The book provides the basics of organization, ideology and architecture, combining different versions of UNIX. The following are considered: kernel architecture (input and output subsystems, memory and process management, as well as the file subsystem), software interface (system calls and basic library functions), user environment (shell shell shell interpreter, basic commands and utilities) and network support (TCPIP family protocols, network subsystem architecture, socket and TLI program interfaces). The second edition includes new material on Linux and FreeBSD operating systems, removed topics that have lost their relevance, adjusted the set of described system calls and library functions, added a glossary of English terms.