Chmod Calculator

Chmod Calculator - Unix/Linux File Permissions Calculator

🔐 Calculate and convert Unix/Linux file permissions between numeric (777, 644) and symbolic (rwxr-xr-x) formats. Visual chmod calculator for understanding file access control.

What is Chmod?

Chmod (change mode) is a Unix/Linux command used to change file and directory permissions. It controls who can read, write, or execute files using a three-digit octal number system.

Permission Types

• Read (r / 4): View file contents or list directory contents
• Write (w / 2): Modify file contents or create/delete files in directory
• Execute (x / 1): Run file as program or access directory

User Categories

• Owner (User): File creator or assigned owner
• Group: Users belonging to the file's group
• Others (World): All other users on the system

How to Calculate Permissions

Each digit is calculated by adding permission values:

• 0: No permissions (---)
• 1: Execute only (--x)
• 2: Write only (-w-)
• 3: Write + Execute (-wx) = 2+1
• 4: Read only (r--)
• 5: Read + Execute (r-x) = 4+1
• 6: Read + Write (rw-) = 4+2
• 7: Full permissions (rwx) = 4+2+1

Common Permission Modes

• 755 (rwxr-xr-x): Owner full access, others read/execute. Standard for executables and scripts.
• 644 (rw-r--r--): Owner read/write, others read only. Default for web files (HTML, CSS, images).
• 600 (rw-------): Owner read/write only. Secure for private config files, SSH keys.
• 777 (rwxrwxrwx): Full access for everyone. DANGEROUS - avoid in production!
• 700 (rwx------): Owner full access only. Maximum privacy for scripts and directories.
• 444 (r--r--r--): Read-only for everyone. Prevents accidental modifications.

Directory vs File Permissions

Permissions work differently for directories:

• Read (r): List directory contents (ls)
• Write (w): Create/delete files in directory
• Execute (x): Enter directory (cd) and access files
• Note: Execute permission is required to access a directory!

Command Usage

Basic chmod syntax:

• chmod 755 file.sh - Set specific permissions
• chmod +x script.sh - Add execute permission
• chmod -w file.txt - Remove write permission
• chmod u+x,g-w file - Add execute for owner, remove write for group
• chmod -R 755 /directory - Recursive (apply to all files/subdirs)

Security Best Practices

• Never use 777: Gives full access to everyone, major security risk
• Principle of least privilege: Grant minimum permissions needed
• Web files: Usually 644 for files, 755 for directories
• Config files: 600 or 400 to prevent unauthorized reading
• SSH keys: Must be 600 or SSH will refuse to use them
• Executable scripts: 700 (private) or 755 (public)

Real-World Examples

• Web Server (Apache/Nginx): Files 644, directories 755
• WordPress: wp-config.php should be 600 or 400
• SSH Private Key: ~/.ssh/id_rsa must be 600
• Cron Jobs: Scripts should be 700 or 750
• Log Files: 640 (owner read/write, group read)

💡 Pro Tip: Use ls -l to view current permissions. The first 10 characters show file type and permissions (e.g., -rwxr-xr-x means regular file with 755 permissions). The first character indicates type: - (file), d (directory), l (symlink).