Effective awk编程指南第三版

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"Effective awk Programming 3rd Edition 是一本关于awk编程语言的专业书籍，由Arnold Robbins撰写。本书针对的是GNU awk 3.1.0或更高版本，为用户提供了一份详尽的指南。书中涵盖了awk编程的各个方面，旨在帮助读者有效地利用awk处理文本和数据。" 在《Effective awk Programming》第三版中，作者深入浅出地讲解了awk编程语言的核心概念、语法和最佳实践。awk是一种强大的命令行工具，常用于数据处理、报告生成和文本分析。以下是一些关键知识点： 1. **awk基础知识**：awk的工作原理，包括模式匹配、动作执行和字段分割。它通过扫描输入文件的每一行，对符合模式的行执行指定的操作。 2. **模式和动作**：awk中的模式可以是正则表达式，用于识别输入行的特定部分。动作是当模式匹配时执行的代码块，可以包含变量赋值、条件语句和循环。 3. **内置变量**：awk提供了许多内置变量，如`$0`表示当前行，`NF`表示当前行的字段数，`NR`表示已读取的行数，这些变量极大地简化了文本处理。 4. **函数和自定义功能**：awk支持内置函数，如`length()`计算字符串长度，`split()`分割字符串，以及用户自定义函数，增强了其灵活性和可扩展性。 5. **文件操作**：awk可以读取和写入文件，允许用户进行复杂的文件处理任务，如合并、过滤、转换数据。 6. **控制结构**：awk支持条件语句（`if...else`）和循环（`for`、`while`），使得程序设计更为灵活。 7. **正则表达式**：awk中的正则表达式用于模式匹配，提供了丰富的元字符和操作符，如`.`匹配任意字符，`^`匹配行首，`$`匹配行尾。 8. **数组**：awk的数组可以用于存储和处理多对多的数据关系，数组索引可以是任何字符串或数值。 9. **awk脚本编写技巧**：书中可能涵盖了如何编写高效、可读性强的awk脚本，包括注释、代码组织和错误处理。 10. **GNU awk扩展**：由于是针对GNU awk的指南，书中会详细介绍一些非标准但非常有用的特性，如函数库、多进程并行处理和自定义记录分隔符。这本书不仅适合awk初学者，也适合有一定经验的用户，通过实例和详细的解释，帮助读者掌握awk的强大功能，提高文本处理效率。不论是在系统管理、数据分析还是日志分析等领域，awk都是一个不可或缺的工具。

Preface

Several kinds of tasks occur repeatedly when working with text ﬁles. You might

want to extract certain lines and discard the rest. Or you may need to make

changes wherever certain patterns appear, but leave the rest of the ﬁle alone. Writ-

ing single-use programs for these tasks in languages such as C, C++, or Pascal is

time-consuming and inconvenient. Such jobs are often easier with awk. The awk

utility interprets a special-purpose programming language that makes it easy to

handle simple data-refor matting jobs.

The GNU implementation of awk is called gawk ; it is fully compatible with the

System V Release 4 version of awk. gawk is also compatible with the POSIX speci-

ﬁcation of the awk language. This means that all properly written awk pr ograms

should work with gawk. Thus, we usually don’t distinguish between gawk and

other awk implementations.

Using awk allows you to:

• Manage small, personal databases

• Generate reports

• Validate data

• Produce indexes and perfor m other document preparation tasks

• Experiment with algorithms that you can adapt later to other computer lan-

guages

9 October 2001 01:40

xvi Preface

In addition, gawk pr ovides facilities that make it easy to:

• Extract bits and pieces of data for processing

• Sort data

• Per form simple network communications

This book teaches you about the awk language and how you can use it effectively.

You should already be familiar with basic system commands, such as cat and ls,

as well as basic shell facilities, such as input/output (I/O) redir ection and pipes.

Implementations of the awk language are available for many differ ent computing

envir onments. This book, while describing the awk language in general, also

describes the particular implementation of awk called gawk (which stands for

“GNU awk”). gawk runs on a broad range of Unix systems, ranging from 80386

PC-based computers up through large-scale systems, such as Crays. gawk has also

been ported to Mac OS X, MS-DOS, Microsoft Windows (all versions) and OS/2

PCs, Atari and Amiga microcomputers, BeOS, Tandem D20, and VMS.

Histor y of awk and gawk

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

Weinberger, and Brian W. Ker nighan. The original version of awk was written in

1977 at AT&T Bell Laboratories. In 1985, a new version made the programming

language more power ful, intr oducing user-deﬁned functions, multiple input

str eams, and computed regular expressions. 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 Language and

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

original Bell Laboratories awk designers provided feedback for the POSIX speciﬁ-

cation.

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

pleted 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. Curr ent development focuses on bug ﬁxes, perfor mance

impr ovements, standards compliance, and occasionally, new features.

* These commands are available on POSIX-compliant systems, as well as on traditional Unix-based

systems. If you are using some other operating system, you still need to be familiar with the ideas of

I/O redir ection and pipes.

9 October 2001 01:40

In May of 1997, Jürgen 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). Chapter 14, Inter networking with gawk,

is condensed from that document. His code ﬁnally became part of the main gawk

distribution with gawk Version 3.1.

See Appendix A, The Evolution of the awk Language, 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 language described in this book is often referr ed 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.Itis

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 !)

Thr oughout 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.

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 pr ogramming language.

* Of particular note is Sun’s Solaris, where /usr/bin/awk is, sadly, still the original version. Use

/usr/xpg4/bin/awk to get a POSIX-compliant version of awk on Solaris.

† Often, these systems use gawk for their awk implementation!

Preface xvii

9 October 2001 01:40

xviii Preface

Primarily, this book explains the features of awk, as deﬁned in the POSIX stan-

dard. It does so in the context of the gawk implementation. While doing so, it also

attempts to describe important differ ences between gawk and other awk

implementations.

Finally, any gawk featur es that are not in the POSIX standard for

awk ar e noted.

This book has the difﬁcult task of being both a tutorial and a refer ence. If you are

a novice, feel free to skip over details that seem too complex. You should also

ignor e the many cross-r efer ences; they are for the expert user and for the online

info version of the document.

Ther e ar e sidebars scattered throughout the book. They add a more complete

explanation of points that are relevant, but not likely to be of interest on ﬁrst read-

ing. All appear in the index, under the heading “advanced features.”

Most of the time, the examples use complete awk pr ograms. In some of the more

advanced sections, only the part of the awk pr ogram that illustrates the concept

curr ently being described is shown.

While this book is aimed principally at people who have not been exposed to

awk, ther e 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, and in Chapter 13, Practical awk Programs, should

be of interest.

Chapter 1, Getting Started with awk, provides the essentials you need to know to

begin using awk.

Chapter 2, Regular Expressions, intr oduces regular expressions in general, and in

particular the ﬂavors supported by POSIX awk and gawk.

Chapter 3, 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 redir ec-

tion is ﬁrst described here.

Chapter 4, Printing Output, describes how awk pr ograms can produce output with

print and printf.

Chapter 5, Expr essions, describes expressions, which are the basic building blocks

for getting most things done in a program.

Chapter 6, Patter ns, 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.

* All such differ ences appear in the index under the entry “differ ences in awk and gawk.”

9 October 2001 01:40

Chapter 7, 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 sort-

ing arrays in gawk.

Chapter 8, Functions, describes the built-in functions awk and gawk pr ovide, as

well as how to deﬁne your own functions.

Chapter 9, Inter nationalization with gawk, describes special features in gawk for

translating program messages into differ ent languages at runtime.

Chapter 10, Advanced Features of gawk, describes a number of gawk-speciﬁc

advanced features. Of particular note are the abilities to have two-way communi-

cations with another process, perfor m TCP/IP networking, and proﬁle your awk

pr ograms.

Chapter 11, Running awk and gawk, describes how to run gawk, the meaning of

its command-line options, and how it ﬁnds awk pr ogram source ﬁles.

Chapter 12, A Library of awk Functions, and Chapter 13, Practical awk Programs,

pr ovide many sample awk pr ograms. Reading them allows you to see awk solving

real problems.

Chapter 14, Inter networking with gawk, provides an in-depth discussion and

examples of how to use gawk for Internet programming.

Appendix A, 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

featur es over time.

Appendix B, Installing gawk, describes how to get gawk, how to compile it under

Unix, and how to compile and use it on differ ent PC operating 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, 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 dir ections for gawk development.

Appendix D, Basic Programming Concepts, provides some very cursory back-

gr ound material for those who are completely unfamiliar with computer program-

ming. Also centralized there is a discussion of some of the issues surrounding

ﬂoating-point numbers.

Appendix E, GNU General Public License, and Appendix F, GNU Free Documenta-

tion License, present the licenses that cover the gawk source code and this book,

respectively.

Preface xix

9 October 2001 01:40

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Effective awk编程指南第三版

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