Understanding Linux Permissions: Demystifying Chmod

If you work on Linux or Unix systems, dealing with chmod is an absolute certainty. Yet, for many developers, generating the correct octal or symbolic value is often a game of trial and error.

The Foundation: Read, Write, Execute

Every file and directory in Linux has three core permissions:

• Read (r): The ability to view the contents of the file or list the contents of a directory.
• Write (w): The ability to modify the file or add/remove files in a directory.
• Execute (x): The ability to run the file as a program or traverse into a directory.

These permissions are assigned to three classes:

• User (Owner): The creator of the file.
• Group: Other users in the file's assigned group.
• Others (Public): Everyone else.

The Octal Number System

Instead of typing rwxr-xr-x, we use numbers. Read is assigned 4, Write is 2, Execute is 1. If you want Read + Execute, you add them: 4 + 1 = 5. If you apply this math to all three classes (Owner, Group, Public), you get a 3-digit number.

• 7: 4 (Read) + 2 (Write) + 1 (Execute)
• 5: 4 (Read) + 1 (Execute)
• 0: No permissions

Thus, chmod 755 simply means Owner gets everything (7), Group gets Read/Execute (5), Public gets Read/Execute (5).

Visualizing Permissions

Our Chmod Calculator provides an interactive matrix. Click on visual checkboxes to instantly generate exactly what you need. Stop guessing numbers and start visualizing permissions!