CHMOD Explained

Interactive chmod Calculator

Select permissions for Owner, Group, and Public:

What is CHMOD?

CHMOD stands for Change Mode. It is a Linux/Unix command used to change the file system permissions for files and directories. Permissions determine who can read, write, or execute a file.

Every file and folder has three types of permissions for three categories of users:

• Owner/User: The person who owns the file.
• Group: Users who are part of the file’s group.
• Others/Public: All other users on the system.

Understanding File Permissions

Permissions are represented in two ways:

1. Symbolic Notation: Example: rw-r--r--
   - First three characters are for the owner, next three for the group, and last three for others.
2. Numeric/Octal Notation: Example: 644
   Each permission type has a numeric value: Read = 4, Write = 2, Execute = 1. The sum gives the octal digit.

How to Use CHMOD

Basic syntax:

chmod [options] permissions filename

Examples:

chmod 644 file.txt

chmod 777 script.sh

chmod 751 program

Common CHMOD Use Cases

• Making a script executable: chmod +x script.sh
• Restricting access to a file: chmod 600 secret.txt
• Setting default permissions for a web directory: chmod 755 /var/www/html
• Granting group members write permission: chmod g+w file.txt

Visual Permission Cheat Sheet

0 = ---  
1 = --x  
2 = -w-  
3 = -wx  
4 = r--  
5 = r-x  
6 = rw-  
7 = rwx