Unix - UNIX File Permission Calculation Using Numeric and Symbolic Methods

UNIX file permissions are one of the most important security features in a multi-user operating system. They control who can access files and directories and what actions are allowed. Learning this concept helps students understand how UNIX protects data and manages user access in real systems.

Owner Permission
Owner permission defines the access rights given to the user who creates the file or directory. The owner can be allowed to read, write, or execute the file depending on how permissions are set. This gives the file creator full control unless restrictions are applied.

Group Permission
Group permission controls access for users who belong to the same group as the file. This is useful when multiple users need shared access to files while still keeping them protected from other users on the system.

Others Permission
Others permission applies to all users who are not the owner and not part of the group. These permissions are usually limited to prevent unauthorized access to important files and directories.

Read Permission
Read permission allows a user to view the contents of a file or list the contents of a directory. Without read permission, the file content cannot be opened or viewed.

Write Permission
Write permission allows a user to modify file contents or create and delete files inside a directory. Improper write permissions can lead to accidental data loss or security issues.

Execute Permission
Execute permission allows a file to run as a program or script. For directories, it allows users to enter the directory and access files inside it.

Numeric Permission Calculation
Numeric permission calculation uses fixed values where read equals 4, write equals 2, and execute equals 1. These values are added together to quickly assign permissions in a single command.

Practical Command Example
The command chmod 755 filename gives full access to the owner and read-execute access to group and others. This method is commonly used to set permissions efficiently.

Where It Is Used
UNIX file permission calculation is widely used in system administration, server management, and multi-user environments to ensure secure and controlled file access.