POSIX and Korn Shell

This tutorial describes the POSIX Shell (/bin/posix/sh) and the Korn Shell (/bin/ksh). POSIX Shell is based on the standard, IEEE P1003.2. Korn Shell is the command interpreter (human interface) written by David Korn at AT&T Bell Laboratories. The tutorial covers the similarities and the differences between these two shells, as well as the new features introduced in the POSIX Shell.

This tutorial addresses new users who are just learning shells, as well as advanced users who are Bourne and C Shell experts. New users should read all of this tutorial. However, at the beginning of each chapter advanced users will be directed to only the areas they need to concentrate on to come up to speed quickly. Both classes of users should finish reading this section.

The kernel is the supervisory part of the HP-UX operating system that keeps track of and assigns system resources to each user. It also coordinates I/O operation, and performs other software functions. A shell is a program that acts as an interface between the system and each user. The shell interprets user commands, then performs system calls to the kernel or loads and runs programs according to the commands given by the user.

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Learning the Korn Shell

by Bill Rosenblatt and Arnold Robbins

The long, tortuous history of the Unix operating system has resulted in systems with all kinds of permutations and combinations of features. This means that whenever you walk up to an unfamiliar Unix system, you need to find out certain things about it in order to use it properly. And even on a given system, you may have a number of choices you can make about what features you want to use.

The most important such decision -- if you get to make it -- is what shell to use. "Shell" is Unix jargon for the program that allows you to communicate with the computer by entering commands and getting responses. The shell is completely separate from the Unix operating system per se; it's just a program that runs on Unix. With other systems such as MS-DOS, Microsoft Windows, Macintosh, OpenVMS, and VM/CMS, the command interpreter or user interface is an integral part of the operating system.

Nowadays there are dozens of different shells floating around, ranging from the original standard, the Bourne shell, to menu-based and graphical interfaces. The most important shells have been the Bourne shell, the C shell, and the Korn shell -- the subject of this book.

Specifically, this book describes the 1993 version of the Korn shell. The 1993 version is distributed with the three major commercial versions of Unix: Solaris, HP-UX, and AIX, albeit as part of the Common Desktop Environment (CDE), in /usr/dt/bin/dtksh. The 1988 version is also available on modern Unix systems, usually as /usr/bin/ksh.

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