What's All This About Multiuser Systems?

Among the many multi words you learned earlier was one that directly concerns how you interact with the computer: multiuser. The goal of a multiuser system is for all users to feel as though they've been given their own personal computer, their own individual UNIX system, although they actually are working within a large system. To accomplish this, each user is given an account—usually based on the person's last name, their initials, or another unique naming scheme—and a home directory, the default place where his or her files are saved. This leads to a bit of a puzzle: When you're working on the system, how does the system know that you're you? What's to stop someone else from masquerading as you, going into your files, prying into private letters, altering memos, or worse?

On a Macintosh or PC, anyone can walk up to your computer when you're not around, flip the power switch, and pry, and you can't do much about it. You can add some security software, but security isn't a fundamental part of the system, which results in an awkward fit between system and software. For a computer sitting on your desk in your office, though, that's okay; the system is not a shared multiuser system, so verifying who you are when you turn on the computer isn't critical. However, if you have that computer attached to a network, you might have it configured to prompt for a password before you can get to your desktop.

But UNIX is a system designed for multiple users, so it is very important that the system can confirm your identity in a manner that precludes others from masquerading as you. As a result, all accounts have passwords associated with them—as with a PIN for a bank card, keep it a secret!—and when you use your password in combination with your account, the computer can be pretty sure that you are who you're claiming to be. For obvious reasons, when you're done using the computer, you always should remember to end your session, or, in effect, to turn off your virtual personal computer when you're done.

In the next hour, you learn your first UNIX commands. At the top of the list are commands to log in to the system, enter your password, and change your password to be memorable and highly secure.