Changing Permissions: `chmod` (Octal Mode)

The Power User's Shortcut

Instead of letters, you can use numbers to set all nine permission bits at once. This is called "octal mode" or "numeric mode." It's faster when you know exactly what final permission state you want.

Each permission has a numeric value:

• Read (r): 4
• Write (w): 2
• Execute (x): 1
• No Permission (-): 0

To calculate the permissions for a category (User, Group, or Others), you add up the values of the permissions you want:

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

You combine three of these numbers to set the full permission string: one digit for User, one for Group, one for Others.

Common permission patterns:

Why octal mode exists:

Octal mode is faster when you know the exact permission state you want. Instead of multiple symbolic commands (chmod u=rwx,g=rx,o=rx), you just type chmod 755. It's also unambiguous—there's no question about what the final state will be.

In infrastructure-as-code (Ansible, Terraform, Dockerfiles), you'll almost always see octal mode because it's explicit and deterministic.

Key Point: In production, octal mode is preferred for idempotency. A configuration management tool running chmod 644 /etc/app/config.yaml will always result in the same permission state, regardless of what the permissions were before. Symbolic mode like chmod g+w depends on the current state, which can lead to drift over time if the file is manipulated by multiple processes. When writing Ansible tasks or Terraform local_file resources, always specify permissions in octal.