The UNIX operating system has always occupied an important position in the scientific and technical communities. Currently, there are many large-scale data management and transaction processing systems on the UNIX platform. Moreover, this OS is the core of the servers of the Internet backbone network. The proposed edition is addressed primarily to programmers already familiar with UNIX, who are going to develop software for this operating system in C. In addition to an overview of basic concepts and terminology, the book provides a description of system file access primitives, UNIX processes, and how to work with them. Interprocess interaction is considered, work with major libraries is covered. The book will also be useful to developers of system software, application and business applications.