$ man --apropos permission
chmod (2) - change permissions of a file
$ man chmod
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.
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).
The letters rwxXst select file mode bits for the affected users: read (r), write (w),
execute (or search for directories) (x)