🐧 chmod Command Guide (Linux File Permissions)

The chmod (change mode) command is used to modify file and directory permissions. It controls who can read, write, or execute a file.

📌 Permission Basics

Each file has three permission groups:

• u (user/owner)
• g (group)
• o (others)

And three permission types:

• r (read) → view file
• w (write) → modify file
• x (execute) → run file

▶️ Basic Syntax

chmod [options] permissions file

🔢 Numeric (Octal) Mode

Permissions use numbers:

• 4 = read
• 2 = write
• 1 = execute

Example:

chmod 755 script.sh

✔ Owner: read, write, execute
✔ Group: read, execute
✔ Others: read, execute

🔤 Symbolic Mode

Add permission

chmod +x script.sh

Remove permission

chmod -w file.txt

Set exact permissions

chmod u=rwx,g=rx,o=r file.txt

📂 Common Examples

Make a script executable

chmod +x script.sh

Secure a private file

chmod 600 file.txt

Full access for owner only

chmod 700 folder

🔁 Apply Recursively

chmod -R 755 /var/www/html

✔ Changes permissions for all files and folders inside

⚠️ Common Issues

Permission denied

• File isn’t executable or writable
• Fix:

chmod +x filename

Script won’t run

• Ensure it has execute permission and correct shebang (#!/bin/bash)

🧠 When to Use chmod

• Running scripts
• Securing sensitive files
• Fixing permission errors
• Managing web server directories

💡 Pro Tip

Avoid 777 unless absolutely necessary—it gives full access to everyone and can create security risks.