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Chmod Calculator

Owner

Group

Public

Octal Permission
000
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Understanding Linux Permissions

In Linux/Unix systems, file permissions are managed using a 3-digit octal code (e.g., 755) or symbolic notation (e.g., rwxr-xr-x). Each digit represents the permissions for a specific group: Owner, Group, and Public (Others).

The values are calculated by adding:
• Read (r) = 4
• Write (w) = 2
• Execute (x) = 1

💡
Expert Tip: The "777" Trap
Never use chmod 777 (full permissions for everyone) on production servers, especially for web directories. It allows any user on the system to modify or delete your files. Stick to 755 for directories and 644 for files to maintain security.

⚠️ Common Mistake: Using 777 for Everything

Setting permissions to 777 (rwxrwxrwx) is a HUGE security risk. It gives everyone full control. For web files, use 644 for files and 755 for directories. Never use 777 in production unless you know exactly why.

Common Permission Codes

  • 777 (rwxrwxrwx): No restrictions. Dangerous.
  • 755 (rwxr-xr-x): Owner has full access; others can only read/execute. Standard for scripts/directories.
  • 644 (rw-r--r--): Owner can read/write; others can only read. Standard for text/config files.
  • 600 (rw-------): Only owner can read/write. Use for SSH keys and sensitive data.
  • 400 (r--------): Read-only for owner. Good for protecting critical backups.
DL
Reviewed by David Lin, DevOps Engineer
Updated November 26, 2025

Frequently Asked Questions

What is chmod 777?

Chmod 777 gives full read, write, and execute permissions to everyone (Owner, Group, and Public). It is generally considered a security risk and should be used with extreme caution.

What is the difference between 755 and 644?

755 (rwxr-xr-x) allows the owner to do everything, while others can only read and execute. It's common for scripts and directories. 644 (rw-r--r--) allows the owner to read/write, while others can only read. It's the standard for regular files.

How do I calculate octal permissions?

Permissions are summed up: Read = 4, Write = 2, Execute = 1. For example, Read+Write = 4+2 = 6. You do this for Owner, Group, and Public to get the 3-digit code (e.g., 644).

What does 'chmod +x' do?

'chmod +x filename' adds execute permission to the file for all users (Owner, Group, and Public). It's equivalent to adding 1 to each digit in the octal code where execute was missing.