5.3 Managing File Permissions and Ownership (Weight: 2)

📘Linux Essentials (LPI 010-160)

1. Linux Permission Model

Every file and directory in Linux has permissions that define what actions users can perform.

Linux permissions are based on three categories:

Category	Description
User (u)	The owner of the file
Group (g)	Members of the file’s group
Others (o)	All other users on the system

This means Linux controls access using three permission sets.

Example structure:

User   Group   Others

Each set has its own permissions.

2. Types of Permissions

Linux permissions consist of three basic types.

Permission	Symbol	Meaning
Read	r	View file contents
Write	w	Modify or change file
Execute	x	Run file as a program or script

These permissions can apply to files and directories, but they behave slightly differently.

3. Viewing Permissions

Permissions are displayed using the ls -l command.

Example:

ls -l

Example output:

-rwxr-xr-- 1 user staff 2048 Mar 10 10:00 script.sh

Breakdown:

-rwxr-xr--

This string represents the file type and permissions.

4. File Type Indicator

The first character indicates the file type.

Symbol	Meaning
–	Regular file
d	Directory
l	Symbolic link

Example:

drwxr-xr-x

This indicates a directory.

5. Permission Structure

The remaining characters are grouped in three sets of three.

Example:

-rwxr-xr--

Breakdown:

Owner   Group   Others
rwx     r-x     r--

Meaning:

Category	Permissions	Meaning
Owner	rwx	Can read, write, execute
Group	r-x	Can read and execute
Others	r–	Can only read

6. Permission Meaning for Files

For files, permissions work like this:

Read (r)

Allows a user to view the contents of a file.

Example IT usage:

Reading configuration files
Viewing log files
Opening documents

Example:

cat config.conf

Write (w)

Allows a user to modify or overwrite a file.

Example IT usage:

Updating configuration files
Editing application scripts
Writing log entries

Example:

nano config.conf

Execute (x)

Allows a file to run as a program or script.

Example IT usage:

Running shell scripts
Executing compiled programs
Starting system tools

Example:

./backup.sh

Without execute permission, the file cannot run as a program.

7. Permission Meaning for Directories

Permissions work differently for directories.

Permission	Meaning for Directory
r	View file names inside directory
w	Create, delete, or rename files
x	Enter or access directory

Read Permission on Directory

Allows listing files inside the directory.

Example:

ls /project

Without read permission, users cannot see the directory contents.

Write Permission on Directory

Allows:

Creating files
Deleting files
Renaming files

Example:

touch newfile.txt
rm oldfile.txt

Execute Permission on Directory

Allows users to enter the directory.

Example:

cd /project

Without execute permission, users cannot access files inside the directory even if they know the filename.

8. Permission Examples in IT Environments

Example 1: Application script

-rwxr-xr-x deploy.sh

Meaning:

Owner can edit and run script
Others can run script but not modify it

Example 2: Configuration file

-rw-r----- database.conf

Meaning:

Owner can edit
Group can read
Others cannot access

Used to protect sensitive configuration settings.

Example 3: Shared project directory

drwxrwxr-x project/

Meaning:

Owner and group can modify files
Others can only read

This is common in development teams.

9. Numeric (Octal) Permissions

Permissions can also be represented using numbers.

Each permission has a numeric value.

Permission	Value
Read	4
Write	2
Execute	1

Permissions are added together.

Example:

Permission	Calculation	Value
rwx	4 + 2 + 1	7
rw-	4 + 2	6
r-x	4 + 1	5
r–	4	4

Example Numeric Permission

Breakdown:

Category	Value	Meaning
Owner	7	rwx
Group	5	r-x
Others	5	r-x

Equivalent to:

rwxr-xr-x

Another example:

Breakdown:

Category	Value	Meaning
Owner	6	rw-
Group	4	r–
Others	4	r–

Equivalent to:

rw-r--r--

10. Changing Permissions (chmod)

Permissions are changed using the chmod command.

Syntax:

chmod permissions file

Using Numeric Mode

Example:

chmod 755 script.sh

Meaning:

rwxr-xr-x

Example:

chmod 644 config.txt

Meaning:

rw-r--r--

Using Symbolic Mode

Permissions can also be modified using symbols.

Symbols used:

Symbol	Meaning
u	user (owner)
g	group
o	others
a	all users

Operators:

Operator	Meaning
+	Add permission
–	Remove permission
=	Set exact permission

Examples

Add execute permission:

chmod +x script.sh

Remove write permission from group:

chmod g-w file.txt

Add read permission for others:

chmod o+r file.txt

Set permission exactly:

chmod u=rwx,g=rx,o=r file.sh

11. Recursive Permission Changes

Permissions can be applied to directories and their contents using the -R option.

Example:

chmod -R 755 project/

This modifies permissions for:

Directory
All files inside
All subdirectories

This is commonly used in web server directories and application deployments.

12. Default Permissions and umask (Basic Concept)

When new files are created, they receive default permissions.

The umask value removes certain permissions.

Typical defaults:

Item	Default Permission
Files	666
Directories	777

The umask subtracts permissions from these defaults.

Example:

umask 022

Results in:

Files:

Directories:

13. Important Commands for the Exam

Command	Purpose
ls -l	View permissions
chmod	Change permissions
umask	View or set default permissions

Examples:

ls -l file.txt
chmod 755 script.sh
chmod +x program.sh
umask

14. Security Importance of Permissions

Permissions help protect systems by:

Preventing unauthorized file access
Protecting configuration files
Limiting execution of programs
Controlling access to application directories

In IT environments, correct permission configuration is essential for:

Web servers
Databases
System scripts
Log files
Shared project directories

15. Key Exam Points to Remember

For the Linux Essentials exam, remember:

Linux permissions use user, group, and others
Permission types are read (r), write (w), execute (x)
Permissions appear in ls -l output
First character shows file type
Numeric permissions use 4, 2, 1 values
chmod changes permissions
Symbolic and numeric modes both exist
Directory permissions behave differently from file permissions
Recursive permissions use -R