GNU GAWK权威指南：有效awk编程用户手册

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GNU Awk是GNU项目下的一个强大的文本处理工具，专用于编程语言awk的一个实现。这本《GAWK: Effective AWK Programming》用户指南，第三版于2009年7月发布，作者是Arnold D. Robbins。它提供了对GNU Awk 3.1.7版本或更高版本的详尽和全面的介绍，旨在帮助读者掌握AWK语言的有效编程技巧。 AWK是一种强大的脚本语言，特别适合在数据流中进行文本处理、搜索和替换操作。该手册涵盖了AWK的基本语法、变量和表达式、流程控制结构（如条件语句、循环）、正则表达式（在AWK中至关重要）以及高级主题，如函数、数组、文件输入输出和内存管理。此外，书中还强调了AWK与其他编程语言的集成，以及如何利用其内置的实用功能进行高效的文本处理工作。本书的特色在于其权威性和实用性，不仅提供理论知识，还有大量示例代码供读者实践。对于初学者来说，它是学习AWK编程的基石；对于经验丰富的程序员，它则是深入理解并提升AWK技能的宝典。同时，手册中包含了GNU Awk与GNU Free Documentation License的兼容性声明，这意味着读者可以在自由软件的框架下自由复制、分发和修改这份文档。版权方面，该书自1989年至2009年，由Free Software Foundation Inc.持有，并遵循GNU Free Documentation License (v1.3或更高版本)。此许可允许用户在特定条件下使用和分享这份文档，包括保持GNU General Public License作为不变节部分，封面文本需包含特定声明，后封面文本也有所规定。《GAWK: Effective AWK Programming》是一本不可或缺的参考文献，无论你是想要学习AWK编程的新手，还是寻求提高AWK编程效率的高手，都能从中找到有价值的信息和实践指导。

4 GAWK: Eﬀective AWK Programming

After eight years, add another part egrep and two more parts C. Document

very well and release.

The name awk comes from the initials of its designers: Alfred V. Aho, Peter J. Wein-

berger and Brian W. Kernighan. The original version of awk was written in 1977 at AT&T

Bell Laboratories. In 1985, a new version made the programming language more powerful,

introducing user-deﬁned functions, multiple input streams, and computed regular expres-

sions. This new version became widely available with Unix System V Release 3.1 (SVR3.1).

The version in SVR4 added some new features and cleaned up the behavior in some of the

“dark corners” of the language. The speciﬁcation for awk in the POSIX Command Lan-

guage and Utilities standard further clariﬁed the language. Both the gawk designers and

the original Bell Laboratories awk designers provided feedback for the POSIX speciﬁcation.

Paul Rubin wrote the GNU implementation, gawk, in 1986. Jay Fenlason completed

it, with advice from Richard Stallman. John Woods contributed parts of the code as

well. In 1988 and 1989, David Trueman, with help from me, thoroughly reworked gawk for

compatibility with the newer awk. Circa 1995, I became the primary maintainer. Current

development focuses on bug ﬁxes, performance improvements, standards compliance, and

occasionally, new features.

In May of 1997, J¨urgen Kahrs felt the need for network access from awk, and with a

little help from me, set about adding features to do this for gawk. At that time, he also

wrote the bulk of TCP/IP Internetworking with gawk (a separate document, available as

part of the gawk distribution). His code ﬁnally became part of the main gawk distribution

with gawk version 3.1.

See

Section A.6 [Major Contributors to gawk], page 263, for a complete list of those who

made important contributions to gawk.

A Rose by Any Other Name

The awk language has evolved over the years. Full details are provided in Appendix A [The

Evolution of the awk Language], page 257. The language described in this book is often

referred to as “new awk” (nawk).

Because of this, many systems have multiple versions of awk. Some systems have an awk

utility that implements the original version of the awk language and a nawk utility for the

new version. Others have an oawk version for the “old awk” language and plain awk for the

new one. Still others only have one version, which is usually the new one.

All in all, this makes it diﬃcult for you to know which version of awk you should run when

writing your programs. The best advice I can give here is to check your local documentation.

Look for awk, oawk, and nawk, as well as for gawk. It is likely that you already have some

version of new awk on your system, which is what you should use when running your

programs. (Of course, if you’re reading this book, chances are good that you have gawk!)

Throughout this book, whenever we refer to a language feature that should be available

in any complete implementation of POSIX awk, we simply use the term awk. When referring

to a feature that is speciﬁc to the GNU implementation, we use the term gawk.

Often, these systems use gawk for their awk implementation!

Preface 5

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 book 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 book 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.

Finally, any

gawk features that are not in the POSIX standard for awk are noted.

This book 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 book. 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 book 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 Chapter 12 [A Library of awk

Functions], page 186, and in Chapter 13 [Practical awk Programs], page 215, should be of

interest.

Chapter 1 [Getting Started with awk], page 11, provides the essentials you need to know

to begin using awk.

Chapter 2 [Regular Expressions], page 24, introduces regular expressions in general, and

in particular the ﬂavors supported by POSIX awk and gawk.

Chapter 3 [Reading Input Files], page 36, describes how awk reads your data. It intro-

duces the concepts of records and ﬁelds, as well as the getline command. I/O redirection

is ﬁrst described here.

Chapter 4 [Printing Output], page 58, describes how awk programs can produce output

with print and printf.

Chapter 5 [Expressions], page 75, describes expressions, which are the basic building

blocks for getting most things done in a program.

Chapter 6 [Patterns, Actions, and Variables], page 96, 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.

Chapter 7 [Arrays in awk], page 119, covers awk’s one-and-only data structure: associa-

tive arrays. Deleting array elements and whole arrays is also described, as well as sorting

arrays in gawk.

All such diﬀerences appear in the index under the entry “diﬀerences in awk and gawk.”

6 GAWK: Eﬀective AWK Programming

Chapter 8 [Functions], page 130, describes the built-in functions awk and gawk provide,

as well as how to deﬁne your own functions.

Chapter 9 [Internationalization with gawk], page 160, describes special features in gawk

for translating program messages into diﬀerent languages at runtime.

Chapter 10 [Advanced Features of gawk], page 169, 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.

Chapter 11 [Running awk and gawk], page 177, describes how to run gawk, the meaning

of its command-line options, and how it ﬁnds awk program source ﬁles.

Chapter 12 [A Library of awk Functions], page 186, and Chapter 13 [Practical awk

Programs], page 215, provide many sample awk programs. Reading them allows you to see

awk solving real problems.

Appendix A [The Evolution of the awk Language], page 257, describes how the awk

language has evolved since ﬁrst release to present. It also describes how gawk has acquired

features over time.

Appendix B [Installing gawk], page 265, 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.

Appendix C [Implementation Notes], page 285, describes how to disable gawk’s exten-

sions, as well as how to contribute new code to gawk, how to write extension libraries, and

some possible future directions for gawk development.

Appendix D [Basic Programming Concepts], page 301, provides some very cursory back-

ground 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], page 307, 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], page 317, and [GNU Free Documentation License],

page 328, present the licenses that cover the gawk source code and this book, respectively.

Typographical Conventions

This book is written using Texinfo, the GNU documentation formatting language. A single

Texinfo source ﬁle is used to produce both the printed and online versions of the documen-

tation. Because of this, the typographical conventions are slightly diﬀerent than in other

books you may have read.

Examples you would type at the command-line are preceded by the common shell pri-

mary and secondary prompts, ‘$’ and ‘>’. Output from the command is preceded by the

glyph “

”. This typically represents the command’s standard output. Error messages, and

other output on the command’s standard error, are preceded by the glyph “

error

”. For

example:

$ echo hi on stdout

hi on stdout

$ echo hello on stderr 1>&2

error

hello on stderr

Preface 7

In the text, command names appear in this font, while code segments appear in the

same font and quoted, ‘like this’. Some things are emphasized like this, and if a point

needs to be made strongly, it is done like this. The ﬁrst occurrence of a new term is usually

its deﬁnition and appears in the same font as the previous occurrence of “deﬁnition” in this

sentence. ﬁle names are indicated like this: ‘/path/to/ourfile’.

Characters that you type at the keyboard look like this. In particular, there are special

characters called “control characters.” These are characters that you type by holding down

both the CONTROL key and another key, at the same time. For example, a Ctrl-d is typed

by ﬁrst pressing and holding the CONTROL key, next pressing the d key and ﬁnally releasing

both keys.

Dark Corners

Dark corners are basically fractal — no matter how much you illuminate, there’s

always a smaller but darker one.

Brian Kernighan

Until the POSIX standard (and The Gawk Manual), many features of awk were either

poorly documented or not documented at all. Descriptions of such features (often called

“dark corners”) are noted in this book with the picture of a ﬂashlight in the margin, as

shown here. They also appear in the index under the heading “dark corner.”

As noted by the opening quote, though, any coverage of dark corners is, by deﬁnition,

something that is incomplete.

The GNU Project and This Book

The Free Software Foundation (FSF) is a nonproﬁt organization dedicated to the production

and distribution of freely distributable software. It was founded by Richard M. Stallman,

the author of the original Emacs editor. GNU Emacs is the most widely used version of

Emacs today.

The GNU

Project is an ongoing eﬀort on the part of the Free Software Foundation

to create a complete, freely distributable, POSIX-compliant computing environment. The

FSF uses the “GNU General Public License” (GPL) to ensure that their software’s source

code is always available to the end user. A copy of the GPL is included in this book for

your reference (see [GNU General Public License], page 317). The GPL applies to the C

language source code for gawk. To ﬁnd out more about the FSF and the GNU Project

online, see the GNU Project’s home page. This book may also be read from their web site.

A shell, an editor (Emacs), highly portable optimizing C, C++, and Objective-C com-

pilers, a symbolic debugger and dozens of large and small utilities (such as gawk), have all

been completed and are freely available. The GNU operating system kernel (the HURD),

has been released but is still in an early stage of development.

Until the GNU operating system is more fully developed, you should consider using

GNU/Linux, a freely distributable, Unix-like operating system for Intel 80386, DEC Alpha,

Sun SPARC, IBM S/390, and other systems.

There are many books on GNU/Linux. One

that is freely available is Linux Installation and Getting Started, by Matt Welsh. Many

GNU stands for “GNU’s not Unix.”

The terminology “GNU/Linux” is explained in the [Glossary], page 307.

8 GAWK: Eﬀective AWK Programming

GNU/Linux distributions are often available in computer stores or bundled on CD-ROMs

with books about Linux. (There are three other freely available, Unix-like operating systems

for 80386 and other systems: NetBSD, FreeBSD, and OpenBSD. All are based on the 4.4-

Lite Berkeley Software Distribution, and they use recent versions of gawk for their versions

of awk.)

The book you are reading is actually free—at least, the information in it is free to

anyone. The machine-readable source code for the book comes with gawk; anyone may take

this book to a copying machine and make as many copies as they like. (Take a moment to

check the Free Documentation License in [GNU Free Documentation License], page 328.)

Although you could just print it out yourself, bound books are much easier to read and

use. Furthermore, the proceeds from sales of this book go back to the FSF to help fund

development of more free software.

The book itself has gone through a number of previous editions. Paul Rubin wrote the

very ﬁrst draft of The GAWK Manual; it was around 40 pages in size. Diane Close and

Richard Stallman improved it, yielding a version that was around 90 pages long and barely

described the original, “old” version of awk.

I started working with that version in the fall of 1988. As work on it progressed, the FSF

published several preliminary versions (numbered 0.x). In 1996, Edition 1.0 was released

with gawk 3.0.0. The FSF published the ﬁrst two editions under the title The GNU Awk

User’s Guide.

This edition maintains the basic structure of Edition 1.0, but with signiﬁcant additional

material, reﬂecting the host of new features in gawk version 3.1. Of particular note is

Section 7.11 [Sorting Array Values and Indices with gawk], page 127, as well as Section 8.1.6

[Bit-Manipulation Functions of gawk], page 150, Chapter 9 [Internationalization with gawk],

page 160, and also Chapter 10 [Advanced Features of gawk], page 169, and Section C.3

[Adding New Built-in Functions to gawk], page 288.

GAWK: Eﬀective AWK Programming will undoubtedly continue to evolve. An electronic

version comes with the gawk distribution from the FSF. If you ﬁnd an error in this book,

please report it! See Section B.5 [Reporting Problems and Bugs], page 281, for information

on submitting problem reports electronically, or write to me in care of the publisher.

How to Contribute

As the maintainer of GNU awk, I once thought that I would be able to manage a collection of

publicly available awk programs and I even solicited contributions. Making things available

on the Internet helps keep the gawk distribution down to manageable size.

The initial collection of material, such as it is, is still available at

ftp://ftp.freefriends.org/arnold/Awkstuff. In the hopes of doing some-

thing more broad, I acquired the awk.info domain.

However, I found that I could not dedicate enough time to managing contributed code:

the archive did not grow and the domain went unused for several years.

Fortunately, late in 2008, a volunteer took on the task of setting up an awk-related web

site http://awk.info and did a very nice job.

If you have written an interesting awk program, or have written a gawk extension that you

would like to share with the rest of the world, please see http://awk.info/?contribute

for how to contribute it to the web site.

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