UNIX provides two levels of security to protect data::

1. System Level Security :: It is provided with username and password to access the system by the respected owner .
2. File Level Security :: After login at the System Level Security successfully , you can give permissions to your files read, write,delete or no access to any user.
   

Added to the security ::

1. UNIX is a case sensitive. ($cat emp is not equal to $cat EMP)
2. It is a CUI operating system because of this reason the unauthorized user must have a knowledge on UNIX Commands,Keywords and Shell Scripting in order to modify the files.