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Technically speaking, the UNIX kernel "is" the operating
system. It provides the basic full time software connection to the
hardware.
By full time, I mean that the kernel is always
running while the computer is turned on. When the system
boots up, the kernel is loaded. Likewise, the kernel is only
exited when the computer is turned off.
The UNIX kernel is built specifically
for a machine when it is installed. It has a record of all the
pieces of hardware it needs to talk to and knows what languages
they speak (how to turn switches on and off to get a desired
result). Thus, a kernel is not easily ported to another
computer. Each individual computer will have its own tailor-
made kernel. And if the computer's hardware configuration
changes during its life, the kernel must be "rebuilt" (told
about the new pieces of hardware).
However, though the connection between the
kernel and the hardawre is "hardcoded" to a specific machine,
the connection betwen the user and the kernel is generic. That
is the beauty of the UNIX kernel. From your perspective,
regardless of how the kernel interacts with the hardware,
no matter which UNIX computer you use, you will have the same
kernel interface to work with. That is beacuse the hardware
is "hidden" by the kernel.
| The kernel also handles memory
management, input and output requests, and process
scheduling for time-shared operations (we'll talk more
about what this means later).
To help it with its work, the
kernel also executes daemon programs which stay alive as
long as the machine is turned on and help perform tasks
such as printing or serving web documents.
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However, the task of hidding the hardware
is a pretty much full time job for the kernel. As such,
it does not have too much time to provide for a fancy
user-friendly interface. Thus, though the kernel is much
easier to talk to than the hardware, the language of
the kernel is still pretty cryptic.
Fortunately, the UNIX operating system has
built in "shells" which wrap around the kernel and provide a
much more user-friendly interface. Let's take a look at shells.
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