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 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 view a list of POSIX standards, visit the IEEE Standards Web page at http://standards.ieee.org/catalog/ olis/posix.html.

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 electronic-mail program are part of the standard set. You 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 Boume-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 X.Org X11.

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