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The GNU Awk User's Guide

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1.1 Using This Book

The term awk refers to a particular program as well as to the language you use to tell this program what to do. When we need to be careful, we call the language "the awk language," and the program "the awk utility." This Web page explains both the awk language and how to run the awk utility. The term awk program refers to a program written by you in the awk programming language.

Primarily, this Web page explains the features of awk, as defined in the POSIX standard. It does so in the context of the gawk implementation. While doing so, it also attempts to describe important differences between gawk and other awk implementations.(3) Finally, any gawk features that are not in the POSIX standard for awk are noted.

This Web page has the difficult task of being both a tutorial and a reference. If you are a novice, feel free to skip over details that seem too complex. You should also ignore the many cross-references; they are for the expert user and for the online Info version of the document.

There are subsections labelled as Advanced Notes scattered throughout the Web page. They add a more complete explanation of points that are relevant, but not likely to be of interest on first reading. All appear in the index, under the heading "advanced features."

Most of the time, the examples use complete awk programs. In some of the more advanced sections, only the part of the awk program that illustrates the concept currently being described is shown.

While this Web page is aimed principally at people who have not been exposed to awk, there is a lot of information here that even the awk expert should find useful. In particular, the description of POSIX awk and the example programs in A Library of awk Functions, and in Practical awk Programs, should be of interest.

Getting Started with awk, provides the essentials you need to know to begin using awk.

Regular Expressions, introduces regular expressions in general, and in particular the flavors supported by POSIX awk and gawk.

Reading Input Files, describes how awk reads your data. It introduces the concepts of records and fields, as well as the getline command. I/O redirection is first described here.

Printing Output, describes how awk programs can produce output with print and printf.

6. Expressions, describes expressions, which are the basic building blocks for getting most things done in a program.

Patterns Actions and Variables, describes how to write patterns for matching records, actions for doing something when a record is matched, and the built-in variables awk and gawk use.

Arrays in awk, covers awk's one-and-only data structure: associative arrays. Deleting array elements and whole arrays is also described, as well as sorting arrays in gawk.

9. Functions, describes the built-in functions awk and gawk provide, as well as how to define your own functions.

Internationalization with gawk, describes special features in gawk for translating program messages into different languages at runtime.

Advanced Features of gawk, describes a number of gawk-specific advanced features. Of particular note are the abilities to have two-way communications with another process, perform TCP/IP networking, and profile your awk programs.

Running awk and gawk, describes how to run gawk, the meaning of its command-line options, and how it finds awk program source files.

A Library of awk Functions, and Practical awk Programs, provide many sample awk programs. Reading them allows you to see awk solving real problems.

The Evolution of the awk Language, describes how the awk language has evolved since first release to present. It also describes how gawk has acquired features over time.

Installing gawk, describes how to get gawk, how to compile it under Unix, and how to compile and use it on different non-Unix systems. It also describes how to report bugs in gawk and where to get three other freely available implementations of awk.

Implementation Notes, describes how to disable gawk's extensions, as well as how to contribute new code to gawk, how to write extension libraries, and some possible future directions for gawk development.

Basic Programming Concepts, provides some very cursory background material for those who are completely unfamiliar with computer programming. Also centralized there is a discussion of some of the issues surrounding floating-point numbers.

The Glossary, defines most, if not all, the significant terms used throughout the book. If you find terms that you aren't familiar with, try looking them up here.

GNU General Public License, and GNU Free Documentation License, present the licenses that cover the gawk source code and this Web page, respectively.


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