User-defined, 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.
One valuable way to learn a new programming language is to read programs in that language. To that end, this chapter and Sample Programs, provide a good-sized body of code for you to read, and hopefully, 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.
Extract Program, 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 author's collection of awk programs, see How To Contribute, for more information.
The programs in this chapter and in Sample Programs, freely use features that are gawk-specific. 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 Nextfile Statement)
to skip any remaining input in the input file.
Nextfile Function,
shows you how to write a function that does the same thing.
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 effect1 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.
[1] 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.