(coreutils.info)Operator Numeric Modes

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27.4 Operator Numeric Modes
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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.  Note: Directory Setuid and Setgid.

   Operator numeric modes are a GNU extension.