What Are Linux File Permissions?

Linux file permissions define which users can read, write, or execute files and directories. They form a core security mechanism in multi-user Linux systems.

Each file and directory has permissions assigned to three different user categories and three basic permission types.

User Categories in Linux

Permissions are evaluated for the following user groups:

• Owner: The user who owns the file
• Group: The group associated with the file
• Others: All other system users

Basic Permission Types

• Read (r): Read file contents or list directory
• Write (w): Modify files or write into directories
• Execute (x): Execute files or access directories

What Is chmod?

The chmod command is used to modify file and directory permissions. It supports symbolic and numeric (octal) notation.

Symbolic Mode

chmod u+x script.sh
chmod g-w file.txt
chmod o+r file.txt

Numeric (Octal) Mode

Each permission has a numeric value:

• Read = 4
• Write = 2
• Execute = 1

chmod 755 script.sh
chmod 644 config.php

What Is chown?

The chown command changes file and directory ownership.

chown user file.txt
chown user:group file.txt
chown -R user:group /var/www

The -R option applies changes recursively.

Why File Permissions Matter

Improper file permissions can introduce serious security vulnerabilities, especially on web servers.

• Unauthorized file modification
• Remote code execution risks
• Data leakage and system compromise

Best Practices

• Avoid unnecessary write permissions
• Never use 777 permissions in production
• Regularly audit file ownership
• Apply the principle of least privilege

From a knowledge base standpoint, Linux file permissions are a fundamental pillar of system security.