The UNIX System

The UNIX Operating System is divided into three major components:

The Kernel

This is the heart of the UNIX Operating System. It performs the tasks that create and maintain the UNIX environment. It keeps track of the disks, tapes, printers, terminals, communication lines and any other devices attached to the computer. It also interfaces between the computer's hardware and the users.

The Shell

This is a program that interfaces between the user and the UNIX Operating System. It listens to the user's terminal and translates the actions requested by the user. There are a number of different Shells that may be used.

Utilities and Application Programs

Utilities are the UNIX Commands. Application Programs, such as Word Processors, Spreadsheets and Database Management Systems, may be installed alongside the UNIX Commands. A user may run a utility or application through the shell.

The Shell

The Shell is a program that provides an interpreter and interface between the user and the UNIX Operating System. It executes commands that are read either from a terminal or from a file. Files containing commands may be created, allowing users to build their own commands. In this manner, users may tailor UNIX to their individual requirements and style.

There are a number of different Shells. Each provides a slightly different interface between the user and the UNIX Operating System. The most important shells are:

sh
The Bourne Shell. This was written by Stephen Bourne of AT&T Bell Laboratories. This is the original shell and is the standard shell used in the UNIX Operating System.

csh
The C Shell. This was written by William Joy at the University of California at Berkley and is available on machines running Berkley UNIX or those with the Berkley enhancements. The shell derives its name from the fact that many of its programming features resemble the syntax of the C programming language. It has several advantages over the Bourne shell, including a history facility that allows the user to call upon previously-used commands as well as edit them for reuse. It also has an alias facility that allows the user to give long command sequences shorthand names. The C shell is quite different from the Bourne shell, in both its usage and its actions.

ksh
The Korn Shell. This was written by David Korn also of AT&T Bell Laboratories. This is a newer shell program that is rapidly becoming one of the most popular of shells. Its format and usage mirror those of the Bourne shell, so that a user may use it without being aware of the differences. However, it has facilities, such as, history and alias, that make it almost as versatile as the C shell.

There are other shells that are less widely used and not available on many machines. For example, there is the Restricted Shell - rsh. This restricts the area of memory the user may access to his or her own directory, thus limiting access to all other users' files.

A command issued by a user may be run in the present shell, or the shell itself may start up another copy of itself in which to run that command. In this way, a user may run several commands at the same time. A secondary shell is called a sub-shell.

When a user logs onto the System, a shell is automatically started. This will monitor the user's terminal, waiting for the issue of any commands. The type of shell used is stored in a file called passwd in the subdirectory. (see Section 3.2). Any other shell may be run as a sub-shell by issuing it as a command. For example, /bin/ksh will run a Korn shell. The original shell will still be running - in background mode - until the Korn shell is terminated.

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