计算机专业英语unit4 B课文

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The UNIX operating system was designed to let a number of programmers access the computer at the same time and share its resources.

The operating system coordinates the use of the computer's resources, allowing one person, for example, to run a spell check program while another creates a document, lets another edit a document while another creates graphics, and lets another user format a document - all at the same time, with each user oblivious to the activities of the others.

The operating system controls all of the commands from all of the keyboards and all of the data being generated, and permits each user to believe he or she is the only person working on the computer.

This real-time sharing of resources makes UNIX one of the most powerful operating systems ever.

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Although UNIX was developed by programmers for programmers, it provides an environment so powerful and flexible that it is found in businesses, sciences, academia and industry. Many telecommunications switches and transmission systems are also controlled by and maintenance systems based on UNIX.

While initially designed for medium-sized minicomputers, the operating system was soon moved to larger, more powerful mainframe computers. As personal computers grew in popularity, versions of found their way into these boxes, and a number of companies produce UNIX-based machines for the scientific and programming communities.

1. The uniqueness of UNIX

The features that made UNIX a hit from the start are

Multitasking capability

Multiuser capability

Portability

UNIX programs

Library of application software

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1.1 Multitasking

Many computers do just one thing at a time, as anyone who uses a PC or

laptop can attest. Try logging onto your company's network while opening your

browser while opening a word processing program. Chances are the processor wiil

freeze for a few seconds while it sorts out the multiple instructions.

UNIX, on the other hand, lets a computer do several things at once, such as

printing out one file while the user edits another file. This is a major feature for

users, since users don't have to wait for one application to end before starting

another one.

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1.2 Multiuser

The same design that permits multitasking permits multiple users to use the

computer. The computer can take the commands of a number of users - determined

by the design of the computer - to run programs, access files, and print documents

at the same time. The computer can't tell the printer to print all the requests

at once, but it does prioritize the requests to keep everything; orderly. It also lets several users access the same document by compartmentalizing the document so that the changes of one user don't override the changes of another user.

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1.3 System portability

A major contribution of the UNIX system was its portability, permitting it to

move from one brand of computer to another with a minimum of code changes. At a time when different computer lines of the same vendor didn't talk to each other - yet alone machines of multiple vendors - that meant a great savings in both hardware and software upgrades.

It also meant that the operating system could be upgraded without having all

the customer's data input again. And new versions of UNIX were backward compatible with older versions, making it easier for companies to upgrade in an orderly manner.

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1.4 UNIX tools

UNIX comes with hundreds of programs that can be divided into two classes:

Integral utilities that are absolutely necessary for the operation of the

computer, such as the command interpreter. .

Tools that aren't necessary for the Operation of UNIX but Provide the

user with additional capabilities, such as typesetting capabilities and E-mail

1.5 UNIX communications

E-mail is commonplace today, but it has only come into its own in the business community within the last 10 years. Not so with UNIX users. who have been enjoying E-mail for several decades.

UNIX E-mail at first permitted users on the same computer to communicate with each other via their terminals. Then users on different machines, even made by different vendors, were connected to support E-mail And finally, UNIX systems around the world were linked into a world wide web decades before the development of today's World Wide Web.

1.6 Applications libraries

UNIX as it is known today didn't just develop overnight. Nor were just a few people responsible far its growth. As soon as it moved from Ball Labs into the universities, every computer programmer worth his or her own salt started developing programs for UNIX, Today there are hundreds of UNIX applications that can be purchased from third-party vendors, in addition to the applications that come with UNIX.

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2. How UNIX is organized

The UNIX system is functionally organized at three levels:

The kernel, which schedules tasks and manages storage;

The shell, which connects and interprets users' commands, calls programs from

memory, and executes them.

The tools and applications that offer additional functionality to the operating

system.

2.1 The kernel

The heart of the operating system, the kernel controls the hardware and turns part of the system on and off at the programmer's command. If you ask the computer to list (Is) all the files in a directory, the kernel tells the computer to read all the files in that directory from the disk and display them on your screen.

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2.2 The shell

There are several types of shell, most notably the command driven Bourne Shell and the

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