An operator numeric mode is a numeric mode that is prefixed by a ‘-’, ‘+’, or ‘=’ operator, which has the same interpretation as in symbolic modes. For example, ‘+440’ enables read permission for the file’s owner and group, ‘-1’ disables execute permission for other users, and ‘=600’ clears all permissions except for enabling read-write permissions for the file’s owner. Operator numeric modes can be combined with symbolic modes by separating them with a comma; for example, ‘=0,u+r’ clears all permissions except for enabling read permission for the file’s owner.
The commands ‘chmod =755 dir’ and ‘chmod 755 dir’ differ in that the former clears the directory dir’s setuid and setgid bits, whereas the latter preserves them. See Directories and the Set-User-ID and Set-Group-ID Bits.
Operator numeric modes are a GNU extension.