Chmod Calculator

Reading and building Unix file permissions

On Unix systems every file has permissions for three groups: the owner (user), the group, and everyone else (other). Each group can have read, write, and execute rights. A chmod calculator converts between the symbolic form you see in ls -l, like rwxr-xr-x, and the octal form you pass to chmod, like 755. Getting this right is the difference between a working script and a "permission denied" error.

The arithmetic behind the digits

Each permission is a bit with a value: read is 4, write is 2, execute is 1. You add them per group. Read plus write plus execute is 4 + 2 + 1 = 7. Read plus execute is 4 + 1 = 5. So 755 means the owner gets 7 (rwx) and group and other each get 5 (r-x), which reads as rwxr-xr-x.

Example

Symbolic:  rw-r--r--
Owner:  rw-  = 4+2   = 6
Group:  r--  = 4     = 4
Other:  r--  = 4     = 4
Octal:  644

Common values:

644  files you edit, world-readable
755  scripts and directories you run or enter
600  private files, owner only
777  everyone can do anything (avoid)

FAQ

Why does a directory need execute? On a directory the execute bit means "you may enter it and access files inside". Without it, read access only lets you list names, not open them.

Is 777 ever fine? Almost never. It lets any user modify the file. Prefer the narrowest permission that works, usually 644 or 755.

What is the leading fourth digit? A four-digit mode like 4755 adds special bits (setuid, setgid, sticky). The octal math is the same idea as the number base converter.