掌握AWK编程语言的基石指南

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AWK（Aho、Weinberger和Kernighan的语言）是一种强大的文本处理工具，以其简洁的语法和功能强大的模式匹配而闻名。这本名为《AWK编程语言》的书籍由Alfred V. Aho、Brian W. Kernighan和Peter J. Weinberger合著，他们是AT&T贝尔实验室的研究人员，该书是Addison-Wesley计算机科学系列的一部分。书中深入探讨了AWK语言的基础和高级特性，旨在帮助读者掌握这个实用且灵活的脚本语言。 AWK语言的设计初衷是为了在文本数据中查找和处理模式，常用于数据分析、报表生成和文件格式转换等场景。它由三个主要部分组成：输入、处理和输出阶段，每个阶段都对应着相应的语句结构。AWK的核心是它的模式匹配引擎，它能够根据指定的正则表达式（或简单模式）筛选和操作输入数据行。学习AWK的关键在于理解其内建变量（如$0, $1, $2等）、控制流结构（如if...else、for循环）、以及数组和函数的使用。此外，本书可能还会介绍AWK的预处理器（AWK的宏系统），它允许用户编写可重用的代码块，提高编程效率。书中还可能包含AWK与其他编程语言（如shell脚本或Python）的比较，以及如何在各种操作系统（如Unix和类Unix系统，包括早期的UNIX~）上使用AWK。对于初学者来说，章节可能会从基本语法开始，逐步引入更复杂的功能，例如正则表达式的深入理解、文件处理和管道操作的集成。作为一本经典教材，它不仅提供了理论知识，还可能包含大量的示例和练习，以便读者通过实践来巩固所学。同时，书中可能有对后续版本的AWK（如PolyAWK）的介绍，这些扩展增强了AWK的功能，使其在实际应用中更加灵活。版权信息表明，这本书是在1988年由AT&T贝尔实验室授权出版，使用了Times Roman和Courier字体，基于第九版UNIX操作系统进行排版。这本权威指南对于希望深入理解并精通AWK语言的专业人士和开发者来说，是一份不可或缺的学习资料。

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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