Linux chmod Permissions Tutorial: Read, Write, Execute Explained

Overview

File permissions control who can read, write, and execute files on a Linux system. Misconfigured permissions are a leading cause of security incidents. This tutorial explains the permission model and shows how to use chmod, chown, and chgrp correctly.

The Permission Model

Every file and directory has three permission sets:

Each set has three permission bits: read (r), write (w), and execute (x).

Reading Permission Strings

ls -l script.sh
-rwxr-xr-- 1 alice devs 1024 Jan 1 12:00 script.sh

Numeric (Octal) Mode

Each permission maps to a number: read = 4, write = 2, execute = 1. Sum the values for each set.

Common Octal Modes

Symbolic Mode

Symbolic mode uses letters and operators instead of numbers.

# Add execute for owner
chmod u+x script.sh

# Remove write for group and others
chmod go-w file.txt

# Set exact permissions for all
chmod u=rwx,g=rx,o= file.sh

Changing Ownership

# Change owner
sudo chown alice file.txt

# Change owner and group
sudo chown alice:developers file.txt

# Change group only
sudo chgrp developers file.txt

# Recursively change ownership
sudo chown -R alice:developers /var/www/site

Special Permissions

Security Best Practices

• Never use chmod 777 on production files or directories. It gives everyone full access.
• Private keys should be 600. SSH refuses to use keys with looser permissions.
• Directories need execute permission to be traversed. A directory with mode 644 cannot be entered even if it is readable.
• Use groups instead of world permissions to share access among a team.
• Audit regularly with find /var/www -perm -o+w to locate world-writable files.