AWK编程语言详解

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"The AWK Programming Language" 《AWK编程语言》是由Alfred V. Aho、Brian W. Kernighan和Peter J. Weinberger三位作者合著的一本经典书籍，他们在AT&T贝尔实验室工作时编写了这本书。这本书是Addison-Wesley出版社计算机科学系列的一部分，由Michael A. Harrison担任咨询编辑。 AWK是一种强大的文本分析工具和编程语言，其名称来源于三位作者的名字首字母。AWK被设计用于处理结构化的文本数据，它允许用户在一行或多行文本中搜索特定模式，并根据这些模式执行操作。这种语言非常适合于数据分析、日志分析、报告生成等任务，是系统管理员和程序员的常用工具。本书详细介绍了AWK语言的基础和高级特性，包括： 1. **模式匹配**：AWK的核心是基于模式的处理，它会自动读取输入文件并逐行处理。用户可以定义模式（正则表达式）来匹配感兴趣的行。 2. **动作与脚本**：匹配到模式后，AWK会执行关联的动作，这通常是一段代码。用户可以通过编写简单的脚本来实现复杂的数据处理任务。 3. **内置变量和函数**：AWK提供了一系列内置变量（如NF表示当前行的字段数，NR表示已读取的行数）和函数，使得在处理文本时更加方便。 4. **数据类型和数组**：AWK支持基本的数据类型，如整型、浮点型和字符串，还提供了数组，允许存储和操作多对键值。 5. **控制结构**：包括条件语句（if...else）、循环（for，while）和其他流程控制结构，使AWK具备了完整的编程能力。 6. **函数定义**：用户可以在AWK程序中定义自己的函数，增加代码的复用性和模块化。 7. **文件输入/输出**：AWK可以读取和写入文件，进行文件操作。 8. **正则表达式**：AWK的模式匹配功能依赖于正则表达式，它是处理文本的强大工具。 9. **程序设计原则**：书中还涵盖了如何编写清晰、高效的AWK程序的指导原则。 10. **索引和例子**：书中包含了一个详细的索引，方便读者查找特定主题，并提供了丰富的示例来解释概念。该书是学习和理解AWK语言的重要参考资料，适合对文本处理感兴趣的程序员、系统管理员和数据分析师阅读。通过这本书，读者可以深入掌握AWK语言，提高处理和解析文本数据的能力。

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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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