File Permissions

File Permissions in Linux

1. Introduction

File permissions in Linux define who can access a file or directory and what actions they can perform. Permissions are a core part of Linux security and protect the system from unauthorized access and accidental damage.

Linux uses a permission-based access control system for files and directories.

2. Why File Permissions are Needed

Protect files from unauthorized users
Prevent accidental deletion or modification
Support multi-user environments
Maintain system security and stability

3. Permission Categories (Who can access)

Linux divides users into three categories:

Category	Symbol	Description
Owner	`u`	User who owns the file
Group	`g`	Users in the same group
Others	`o`	All other users

4. Permission Types (What actions are allowed)

Permission	Symbol	Meaning (File)	Meaning (Directory)
Read	`r`	View file content	List directory
Write	`w`	Modify file	Create/delete files
Execute	`x`	Run file	Access directory

5. Viewing File Permissions

Use:

ls -l

Example output:

-rwxr-xr--

Permission Breakdown

-   rwx   r-x   r--
|    |     |     |
|  Owner  Group Others
|
File type

6. File Types (First Character)

Symbol	File Type
`-`	Regular file
`d`	Directory
`l`	Symbolic link
`c`	Character device
`b`	Block device

7. Changing Permissions – `chmod`

1. Symbolic Mode

chmod u+x file.txt
chmod g-w file.txt
chmod o+r file.txt
chmod u+rwx,g+rx,o-r file.txt

2. Numeric (Octal) Mode

Value	Permission
4	Read
2	Write
1	Execute

Examples:

chmod 755 file.sh
chmod 644 file.txt

Meaning:

7 → rwx
6 → rw-
5 → r-x
4 → r–

8. Changing Ownership – `chown`

chown user file.txt
chown user:group file.txt

9. Special Permissions (Advanced – Exam Important)

1. SUID (Set User ID)

Executes file with owner’s permission
Symbol: s

chmod u+s file

2. SGID (Set Group ID)

Executes with group permission
New files inherit group

chmod g+s directory

3. Sticky Bit

Only owner can delete files in directory
Used in /tmp

chmod +t directory

10. Permissions on Directories (Key Difference)

r → list files
w → create/delete files
x → enter directory

📌 Execute permission is mandatory to access a directory.

11. Default Permissions & `umask`

umask sets default permissions

umask

Example:

umask = 022
File permission → 644
Directory permission → 755

12. Real-World Example

ls -l /etc/passwd

Readable by all
Writable only by root

13. Security Best Practices

Avoid 777 permissions
Use sudo carefully
Assign minimum required permissions
Use groups efficiently

14. File Permissions Summary Table

Command	Purpose
`ls -l`	View permissions
`chmod`	Change permissions
`chown`	Change owner
`umask`	Default permissions

15. Conclusion

File permissions are a fundamental security mechanism in Linux. By controlling access at the file and directory level, Linux ensures safe, stable, and multi-user operation, making it ideal for servers and enterprise systems.