Appendix A. Administrative Shell Programming

The purpose of this appendix is to review major Bourne shell (sh) programming features. It is not intended as a comprehensive treatment of shell programming or of the features of the various shells. Rather, it will enable you to understand and modify the system administration scripts on your system, most of which are Bourne shell scripts (although this is slowly changing).

In the course of this appendix, we will look at many examples drawn from actual system scripts, as well as some other simple examples to illustrate basic features. Some of the latter examples use shell commands executed at the command prompt (although the corresponding commands could obviously appear just as easily in scripts).^[1]

^[1] Not all examples will necessarily run in every sh implementation.

With the exception of AIX and Linux, the Unix versions we are considering use the Bourne shell for system scripts. AIX uses the Korn shell (ksh), and Linux uses the Bourne-Again shell (bash). Linux system scripts also frequently use bash features that are not part of the standard shell. Since they are extensions to sh, however, the most important of these features are now described in this appendix.^[2] When I mention bash features here, I am doing so in a descriptive sense only not in an historical sense in comparison to what is offered in the standard Bourne shell. The feature in question may also be present in other shells and may very well have originated in a shell other than bash.

^[2] This discussion covers bash Version 2.04 or later.

The books UNIX is a Nutshell: System V Edition, by Arnold Robbins, and Learning the bash Shell, by Cameron Newham and Bill Rosenblatt (both published by O'Reilly & Associates) are excellent references for sh and bash, respectively.

Discussing Korn shell features is beyond the scope of this appendix; consult the book Learning the Korn Shell by Bill Rosenblatt and Arnold Robbins (O'Reilly & Associates) for a detailed discussion of this shell.

As much as possible, the examples in this appendix come from actual system scripts. Thus, while there are many useful techniques illustrated in the examples, they should not generally be viewed as recommendations of shell programming style, and many readers may quibble or disagree with them at some points.