Chmod Calculator

About Chmod Calculator

The chmod command in Unix/Linux changes file and directory permissions. Permissions control who can read, write, and execute files. This calculator makes it easy to convert between numeric (octal) and symbolic notation.

Understanding Permission Bits

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

Each digit in the numeric notation is the sum of permissions: 7 = rwx (4+2+1), 5 = r-x (4+0+1), 4 = r-- (4+0+0)

Common Permission Patterns

• 644: Standard file — owner reads/writes, everyone else reads only
• 755: Executable/directory — owner has full access, others can read and execute
• 600: Private file — only owner can read/write (SSH keys, configs with secrets)
• 700: Private directory — only owner can access
• 777: Full access for everyone — rarely appropriate, security risk

FAQ

When should I use 755 vs 644?

Use 755 for directories and executable files (scripts, binaries). Use 644 for regular files (documents, configs, source code). The key difference is the execute bit — directories need it for access, files need it to run as programs.

Why is 777 dangerous?

777 means anyone on the system can read, modify, and execute the file. On a web server, this could allow attackers to modify your scripts. Use the most restrictive permissions that still allow your application to function.

What's the difference between numeric and symbolic notation?

Numeric (755) sets all permissions at once. Symbolic (u+x, g-w) modifies specific permissions without changing others. Numeric is faster for setting exact permissions; symbolic is better for targeted changes.