To ensure security and organization on a system with many users, UNIX machines employ a system of user accounts. The user accounting features of UNIX provide a basis for analysis and control of system resources, preventing any user from taking up more than his or her share, and preventing unauthorized people from accessing the system. Every user of a UNIX system must get permission by some access control mechanism.

Logging in

Logging in to a UNIX system requires two pieces of information: A username, and a password. When you sit down for a UNIX session, you are given a login prompt that looks like this:
login:
Type your username at the login prompt, and press the return key. The system will then ask you for your password. When you type your password, the screen will not display what you type.

Your username

Your username is assigned by the person who creates your account. At ISU, the standard username is the first four letters of your last name concatenated with the first four letters of your first name.

Your username must be unique on the system where your account exists since it is the means by which you are identified on the system.

Your password

When your account is created, a password is assigned. The first thing you should do is change your password, using the passwd utility. To change your password, type the command
passwd
after you have logged in. The system will ask for your old password, to prevent someone else from sneaking up, and changing your password. Then it will ask for your new password. You will be asked to confirm your new password, to make sure that you didn't mistype. It is very important that you choose a good password, so that someone else cannot guess it. Here are some rules for selecting a good password:
  • Do not use any part of your name, your spouse's name, your child's name, your pet's name, or anybody's name. Do not use any backward spellings of any name, either.
  • Do not use an easily-guessable number, like your phone number, your social security number, your address, license plate number, etc.
  • Do not use any word that can be found in an English or foreign-language dictionary.
  • Do not use all the same letter, or a simple sequence of keys on the keyboard, like qwerty.
  • Do use a mix of upper-case and lower-case letters, numbers, and control characters.
  • Do use at least six characters.

If you have accounts on multiple machines, use a different password on each machine. Do not choose a password that is so difficult to remember that you must write it down.

Logging Out

When you're ready to quit, type the command
exit
Before you leave your terminal, make sure that you see the login prompt, indicating that you have successfully logged out. If you have left any unresoved processes, the UNIX system will require you to resolve them before it will let you log out. Some shells will recognize other commands to log you out, like "logout" or even "bye".

It is always a good idea to clear the display before you log out, so that the next user doesn't get a screenful of information about you, your work, or your user account. You can type the command

clear
right before you log out, or you can press the return key until all the information is scrolled off the screen.