Classic Shell Scripting

Software Tools Principles

Over the course of time, a set of core principles developed for designing and writing software tools. You will see these exemplified in the programs used for problem solving throughout this book. Good software tools should do the following things:

Do one thing well

In many ways, this is the single most important principle to apply. Programs that do only one thing are easier to design, easier to write, easier to debug, and easier to maintain and document. For example, a program like grep that searches files for lines matching a pattern should not also be expected to perform arithmetic.

A natural consequence of this principle is a proliferation of smaller, specialized programs, much as a professional carpenter has a large number of specialized tools in his toolbox.

Process lines of text, not binary

Lines of text are the universal format in Unix. Datafiles containing text lines are easy to process when writing your own tools, they are easy to edit with any available text editor, and they are portable across networks and multiple machine architectures. Using text files facilitates combining any custom tools with existing Unix programs.

Use regular expressions

Regular expressions are a powerful mechanism for working with text. Understanding how they work and using them properly simplifies your script-writing tasks.

Furthermore, although regular expressions varied across tools and Unix versions over the years, the POSIX standard provides only two kinds of regular expressions, ...