from:
http://www.bell-labs.com/history/unix/tutorial.html<H2>An Overview of the UNIX* Operating System</H2>
<P>The UNIX<A HREF="#" TITLE="UNIX is a registered trademark of the
Santa Cruz Operation"
onClick="MM_openBrWindow('trademark.html','trademark','scrollbars=yes,resizable=yes,width=200,height=200')">*</A>
operating system was designed to let a number of programmers access the
computer at the same time and share its resources.</P>
<P>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.</P>
<P>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.</P>
<P>This real-time sharing of resources make UNIX one of the most
powerful operating systems ever.</P>
<P>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 also are
controlled by administration and maintenance systems based on
UNIX.</P>
<P>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
UNIX found their way into these boxes, and a number of companies
produce UNIX-based machines for the scientific and programming
communities.</P>
<H3>The uniqueness of UNIX</H3>
<P>The features that made UNIX a hit from the start are:</P>
<UL>
<LI><SPAN CLASS="fixLI">Multitasking capability</SPAN></LI>
<LI><SPAN CLASS="fixLI">Multiuser capability</SPAN></LI>
<LI><SPAN CLASS="fixLI">Portability</SPAN></LI>
<LI><SPAN CLASS="fixLI">UNIX programs</SPAN></LI>
<LI><SPAN CLASS="fixLI">Library of application software</SPAN></LI>
</UL>
<H4>Multitasking</H4>
<P>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 will freeze for a few seconds while it
sorts out the multiple instructions.</P>
<P>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.</P>
<H4>Multiusers</H4>
<P>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.</P>
<P>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.</P>
<H4>System portability</H4>
<P>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.</P>
<P>It also meant that the operating system could be upgraded without
having all the customer's data inputted again. And new versions of
UNIX were backward compatible with older versions, making it easier
for companies to upgrade in an orderly manner.</P>
<H4>UNIX tools</H4>
<P>UNIX comes with hundreds of programs that can divided into two
classes:</P>
<UL>
<LI><SPAN CLASS="fixLI"><STRONG>Integral utilities</STRONG> that are absolutely
necessary for the operation of the computer, such as the command
interpreter, and</SPAN></LI>
<LI><SPAN CLASS="fixLI"><STRONG>Tools</STRONG> that aren't necessary for the operation
of UNIX but provide the user with additional capabilities, such as
typesetting capabilities and e-mail.</SPAN></LI>
</UL>
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<P CLASS="caption"><STRONG>Tools can be added or removed from a UNIX
system, depending upon the applications required.</STRONG></P>
</DIV>
<H4>UNIX Communications</H4>
<P>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.</P>
<P>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.</P>
<H4>Applications libraries</H4>
<P>UNIX as it is known today didn't just develop overnight. Nor were
just a few people responsible for it's growth. As soon as it moved
from Bell Labs into the universities, every computer programmer
worth his or her own salt started developing programs for UNIX.</P>
<P>Today there are hundreds of UNIX applications that can be
purchased from third-party vendors, in addition to the applications
that come with UNIX.</P>
<H3>How UNIX is organized</H3>
<P>The UNIX system is functionally organized at three levels:</P>
<UL>
<LI><SPAN CLASS="fixLI">The kernel, which schedules tasks and manages storage;</SPAN></LI>
<LI><SPAN CLASS="fixLI">The shell, which connects and interprets users' commands, calls
programs from memory, and executes them; and</SPAN></LI>
<LI><SPAN CLASS="fixLI">The tools and applications that offer additional functionality
to the operating system</SPAN></LI>
</UL>
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<P CLASS="caption"><STRONG>The three levels of the UNIX system:
kernel, shell, and tools and applications.</STRONG></P>
</DIV>
<H4>The kernel</H4>
<P>The heart of the operating system, the kernel controls the
hardware and turns part of the system on and off at the programer's
command. If you ask the computer to list (<I>ls</I>) 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.</P>
<H4>The shell</H4>
<P>There are several types of shell, most notably the command driven
Bourne Shell and the C Shell (no pun intended), and menu-driven
shells that make it easier for beginners to use. Whatever shell is
used, its purpose remains the same -- to act as an interpreter
between the user and the computer.</P>
<P>The shell also provides the functionality of "pipes,"
whereby a number of commands can be linked together by a user,
permitting the output of one program to become the input to another
program.</P>
<H4>Tools and applications</H4>
<P>There are hundreds of tools available to UNIX users,
although some have been written by third party vendors for specific
applications. Typically, tools are grouped into categories for
certain functions, such as word processing, business applications,
or programming.</P>
