http://www.opensourceproject.org.cn/article.php?id=456 “Symbolic Mode The format of a symbolic mode is `[ugoa...][[+-=][rwxXs tugo...]...][,...]'.
Multiple symbolic operations 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) 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.
all users (a) is effectively user + group + others
The operator '+' causes the permissions selected to be added to the existing permissions of each file; '-' causes them to be removed; and '=' causes them to be the only permissions that the file has.
The letters 'rwxXstugo' select the new permissions for the affected users:
Read (r), Write (w), Execute (or access for directories) (x), Execute only if the file is a directory or already has execute permission for some user (X), Set user or group ID on execution (s), Save program text on swap device (t), The permissions that the user who owns the file currently has for it (u), The permissions that other users in the file's group have for it (g), Permissions that other users not in the file's group have for it (o).