The book is an introduction to the UNIX software environment and is addressed to those who want to learn how to program using all the tools that come with the operating system. Considered are login, work with files (cat, mv, cp, rm) and directories (cd, mkdir,...), environment basics (variables, masks), filters (grep, sed, awk), shell programming (loops, signals, arguments, standard I/O), introduction to system calls (read, write, open, creat,...), introduction to programming using lex, uss and make, working with documentation using troff, tbl and eqn. The examples given are not invented specifically for this book - some of them later became part of the set of programs used every day. The programs are written in C. It is assumed that the reader knows or at least learns this language. Reading this book by both beginners and experienced users will help you understand how to make working with the system effective and enjoyable.