By Norman J. Buchanan and Douglas M. Gingrich
What is a shell?
A shell is a command interpreter. While this is certainly true it likely doesn't enlighten the reader any further. A shell is an entity that takes input from the user and deals with the computer rather than have the user deal directly with the computer. If the user had to deal directly with the computer he would not get much done as the computer only understands strings of 1's and 0's. While this is a bit of a misrepresentation of what the shell actually does (the idea of an operating system is neglected) it provides a rough idea that should cause the reader to be grateful that there is such a thing as a shell. A good way to view a shell is as follows. When a person drives a car, that person doesn't have to actually adjust every detail that goes along with making the engine run, or the electronic system controlling all of the engine timing and so on. All the user (or driver in this example) needs to know is that D means drive and that pressing accelerator pedal will make the car go faster or slower. The dashboard would also be considered part of the the shell since pertinent information relating to the user's involvement in operating the car is displayed there. In fact any part of the car that the user has control of during operation of the car would be considered part of the shell. I think the idea of what a shell is coming clear now. It is a program that allows the user to use the computer without him having to deal directly with it. It is in a sense a protective shell that prevents the user and computer from coming into contact with one another.
Basic Unix primer
While it assumed that the reader has a familiarity of Unix, a brief overview can only enhance previous knowledge. Unix comes in a variety of constantly changing flavors (SUNOS, HPUX, BSD and Solaris, just to name a few). Each of these Unix types will have small variations from all of the others. This may seem a bit discouraging at first, but in reality each version of Unix has more in common with all of the others than differences. The ls, for example, will give a listing of the current directory in any Unix environment. The changes or semantics local to any particular brand of Unix should be explained in the man pages that come with that particular system. The purpose of this book is not to explore the differences between differnt Unix flavors but rather to assume that they are all equivalent and look at how the different shells behave. Hence, the rest of the book assumes a kind of generic Unix operating system (except where explicitly stated otherwise).
While it assumed that the reader has a familiarity of Unix, a brief overview can only enhance previous knowledge. Unix comes in a variety of constantly changing flavors (SUNOS, HPUX, BSD and Solaris, just to name a few). Each of these Unix types will have small variations from all of the others. This may seem a bit discouraging at first, but in reality each version of Unix has more in common with all of the others than differences. The ls, for example, will give a listing of the current directory in any Unix environment. The changes or semantics local to any particular brand of Unix should be explained in the man pages that come with that particular system. The purpose of this book is not to explore the differences between differnt Unix flavors but rather to assume that they are all equivalent and look at how the different shells behave. Hence, the rest of the book assumes a kind of generic Unix operating system (except where explicitly stated otherwise).