Symbolic modes represent changes to files' mode bits as
operations on single-character symbols. They allow you to modify either
all or selected parts of files' mode bits, optionally based on
their previous values, and perhaps on the current
umask as well
(see Umask and Protection).
The format of symbolic modes is:
where perms is either zero or more letters from the set ‘rwxXst’, or a single letter from the set ‘ugo’.
The following sections describe the operators and other details of symbolic modes.