Symbolic modes represent changes to files' permissions as
operations on single-character symbols. They allow you to modify either
all or selected parts of files' permissions, optionally based on
their previous values, and perhaps on the current umask as well
(see Umask and Protection).
The format of symbolic modes is:
[ugoa...][[+-=][rwxXstugo...]...][,...]
The following sections describe the operators and other details of symbolic modes.