~r<br >Introduction<br > Most UNIX systems， including AT&T s System V (SV)， XENIX， Berkeley<br > Software Distribution (BSD)， Ultrix， and Su nOS， are distributed with over 200<br > programs， commonly called commands or utilities. By combining two or<br > more of these utilities with some of the features of the shell (one of the 200<br > programs)， UNIX users can create their own programs of commands. These<br > can perform almost any kind of task， from the relatively simple one of<br > enabling the user to use an oft-repeated group of commands by typing just<br > one command， to handling the complex functions of a database management<br > system. Thus UNIX users can write programs without being programmers.<br > The UNIX shell provides a simple but powerful mechanism that enables<br > nonprogrammers to write programs: the shellscript. A shell script is a text file<br > that consists of UNIX commands， possibly used with such programming<br > constructs as loops， if statements， and variables. The constructs are actually<br > quite simple--the power of the shell script comes from the UN IX commands，<br > because each command is a program in its own right. When you create a shell<br > script that does a particular task， you are creating a custom tool for your own<br > use--a tool that saves you from having to remember complicated sequences<br > of commands， helps prevent typing errors， and automates frequently used<br > tasks. (People in the UNIX world frequently share their tools with coworkers<br > and others by electronically mailing copies to requestors and by sending<br > copies to "archive" sites where they may be downloaded.)<br > To become proficient in writing your own tools， you have to know a<br > little about the shell s programming constructs and a lot about the individual<br > commands supplied with your system.<br > Many books have been written on shell programming， and a few on the<br > utilities. This book contains a tutorial on using the commands of AT&T UNIX<br > System V Release 4， and a reference section for those commands. The<br > emphasis， of course， is on the commands that are most useful in shell scripts.<br > l provide abundant examples and give a number of helpful hints.<br > xiii<br >