Categories

Linux permissions are divided into three categories.

Category	Description
User (u)	File owner
Group (g)	Members of the assigned group
Others (o)	Everyone else

Together these determine who can access a file.

Permission Types

Linux supports three basic permissions.

Permission	Symbol	Value
Read	r	4
Write	w	2
Execute	x	1

These values are used when setting numeric permissions.

Read Permission (r)

Read permission allows a user to view a file.

Example:

report.txt

Permission:

r--

Users can:

Open the file
Read its contents
Copy the file

Write Permission (w)

Write permission allows modifications.

Example:

rw-

Users can:

Edit files
Save changes
Delete contents

Execute Permission (x)

Execute permission allows a file to run as a program or script.

Example:

rwx

Typical executable files include:

Bash Scripts
Python Programs
Shell Utilities

Directories also use execute permission to allow users to enter them.

Viewing Permissions

Display permissions.

ls -l

Example:

-rwxr-xr--

student developers report.sh

Understanding Permission Output

Example:

-rwxr-xr--

Breakdown:

-

Regular File

rwx

Owner

r-x

Group

r--

Others

This means:

Owner:

Read
Write
Execute

Group:

Read
Execute

Others:

Read Only

Directory Permissions

Directories use permissions differently.

Permission	Meaning
Read	View directory contents
Write	Create/Delete files
Execute	Enter the directory

Without execute permission, users cannot access the directory even if they can read its contents.

Numeric Permissions

Linux also supports numeric notation.

Value	Permission
7	rwx
6	rw-
5	r-x
4	r–
3	-wx
2	-w-
1	–x
0	—

Common Permission Values

Numeric	Symbolic	Meaning
777	rwxrwxrwx	Full access for everyone
755	rwxr-xr-x	Common for directories
744	rwxr–r–	Owner full control
700	rwx——	Private directory
644	rw-r–r–	Common for files
600	rw—––	Sensitive files

Understanding these values is essential for Linux administration.

Changing Permissions

Use chmod.

Example:

chmod 755 script.sh

Or use symbolic notation.

chmod u+x script.sh

Remove write permission.

chmod g-w report.txt

Changing Ownership

Use chown.

Example:

sudo chown student report.txt

Change owner and group.

sudo chown student:developers report.txt

Changing Groups

Use chgrp.

Example:

sudo chgrp developers report.txt

Default Permissions (umask)

Linux automatically assigns default permissions to new files and directories.

Display the current umask.

umask

Typical output:

The umask removes permissions from the default values.

Example defaults:

Files:

Directories:

The umask determines the final permissions.

Special Permissions

Linux supports three special permissions.

SUID (Set User ID)

Programs run using the owner’s privileges.

Example:

chmod u+s filename

SGID (Set Group ID)

Files inherit the directory’s group ownership.

Example:

chmod g+s directory

Useful for collaborative projects.

Sticky Bit

Only the file owner can delete files in the directory.

Common example:

/tmp

Set Sticky Bit:

chmod +t directory

Permission Examples

Example 1

Private SSH key:

Only the owner can read and write.

Example 2

Shell script:

Owner can modify it, everyone can execute it.

Example 3

Website file:

Owner can edit.

Others can read.

Permissions in Cloud Computing

Linux permissions protect cloud resources.

Examples:

SSH keys
Application files
Configuration files
Kubernetes manifests
Terraform files

Cloud Engineers frequently use chmod to secure sensitive credentials.

Permissions in Cybersecurity

Security teams investigate:

World-writable files
Incorrect ownership
Privilege escalation risks
SUID binaries
Sensitive configuration files

Misconfigured permissions are a common security weakness.

Common Commands

Display permissions:

ls -l

Display hidden files:

ls -la

Change permissions:

chmod

Change ownership:

chown

Change group:

chgrp

Display current umask:

umask

Real-World Example

A DevOps Engineer deploys a web application.

Application Files

↓

Owner

www-data

↓

Permissions

644

↓

Configuration Files

600

↓

Deployment Script

755

Correct permissions help protect the application while allowing it to function properly.

Best Practices

As a Linux administrator:

Follow the Principle of Least Privilege.
Avoid using 777 permissions.
Secure sensitive files with 600.
Use groups for shared access.
Review file ownership regularly.
Protect SSH keys.
Audit SUID and SGID files.
Monitor permission changes.

Proper permission management significantly improves Linux security.

Key Takeaways

After completing this lesson, you should understand:

Linux file ownership.
User, Group, and Others permissions.
Read, Write, and Execute permissions.
Symbolic and numeric permission notation.
chmod, chown, chgrp, and umask.
Special permissions (SUID, SGID, Sticky Bit).
Linux permission best practices.

Summary

Linux File Permissions are one of the most important security mechanisms in the operating system.

By controlling who can read, modify, and execute files, Linux protects users, applications, and critical system resources from unauthorized access.

Throughout your career, you’ll use these concepts to secure cloud servers, deploy applications, protect sensitive data, and troubleshoot permission-related issues in enterprise environments.

Mastering Linux permissions is a foundational skill for Cloud Computing, Cybersecurity, DevOps, and Linux Administration.

Next Lesson

➡️ Lesson 06 — Linux Processes

In the next lesson, you’ll learn how Linux manages running processes, understand process states and priorities, monitor system activity, and use commands like ps, top, htop, kill, and nice to manage applications and troubleshoot performance issues.