A symbolic mode has the form:
The who part is a combination of the letters u (for user's permissions), g (group) and o (other). The letter a stands for ugo. If who is omitted, the default is a but the setting of the file creation mask (see umask(2)) is taken into account.
Op can be + to add permission to the file's mode, - to take away permission and = to assign permission absolutely (all other bits will be reset).
Permission is any combination of the letters r (read), w (write), x (execute), s (set owner or group id) and t (save text - sticky). Letters u, g or o indicate that permission is to be taken from the current mode. Omitting permission is only useful with = to take away all permissions.
The first example denies write permission to others, the second makes a file executable:
Multiple symbolic modes separated by commas may be given. Operations are performed in the order specified. The letter s is only useful with u or g.
Only the owner of a file (or the super-user) may change its mode.