The format of a symbolic mode is [ugoa...][[+-=][perms...]...],
where perms is either zero or more letters from the set rwxXst,
or a single letter from the set ugo. Multiple symbolic modes can
be given, separated by commas. A combination of the letters ugoa
controls which users’ access to the file will be changed: the
user who owns it (u), other users in the file’s group (g), other
users not in the file’s group (o) , or all users (a). If none of
these are given, the effect is as if a were given, but bits that
are set in the umask are not affected. The operator + causes the
selected file mode bits to be added to the existing file mode
bits of each file; - causes them to be removed; and = causes
them to be added and causes unmentioned bits to be removed
except that a directory’s unmentioned set user and group ID bits
are not affected. The letters rwxXst select file mode bits for
the affected users: read (r), write (w), execute (or search for
directories) (x), execute/search only if the file is a directory
or al