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Although the preceding commands and features are useful for day-to-day work with UNIX, the real power of this operating system comes from the user s ability to create shell scripts. In this section, you ll start slowly by building a simple shell program, and you ll proceed to build up your confidence and skill level as you move along into branching, looping, and all that good stuff.

1. This script is based on an idea by Heiner Stevens. You can find the original implementation at http:// shelldorado.com/scripts/cmds/timeout.txt/.

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A shell script (or shell program) is simply a file containing a set of commands. The shell script looks just like any regular UNIX file, but it contains commands that can be executed by the shell. Although you ll learn mostly about Korn shell programming here, Bourne and C shell programming are similar in many ways. If you want to make the Korn shell your default shell, ask your system administrator to set it up by changing the shell entry for your username in the /etc/passwd file. Before you begin creating a shell program, you should understand that shell programs don t contain any special commands that you can t use at the command prompt. In fact, you can type any command in any shell script at the command prompt to achieve the same result. All the shell program does is eliminate the drudgery involved in retyping the same commands every time you need to perform a set of commands together. Shell programs are also easy to schedule on a regular basis.

You learned earlier in this chapter how shell variables are used to set up your UNIX environment. It s common to set variables within shell programs, so that these variables will hold their values for as long as the shell program executes. If you re running the shell program manually, you can set the shell variables in the session you re using, and there s really no need for separate specification of shell variables in the shell program. However, you won t always run a shell program manually that defeats the whole purpose of using shell programs in the first place. Shell programs are often run as part of the cron job, and they could be run from a session that doesn t have all the environmental variables set correctly. By setting shell variables in the program, you can make sure you re using the right values for key variables such as PATH, ORACLE_SID, and ORACLE_HOME.

In order to write good shell scripts, you must understand how to use the test command. Most scripts involve conditional (if-then, while-do, until-do) statements. The test command helps in determining whether a certain condition is satisfied or not. The test command evaluates an expression and returns a 0 value if the condition is true; otherwise it returns a value greater than 0, usually 1.

tlb" While information in a header file is used only at compile time, information in a COM type library is also used for different runtime features Based on information in type libraries, COM can dynamically create proxies for remote procedure calls and dynamically invoke functions for scripting scenarios Due to the required runtime availability, the type library is often embedded in the COM server #import can also extract a type library from an existing COM server, as follows: #import "AComServerWithAnEmbeddedTypeLibrarydll" For NET assemblies, a description of the assembly itself, as well as its contents, is a mandatory part of the assembly; not an optional part, as in COM In NET, this description is called metadata Metadata in NET is mandatory because it is required by runtime services like garbage collection Most (but not all) metadata is bound to the NET types defined by an assembly.

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