The first program encountered when using a UNIX system is the program that checks the user's identity. It first asks the user his username which uniquely identifies the user and then asks the user for his password. The password is compared to the password stored in the system's password file and if it has been entered correctly the user's chosen shell is started.
Each user has a unique user identification (UID) and can be a member of many groups, each identified by a group identification (GID). This determines which files he can access and what privileges he is allowed. Some programs, especially system programs can only be executed by members of a certain group. Very critical programs can only be executed by the super-user, who is the most powerful user in a UNIX system. The super-user is allowed to do everything and can access all files. He always has the UID 0.
A user can change his password at any time. This feature should be used regularly. Even if a secure password has been chosen it is possible to break it and gain illegal access by brute force code breaking within about eight weeks.
As a former systems administrator of this College used to put it: ``Passwords are like toothbrushes: You change them frequently and you don't share them.''
To make passwords secure they should not be names or words in a dictionary. Many system administrators will, for the sake of security, occasionally try and break the users passwords by testing them against the spelling dictionary. To make a password secure it should also be something which is not easy to guess. A user's telephone number, registration plate or date of birth are favourite things crackers try first when trying to break into a system. A good password should contain a number or a capital letter in an unusual place. Good passwords are for instance: "comp5uter" or "queRy". Foreign words are also effective as they will be very hard to break using a standard dictionary.