AWK编程语言权威指南：阿尔弗雷德·霍和布莱恩·科恩尼汉合著

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《AWK编程语言》是一本由阿尔弗雷德·A·霍(Alfred V. Aho)、布莱恩·W·科恩尼汉(Brian W. Kernighan)和彼得·J·韦恩伯格(Peter J. Weinberger)共同撰写的经典技术书籍，最初由AT&T贝尔实验室编写，后来成为计算机科学领域的重要教材。该书是Addison-Wesley出版社出版的系列之一，旨在介绍AWK编程语言，这是一种强大的文本处理工具，特别适合于数据解析、脚本编写和格式化输出。 AWK（Ardour, Wish, Korn, or Gawk）是一种编程语言，它起源于Unix系统，主要用于处理文本数据。本书以其简洁的语法和高效的文本处理能力为核心，向读者展示了如何利用AWK进行字符串匹配、字段分割、逻辑运算和流程控制等操作。书中包含丰富的例子和实战，使得学习者能够快速上手并掌握AWK的基本原理和高级技巧。章节一可能介绍了AWK语言的基础概念，包括变量声明、模式匹配、命令和动作的组合，以及基本的控制结构如if-else语句和循环。AWK的工作原理通常包括三个主要部分：扫描器、处理器和打印机。扫描器负责读取输入行，处理器则根据模式执行相应的动作，而打印机负责输出处理结果。书中还可能涵盖了AWK的内置函数和正则表达式，这对于处理复杂的文本数据至关重要。此外，书中提到的PolyAWK和The Toolbox Language可能是扩展或变体版本，可能包含额外的功能或不同的编程风格，以满足不同用户的需求。版权信息表明，这本书是在1988年由AT&T贝尔实验室出版，并在同年发行，使用了Times Roman和Courier字体，通过Autologic APS-5照片排版机和DEC VAX 8550计算机，运行第九版的UNIX操作系统进行排版。这体现了当时的技术背景和AWK语言与主流系统的兼容性。《AWK编程语言》是一本深入浅出的指南，不仅适合初学者系统学习AWK，也对已经在使用该语言的专业人士具有参考价值，因为它提供了丰富的实践经验和理论基础，帮助读者更好地理解和利用AWK进行文本处理任务。

AN A

TUTORIAL

CHAPTER

$ awk

'$3

0 {

} ,

Beth

4.00

Beth

Dan

3.75

Dan

Kathy

3.75

Kathy

3.75

Kathy

The

heavy

characters

are

what the computer printed.

This behavior makes it easy to experiment with awk: type your program,

then type

data

it and see what happens. We again encourage you to try the

examples and variations on them.

Notice

that

the program

enclosed

single quotes on the command line.

This protects characters like $

the program from being interpreted by the

shell and also allows the program to be longer than one line.

This arrangement

convenient when the program

short (a

few

lines).

the program

long, however,

more convenient to put it into a separate file,

say

progfile,

and type the command line

awk

-f

progfile

optional list

input files

The

-f

option instructs awk to fetch the program from the named file. Any

filename can be used

place

progfile.

Errors

you make an error

an awk program, awk will give you a diagnostic

message. For example, if you mistype a brace, like this:

awk

'$3

0 [

emp.data

you will get a message like this:

awk:

syntax

error

source

line

context

>>>

[ <<<

extra

}

missing

awk:

bailing

out

source

line

"Syntax error" means

that

you have made a grammatical error that was

detected

the place marked

>>>

<<<.

"Bailing

out"

means

that

recovery was attempted. Sometimes

you

get a little more help about what the

error was, such as a report

mismatched braces or parentheses.

Because

the syntax error, awk did not try to execute this program. Some

errors, however, may not be detected until your program

running. For exam-

ple, if you attempt to divide a number

zero, awk will stop its processing and

report the input line number and the line number

the program

which the

division was attempted.

SECTION

1.2

SIMPLE OUTPUT 5

1.2 Simple Output

The

rest

this chapter contains a collection

short, typical awk programs

based on manipulation

the emp o

data

file above. We'll explain briefly

what's going on, but these examples are meant mainly to suggest useful opera-

tions

that

are

easy to do with awk - printing fields, selecting input, and

transforming data. We

are

not showing everything

that

awk

can

do by any

means, nor

are

going into many details about the specific things presented

here. But by the end

this chapter, you will be able to accomplish quite a bit,

and you'll find it much easier to read the later chapters.

will usually show just the program, not the whole command line. In

every case, the program can be run either by enclosing it in quotes as the first

argument

the

awk

command, as shown above, or by putting it

a file and

invoking awk on

that

file with the

-f

option.

There

are

only two types

data

in awk: numbers and strings

characters.

The emp 0

data

file

typical

this kind of information - a mixture

words

and numbers separated

blanks

and/or

tabs.

Awk reads its input one line

a time and splits each line into fields, where,

default, a field

a sequence

characters

that

doesn't contain any blanks or

tabs. The first field

the current input line

called

$1,

the second

$2,

and

forth.

The

entire line

called $0. The number

fields can vary from line to

line.

Often, all we need to do

print some or all

the fields

each line,

perhaps performing some calculations. The programs

this section

are

all of

that

form.

Printing Every Line

action has

pattern, the action

performed for all input lines. The

statement

by itself prints the current input line, so the program

{

}

prints all

its input on the standard output. Since

the whole line,

{

}

does the same thing.

Printing Certain Fields

More than one item can be printed on the same output line with a single

statement. The program to print the first and third fields

each input

line

{

$1,

}

With

emp.

data

as input, it produces

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