A.1 UNIX/Linux Operating System: An Overview

The UNIX operating system [1, 2] was designed in the 1970s with the goal of providing:

• Simultaneous computer access to a multiplicity of users;

• Easy sharing of information.

The operating system soon became popular among universities and research groups, as it provided a good environment for program development, network transactions, and information sharing. Another remarkable reason for UNIX's immediate success was that it is written in the C high-level language. This makes the operating system highly portable. The other operating systems, in fact, are typically implemented in low-level languages, thus they are strictly tied to the specific platform for which they were developed.

The simplicity and clarity of UNIX programs and the widespread knowledge of the C language tempted many developers to enhance UNIX in their own way. This generated both the enhancement of the operating system with a lot of utilities and services oriented to the end user and the birth of a number of UNIX different dialects, running on a variety of platforms, from microprocessors to mainframes. To make order in the myriad of UNIX versions and dialects, the IEEE formed a committee to define standard specifications on operating systems. This work produced a well-known family of standards, called POSIX [3], which still contains the guidelines governing new generation UNIX systems. POSIX documents are not available online but can be purchased in printed form from the IEEE Computer Society [2].

Linux is a completely free reimplementation of UNIX, following most of...