1.1 UNIX Shells

The core of the UNIX operating system is the kernel . It can be thought of as a piece of software that handles all the communications between user software and computer hardware. It is the kernel that decides how to communicate with peripheral devices, how to share time among users of the system, how to allocate memory for different programs running, and how to utilize the processor and other resources. The kernel keeps records of all programs (commonly called processes ) running on the system and shares time among these processes according to a well-defined policy.

Users of the UNIX system don't have a direct interaction with the kernel. Instead, the user always works with a program called the UNIX shell. A shell can be considered a command interpreter. The shell takes user commands, interprets them, and takes the necessary action to execute them. It also provides the output of these commands to the user.

The most commonly used shells are listed in Table 1-1. All of these shells are available on HP-UX.

Table 1-1. Most Frequently Used UNIX Shells

Name	Description
Bourne shell	Available on all UNIX platforms. It is the oldest and most widely used shell.
C shell	It uses a C language-like syntax for shell programming.
Korn shell	It has more complex and advanced features than the Bourne or C shell and takes more time to master.
POSIX shell	IEEE POSIX (Portable Operating System Interface) compliant shell. It is the default and primary shell for all users on HP-UX.

Each of these shells has its own capabilities and limitations. The default shell used on HP-UX systems is the POSIX shell, although the user is free to choose any of the available shells.

Note

Throughout the book, whenever I refer to "shell," it means the default HP-UX POSIX shell. The capabilities of the other three shells will be discussed as we move forward to Chapter 9.

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