UNIX uses shells to accept commands given by the user, there are quite a few different shells available. The various shells all have built in functions which allow for the creation of shell scripts, that is, the stringing together of shell commands and constructs to automate what can be automated in order to make life easier for the user. By default a normal command accepts input from standard input, which we abbreviate to stdin, standard input is the command line in the form of arguments passed to the command. By default a normal command directs its output to standard output, which we abbreviate to stdout, standard output is usually the console display. This book is designed for operating system programmers who want to familiarize themselves with the UNIX environment. Knowledge of OS concepts is needed for this book, but no prior knowledge or experience of UNIX itself is required.