Linux as a UNIX Platform

Like other UNIX systems, Linux is a multiuser, multitasking operating system, which means that it enables multiple users to log in and to run more than one program at the same time.

UNIX was developed in the early 1970s at AT&T Bell Laboratories. Its development came on the heels of another operating system called MULTICS; developers are said to have come up with the name UNIX by changing the MULT in MULTICS to UN (meaning one). Bell Laboratories continued to develop UNIX and released several versions: System III, followed by System V Release 1, or SVR1, and SVR2, SVR3, and SVR4.

As it maintained and enhanced UNIX, Bell Laboratories distributed source code to educational institutions. The University of California at Berkeley2 (UC Berkeley) was one of the schools that received a copy of UNIX and added many new features to the operating system. Eventually, UC Berkeley released its version of UNIX, called Berkeley Software Distribution (BSD) UNIX. The most widely used versions of BSD UNIX are 4.3 and 4.4 (known as 4.4BSD).

By the time 4.4BSD UNIX came out, UC Berkeley realized that there was very little original Bell Laboratories UNIX code in the source code. Soon, several groups wrote new code to replace the small amount of leftover Bell Laboratories code and adapted BSD UNIX to the Intel 386 processor. This resulted in the FreeBSD and NetBSD versions of freely available BSD UNIX for Intel PCs.

Note that UNIX System V, Release 4—SVR4—combines all features of System V and BSD UNIX.

POSIX Compliance

Linux is designed to comply with IEEE Std 1003.1 1996 Edition (POSIX). This standard defines the functions that applications written in the C programming language use to access the services of the operating system—for tasks ranging from opening a file to allocating memory. On March 8, 1996, the Computer Systems Laboratory of the National Institute of Standards and Technology (NIST), a U.S. government agency, confirmed that Linux version 1.2.13, as packaged by Open Linux Ltd., conforms to the POSIX standard. To see a list of POSIX-validated products, point your Web browser to http://www.nist.gov/itl/div897/ctg/posix/finalreg4.htm. Note that the NIST POSIX testing program ended on December 31, 1997. Of course, POSIX compliance, while commendable, is not synonymous with a high-quality operating system.

Along with POSIX conformance, Linux includes many features of other UNIX standards, such as the System V Interface Document (SVID) and the Berkeley Software Distribution (BSD) version of UNIX. Linux takes an eclectic approach, picking the most-needed features of several standard flavors of UNIX.

POSIX stands for Portable Operating System Interface (abbreviated as POSIX to make it sound like UNIX). The Institute of Electrical and Electronics Engineers (IEEE) began developing the POSIX standards to promote the portability of applications across UNIX environments. POSIX is not limited to UNIX, however. Many other operating systems, such as Hewlett-Packard OpenVMS and Microsoft Windows NT/2000/XP, implement POSIX—in particular, the IEEE Std. 1003.1 1996 Edition, or POSIX.1, which provides a source-level C-language Application Program Interface (API) to the services of the operating system, such as reading and writing files. POSIX.1 has been accepted by the International Organization for Standardization (ISO) and is known as the ISO/IEC 9945-1:1996 standard.

Incidentally, the term POSIX is used interchangeably with the IEEE 1003 and 2003 family of standards. There are several other IEEE standards besides the 1003 and 2003 family—such as 1224 and 1228—that also provide APIs for developing portable applications. For the latest information on all IEEE standards, visit the IEEE Standards Home Page at http://standards.ieee.org/. To read summary information about the POSIX standards, visit the IEEE Web page at http://standards .ieee.org/reading/ieee/std_public/description/posix/.

In addition to POSIX (IEEE 1003.1) compliance, Linux supports the IEEE 1003.2 standard, which focuses on the operating system’s command interpreter (commonly referred to as the shell) and a standard set of utility programs. If you know UNIX or you’ve had some exposure to it, you know that UNIX takes a tools-oriented view of the operating system. It provides a tool for almost anything you might want to do, and the shell enables you to combine several tools to perform tasks more complicated than those the basic tools handle. The IEEE 1003.2 standard maintains this tools-oriented view, providing the following features:

A shell with a specified set of built-in commands and a programming syntax that can be used to write shell programs, or scripts
A standard set of utility programs—such as sed, tr, and awk—that shell scripts and applications can call. Even the vi editor and the mail electronic-mail program are part of the standard set. You will learn more about these utilities in Chapters 8, 10, and 11.
A set of C functions, such as system and getenv, that applications can use to access features of the shell
A set of utilities, such as Perl and Tcl, for developing shell applications

The default Linux shell is called Bash, which stands for Bourne-Again Shell—a reference to the Bourne shell, which has been the standard UNIX shell since the early days of UNIX. Bash incorporates many of the features IEEE 1003.2 requires and then some. It essentially inherits the features and functionality of the Bourne shell. In case of any discrepancy between the Bourne shell and IEEE 1003.2, Bash follows IEEE 1003.2. For stricter IEEE 1003.2 compliance, Bash even includes a POSIX mode.

All in all, Linux serves as a good platform for learning UNIX because it offers a standard set of UNIX commands (the IEEE 1003.2 standard, as well as the best features of both System V and BSD UNIX).

Linux’s support for POSIX and other common UNIX system calls (the functions that applications call) makes it an excellent system for software development. Another ingredient of modern workstation software, the X Window System, is also available in Linux in the form of XFree86.

Linux Standard Base (LSB)

Linux has become important enough that there is now a standard for Linux called the Linux Standard Base, or LSB for short. LSB is a set of binary standards that should help reduce variations among the Linux distributions and promote portability of applications. The idea behind LSB is to provide application binary interface (ABI) so that software applications can run on any Linux (or other UNIX) systems that conform to the LSB standard. The LSB specification references the POSIX standards as well as many other standards such as the C programming language standard and the X Window System version 11 release 6 (X11R6). LSB version 1.2 (commonly referred to as LSB 1.2) was released on June 28, 2002. LSB 1.3 went through a public review in early November 2002.

The LSB specification is organized into two parts—a common specification that remains the same across all types of processors and a set of hardware-specific specifications, one for each type of processor architecture. For example, LSB 1.2 has architecture-specific specifications for Intel 32-bit (IA32) and Power PC 32-bit (PPC32) processors. LSB 1.3 adds a specification for the Intel 64-bit (IA64) architecture, in addition to the ones for IA32 and PPC32.

There is an LSB certification program, and by now a number of Linux systems, such as Red Hat Linux 8.0, Mandrake Linux ProSuite 9.0, SuSE Linux 8.1, and UnitedLinux 1.0 LSB, are certified to be LSB 1.2–compliant IA32 runtime environments.

To learn more about LSB, visit http://www.linuxbase.org/. The latest list of LSB-certified systems is available at http://www.opengroup.org/lsb/cert/cert_prodlist.tpl.