Classic Shell Scripting - Arnold Robbins [13]
For example, the netpbm suite of programs[6] manipulate image files stored in a Portable BitMap format.[7] These files contain bitmapped images, described using a well-defined format. Each tool reads PBM files, manipulates the contained image in some fashion, and then writes a PBM format file back out. This makes it easy to construct a simple pipeline to perform complicated image processing, such as scaling an image, then rotating it, and then decreasing the color depth.
Let someone else do the hard part
Often, while there may not be a Unix program that does exactly what you need, it is possible to use existing tools to do 90 percent of the job. You can then, if necessary, write a small, specialized program to finish the task. Doing things this way can save a large amount of work when compared to solving each problem fresh from scratch, each time.
Detour to build specialized tools
As just described, when there just isn't an existing program that does what you need, take the time to build a tool to suit your purposes. However, before diving in to code up a quick program that does exactly your specific task, stop and think for a minute. Is the task one that other people are going to need done? Is it possible that your specialized task is a specific case of a more general problem that doesn't have a tool to solve it? If so, think about the general problem, and write a program aimed at solving that. Of course, when you do so, design and write your program so it follows the previous rules! By doing this, you graduate from being a tool user to being a toolsmith, someone who creates tools for others!
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[5] For those who are really worried, the -i option to rm forces rm to prompt for confirmation, and in any case rm prompts for confirmation when asked to remove suspicious files, such as those whose permissions disallow writing. As always, there's a balance to be struck between the extremes of never prompting and always prompting.
[6] The programs are not a standard part of the Unix toolset, but are commonly installed on GNU/Linux and BSD systems. The WWW starting point is http://netpbm.sourceforge.net/. From there, follow the links to the Sourceforge project page, which in turn has links for downloading the source code.
[7] There are three different formats; see the pnm(5) manpage if netpbm is installed on your system.
Summary
Unix was originally developed at Bell Labs by and for computer scientists. The lack of commercial pressure, combined with the small capacity of the PDP-11 minicomputer, led to a quest for small, elegant programs. The same lack of commercial pressure, though, led to a system that wasn't always consistent, nor easy to learn.
As Unix spread and variant versions developed (notably the System V and BSD variants), portability at the shell script level became difficult. Fortunately, the POSIX standardization effort has borne fruit, and just about all commercial Unix systems and free Unix workalikes are POSIX-compliant.
The Software Tools principles as we've outlined them provide the guidelines for the development and use of the Unix toolset. Thinking with the Software Tools mindset will help you write clear shell programs that make correct use of the Unix tools.
Chapter 2. Getting Started
When you need to get some work done with a computer, it's best to use a tool that's appropriate to the job at hand. You don't use a text editor to balance your checkbook or a calculator to write a proposal. So too, different programming languages meet different needs when it comes time to get some computer-related task done.
Shell scripts are used most often for system administration tasks, or for combining existing programs to accomplish some small, specific job. Once you've figured out how to get the job done, you can bundle up the commands into a separate program, or script, which you can then run directly. What's more, if it's