The UNIX operating system consists of three main layers - the kernel layer, shell layer, and utilities/applications layer. The kernel handles processes, memory, files, devices, and networking. The shell sits between the user and kernel and interprets commands. Utilities and applications make up the top layer and include programs like word processors. UNIX is a portable, multiuser, multitasking operating system used widely by programmers, businesses, universities, and governments for its stability and ability to perform many tasks simultaneously.

1. Structure of Unix Operating System

2. The UNIX operating system (OS) consists of a kernel layer, a shell
layer and a utilities and applications layer. These three layers create
a portable, multiuser, multitasking operating system. There are
multiple versions of the OS, but every version has the exact same
structure. UNIX is used by programmers, businesses, universities
and governments because of its stability and its ability to perform
many tasks simultaneously.

4. UNIX is used by programmers and businesses because of its stability
Basics
The UNIX operating system is a multiuser, multitasking operating system
originally designed for programmers by AT&T employees at Bell Laboratories
in 1969. The terms multiuser and multitasking mean that many different users
can perform tasks simultaneously on one machine. Each user interacts with
her own instance of the shell, and can start one or more applications within
that shell.
Kernel
The Kernel is the heart of the UNIX OS. It is a software application that
provides the interface between the hardware and the user. It handles the
process, memory, file, device and network management for the operating
system. The kernel is responsible for ensuring that all system and user tasks
are performed concurrently.

5. Shell
The shell is the program that sits between the user and the kernel. It
is the interpreter that translates the commands that are typed into
the terminal session. Users can type commands directly into the
terminal, or they can create a text file containing a series of
commands that can be sent to the shell. The series of commands are
called a shell script.
There are multiple shells that are used by the UNIX OS. They include
the Bourne shell (sh), the C shell (csh), the Korn shell (ksh) and the
Bourne Again shell (bash). Each shell has own set of shell
commands. Operating system commands are the same across all the
shells.

6. The initial shell that the user logs into is defined by the system
administrator. The user can change her default shell by using the
"chsh" command. Users may want to change their shells in order to
use particular features that are available in one shell or not
another, or they may simply prefer a particular shell environment.
Utilities and Application
The final layer of the UNIX OS is the Utilities and Applications layer.
This layer includes the commands, word processors, graphic
programs and database management programs. Traditionally, these
programs were accessed by typing the commands to start the
program on the command line. They can still be accessed in this
way, but they can now also be accessed through the GUI.

7. Some key features of the Unix architecture concept are:
Unix systems use a centralized operating system kernel which
manages system and process activities.
Unix systems are preemptively multitasking: multiple processes can
run at the same time, or within small time slices and nearly at the
same time, and any process can be interrupted and moved out of
execution by the kernel. This is known as thread management.
Files are stored on disk in a hierarchical file system, with a single top
location throughout the system (root, or "/"), with both files and
directories, subdirectories, sub-subdirectories, and so on below it.

8. The UNIX operating system supports the following features and
capabilities:
Multitasking and multiuser
Programming interface
Use of files as abstractions of devices and other objects
Built-in networking (TCP/IP is standard).