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.
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.
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.
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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.
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.
There are multiple versions of the UNIX operating system. Proprietary versions include Sun Microsystem's Solaris, SCO UNIX, IBM's AIX and Hewett Packard's HP-UX. FreeBSD, NetBSD and OpenBSD are open source versions of Unix. While unknown to many mainstream users, Apple OS X is also proprietary version of UNIX. A feature that sets OS X apart from other UNIX variants is its ability to run applications usually written for PCs running Windows, such as Microsoft Office and Adobe Photoshop. Linux is a UNIX-like operating system. It has the same structure as UNIX, but it was written using none of the original UNIX code base.