A mode can be numerically formed as the OR of the following octal values (a leading 8r is ignored):

0400

read by owner

0200

write by owner

0100

execute (search in directory) by owner

0070

read, write, execute (search) by group

0007

read, write, execute (search) by others

A symbolic mode has the form:

[who] op permission

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.

The op field can be: + to add permission to the file's mode, - to take away permission, = to assign permission absolutely (all other bits being reset).

The permission field is any combination of the letters r (read), w (write), x (execute), a (append only), l (exclusive access), and t (temporary, not archived).