This is a completely updated edition of the classic guide to shell programming on Unix systems. Following the presentation methodology adopted in the original edition, the authors of the book focus on the standard POSIX shell, explaining the features of developing useful programs in this convenient environment in order to extract maximum benefit from the potential inherent in the basis of Unix and similar operating systems. After a brief overview of the commands Unix, the book goes into detail about the step-by-step process of creating shell programs or scripts, debugging them, and how they work in a shell environment. All basic shell features are explained with many practical examples, making it easy to write shell scripts for specific applications. The book also covers the basic features of the Korn and Bash shells.