Chmod Calculator

Every file and directory on Linux, macOS and other Unix-like systems has three sets of permissions: one for the owner (user), one for the group, and one for others (everyone else). Each set can grant read (r), write (w) and execute (x) access. The chmod command changes these permissions.

In octal notation each permission has a value — read = 4, write = 2, execute = 1 — and the values are added per set. So 7 (4+2+1) means rwx, 5 (4+1) means r-x and 4 means read-only. The three digits are written owner-group-others, which is why chmod 755 gives the owner full access while everyone else can read and execute. Symbolic notation such as u=rwx,g=rx,o=rx expresses exactly the same thing.

An optional leading fourth digit sets the special bits: setuid (4) runs an executable with the file owner's privileges, setgid (2) runs it with the group's privileges or makes new files in a directory inherit its group, and the sticky bit (1) on a directory such as /tmp only lets users delete their own files. In ls -l output they appear as s/S in the execute slots of owner and group, and t/T in the others slot — lowercase when execute is also set, uppercase when it is not.

For directories, read lets you list contents, write lets you create and delete entries, and execute lets you enter (cd) and access files inside. Typical safe defaults are 644 for regular files, 755 for directories and scripts, and 600 for private keys. Avoid 777: world-writable files can be modified by any user or compromised process on the system.

Permission modes are octal numbers, so each digit is exactly three bits. To see a mode such as 4755 in binary or decimal, paste it into the octal field of the number base converter.