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COMPUTING SCIENCE

Qwerks of History

Brian Hayes

Roots

When Unix was young, computers were scarce and expensive, which meant that each machine had many users. Dozens of terminals would be wired to a single CPU, and it was the function of time-sharing software like Unix to create the illusion that you had a computer of your own. Part of this magic trick lay in arranging for the processor to switch tasks so rapidly that it could run snippets of your program in between other users' keystrokes. Another aspect was walling off each user in a separate compartment, so that no one could trespass on anyone else's space, either deliberately or inadvertently.

Computers are no longer scarce. In many offices and even in some households, they outnumber people. The challenge today is not dividing a single CPU among multiple users but helping each individual manage and coordinate the resources of multiple machines. Where is the latest version of that document you were looking for—on the file server, on your laptop, on the PC at home? These days it could even be on your keychain. A time-sharing operating system is not the obvious solution to such problems. And yet Unix and its offspring have adapted themselves surprisingly well to this new environment.

Figure 3. Trees of nested directoriesClick to Enlarge Image

Even on a computer with just a single user, the task-switching mechanism remains vitally important: It allows you to keep dozens of programs active at the same time. And the barriers originally meant to isolate users from one another now prevent a rogue program from clobbering other software. Admittedly, the multiuser security apparatus can seem a little excessive—a little qwerky—when you're working all by yourself. In the Unix social milieu, each user has a private domain; groups of users can share resources; there's a public space accessible to all; and there's an inner sanctum off limits to everyone but the "superuser," also known as "root." When you try to delete a file and your laptop curtly objects, "You don't have permission to do that," you will probably not run down the hall looking for someone with root privileges.

But the scheme of file permissions, developed for the hub-and-spoke world of time-sharing computers, turns out to work reasonably well in network file systems, which create a decentralized information universe. A network file system frees you to wander from one machine to another within a local network, taking your entire computing environment with you, including your private documents and programs.





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