Gawk/Library-Functions

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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,⁶⁵ 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.
• A number of programs use nextfile (see section The nextfile Statement) to skip any remaining input in the input file.

• 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 effect⁶⁶ 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.

• Library Names		How to best name private global variables in library functions.
• General Functions		Functions that are of general use.
• Data File Management		Functions for managing command-line data files.
• Getopt Function		A function for processing command-line arguments.
• Passwd Functions		Functions for getting user information.
• Group Functions		Functions for getting group information.
• Walking Arrays		A function to walk arrays of arrays.
• Library Functions Summary		Summary of library functions.
• Library Exercises		Exercises.

Footnotes

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Sadly, over 35 years later, many of the lessons taught by this book have yet to be learned by a vast number of practicing programmers.

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The effects are not identical. Output of the transformed record will be in all lowercase, while IGNORECASE preserves the original contents of the input record.

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