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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 Directory Setuid and Setgid.
Operator numeric modes are a GNU extension.