What Is UNIX?

UNIX is a computer operating system.
An operating system is the program that controls all the other parts of a computer system, both the hardware and the software. It allocates the computer's resources and schedules tasks. It allows you to make use of the facilities provided by the system. Every computer requires an operating system.
UNIX is a multi-user, multi-tasking operating system. Multiple users may have multiple tasks running simultaneously. This is very different from PC operating systems such as MS-DOS or MS-Windows (which allows multiple tasks to be carried out simultaneously but not multiple users).
UNIX is a machine independent operating system. Not specific to just one type of computer hardware. Designed from the beginning to be independent of the computer hardware.
UNIX is a software development environment. Was born in and designed to function within this type of environment.
The "UNIX" trademark, previously owned by AT&T and then deeded to UNIX Systems Laboratories (USL), an AT&T subsidiary, passed to Novell when it acquired USL. After a brief period of negotiations with rival Unix vendors, namely, Sun Microsystems, Santa Cruz Operation, International Business Machines, and Hewlett-Packard, Novell granted exclusive licensing rights of the UNIX trademark to X/Open Co. Ltd., an Open Systems industry standards branding agent based in the United Kingdom.

History of UNIX

1969: Developed at AT&T Bell Labs in Murray Hill, New Jersey, one of the largest research facilities in the world. Created in an environment when most computer jobs were fed into a batch system.
Developed by researchers who needed a set of computing tools to help them with their projects and their collaborators. Allowed a group of people working together on a project to share selected data and programs.
1975: AT&T makes UNIX widely available - offered to educational institutions at minimal cost. Becomes popular with university computer science programs. AT&T distributes standard versions in source form: Version 6 (1975), Version 7 (1978), System III (1981).
1984 to date: University of California, Berkeley adds major enhancements, creates Berkeley Standard Distribution (BSD)
1984 to date: Many Berkeley features incorporated into new AT&T version: System V
UNIX has become the operating system of choice for engineering and scientific workstations.
Two variations maintain popularity today, AT&T System V based and the Berkeley Standard Distribution.
Current versions (1/95)are System V release 4.2 .and 4.4 BSD
Work is in progress to develop a Portable Operating System specification based on UNIX (IEEE POSIX committee).

UNIX Philosophy

Make each program do one thing well. Reusable software tools: 1 tool = 1 function
Expect the output of every program to become the input of another, yet unknown, program to combine simple tools to perform complex tasks
Prototyping: get something small working as soon as possible and modify it incrementally until it is finished
Use terse commands and messages: reduces typing and screen output

operating system code is written in C language rather than a specific assembly language
operating system software can be easily moved from one hardware system to another
UNIX applications can be easily moved to other UNIX machines. Porting is usually as simple as transfer of the source and a recompile

The core of the UNIX system. Loaded at system start up (boot). Memory-resident control program.
Manages the entire resources of the system, presenting them to you and every other user as a coherent system. Provides service to user applications such as device management, process scheduling, etc.
Example functions performed by the kernel are:

managing the machine's memory and allocating it to each process.
scheduling the work done by the CPU so that the work of each user is carried out as efficiently as is possible.
accomplishing the transfer of data from one part of the machine to another
interpreting and executing instructions from the shell
enforcing file access permissions

You do not need to know anything about the kernel in order to use a UNIX system. These details are provided for your information only.

Whenever you login to a Unix system you are placed in a shell program. The shell's prompt is usually visible at the cursor's position on your screen. To get your work done, you enter commands at this prompt.
The shell is a command interpreter; it takes each command and passes it to the operating system kernel to be acted upon. It then displays the results of this operation on your screen.
Several shells are usually available on any UNIX system, each with its own strengths and weaknesses.
Different users may use different shells. Initially, your system adminstrator will supply a default shell, which can be overridden or changed. The most commonly available shells are:

Each shell also includes its own programming language. Command files, called "shell scripts" are used to accomplish a series of tasks.

AT&T distributes System V for their computers. System V is also the basis for several commercial implementations including:

BSD, from the University of California Berkeley, has undergone extensive modification and enhancement in the university environment.
BSD is available directly from UCB and in a number of commercial versions including: Sun, Apollo, DEC Ultrix, Gould UTX/32.
System V and BSD contain a large set of commands in common. Some of these commands, however, support different options and have different default behaviors and output formats. ex: ls, stty, mail, grep
Each version also has its own unique utilities. Some very common tasks, such as browsing a file, are performed by totally different utilities: System V uses "pg" whereas BSD uses "more".