1. What is CHMOD?

CHMOD (Change Mode) is a Unix/Linux command that sets the permissions of files and directories. Permissions determine who can read, write, or execute the file.

2. Understanding 4-Digit CHMOD

The 4-digit CHMOD format includes:

Where each digit is an octal number (0-7) representing different permission sets.

3. Permission Breakdown

Special Bits (First Digit):

• 0 - No special permissions
• 1 - Sticky bit (restricts file deletion)
• 2 - SGID (Set Group ID)
• 4 - SUID (Set User ID)

Standard Permissions (Last 3 Digits):

• 0 - --- (no permissions)
• 1 - --x (execute only)
• 2 - -w- (write only)
• 3 - -wx (write + execute)
• 4 - r-- (read only)
• 5 - r-x (read + execute)
• 6 - rw- (read + write)
• 7 - rwx (read + write + execute)

4. Using the Calculator

Instructions: Select the desired permissions for each category (special, user, group, other) and click Calculate to get the 4-digit CHMOD value and symbolic notation.

5. Frequently Asked Questions (FAQ)

Q1: What's the difference between 3-digit and 4-digit CHMOD?
A: The 4-digit format includes special permissions (first digit) while the 3-digit format only includes user, group, and other permissions.

Q2: When should I use special permissions?
A: Special permissions are used for specific cases: sticky bit for shared directories, SGID for group inheritance, SUID for executing with owner privileges.

Q3: What are common CHMOD values?
A: Common values include 0755 (rwxr-xr-x) for executables, 0644 (rw-r--r--) for regular files, and 1777 (rwxrwxrwt) for shared temp directories.

Q4: How do I apply CHMOD in Linux?
A: Use the command: chmod 0755 filename or chmod u=rwx,g=rx,o=rx filename

Q5: Is 777 permission safe?
A: Generally no - 777 (rwxrwxrwx) gives everyone full access and is a security risk for most files.