Part II:
Problem Solving with awk ¶

10 A Library of awk Functions ¶

User-Defined Functions describes how to write your own awk functions. Writing functions is important, because it allows you to encapsulate algorithms and program tasks in a single place. It simplifies programming, making program development more manageable and making programs more readable.

In their seminal 1976 book, Software Tools,⁶⁷ Brian Kernighan and P.J. Plauger wrote:

Good Programming is not learned from generalities, but by seeing how significant programs can be made clean, easy to read, easy to maintain and modify, human-engineered, efficient and reliable, by the application of common sense and good programming practices. Careful study and imitation of good programs leads to better writing.

In fact, they felt this idea was so important that they placed this statement on the cover of their book. Because we believe strongly that their statement is correct, this chapter and Practical awk Programs, provide a good-sized body of code for you to read and, we hope, to learn from.

This chapter presents a library of useful awk functions. Many of the sample programs presented later in this Web page use these functions. The functions are presented here in a progression from simple to complex.

Extracting Programs from Texinfo Source Files presents a program that you can use to extract the source code for these example library functions and programs from the Texinfo source for this Web page. (This has already been done as part of the gawk distribution.)

If you have written one or more useful, general-purpose awk functions and would like to contribute them to the awk user community, see How to Contribute, for more information.

The programs in this chapter and in Practical awk Programs, freely use gawk-specific features. Rewriting these programs for different implementations of awk is pretty straightforward:

• Diagnostic error messages are sent to /dev/stderr. Use ‘| "cat 1>&2"’ instead of ‘> "/dev/stderr"’ if your system does not have a /dev/stderr, or if you cannot use gawk.
• Finally, some of the programs choose to ignore upper- and lowercase distinctions in their input. They do so by assigning one to IGNORECASE. You can achieve almost the same effect⁶⁸ by adding the following rule to the beginning of the program:

  # ignore case
  { $0 = tolower($0) }
  

  Also, verify that all regexp and string constants used in comparisons use only lowercase letters.

• Naming Library Function Global Variables
• General Programming
• Data file Management
• Processing Command-Line Options
• Reading the User Database
• Reading the Group Database
• Traversing Arrays of Arrays
• Summary
• Exercises