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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, and tools that aren't necessary for the operation of UNIX but provide the user with additional capabilities, such as typesetting capabilities and e-mail; the tools can be added or removed from a UNIX system, depending upon the applications required. .
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; and the tools and applications that offer additional functionality to the operating system. .
The kernel, the heart of the operating system, controls the hardware and turns part of the system on and off at the programmer's command. If you ask the computer to list (ls) 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. .
There are several types of shell, most notably the command driven Bourne Shell and the C Shell, 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. 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. .
There are hundreds of tools available to UNIX users and they are typically grouped into categories for certain functions, such as word processing, business applications, or programming. .
LINUX, a 1991 version of UNIX, was developed by Linus Torvalds, a student at the time, and by hundreds of volunteer programmers around the world. This system can be downloaded at absolutely no charge from the Internet.
