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三位计算机科学家合作编写的经典书籍，他们在AT&T贝尔实验室共同完成了这部作品。这本书是Addison-Wesley计算机科学系列的一部分，由Michael A. Harrison担任咨询编辑。出版于1988年，至今仍被视为AWK语言的重要参考资料。 AWK是一种强大的文本分析工具，它以三位作者的名字命名：AWK是取自Alfred V. Aho、Brian W. Kernighan和Peter J. Weinberger三人姓氏的首字母。该语言设计用于处理结构化数据，特别适合在大型文本文件中进行模式匹配和数据提取。AWK通过模式空间、动作和规则来操作文本，使程序员能够高效地编写简洁的脚本解决复杂的数据处理任务。书中详细介绍了AWK语言的核心概念，包括： 1. **模式匹配**：AWK根据预定义的模式（正则表达式）扫描输入行，只有匹配到模式的行才会执行相应的动作。 2. **内置变量**：AWK提供了许多内置变量，如NR（记录编号）、NF（字段数量）和$0（整个输入行），这些变量极大地简化了数据处理。 3. **字段分隔符**：默认情况下，AWK使用空格或制表符作为字段分隔符，但可以通过改变内置变量FS来自定义分隔符。 4. **动作**：当模式匹配成功时，可以执行指定的动作，如打印、修改或计算字段值。 5. **函数库**：AWK包含一套内置函数，如length()、split()和printf()，用于处理字符串、数组和数值计算。 6. **程序结构**：AWK支持条件语句（if-else）和循环语句（for和while），可以编写复杂的控制流程。 7. **数组**：AWK的数组是关联数组，允许用任何字符串作为索引，这在处理复杂的数据结构时非常有用。 8. **模式和动作的组合**：多个模式和动作可以组合在一起，形成复合模式，实现更复杂的逻辑。 9. **正则表达式**：AWK中的模式匹配基于正则表达式，提供了一种强大而灵活的方式来定义匹配规则。书中还涵盖了如何在实际工作中应用AWK，包括数据过滤、报告生成、文本转换等场景。通过实例和练习，读者可以逐步掌握AWK语言的精髓，提高处理文本数据的能力。《AWK编程语言》是一本深入浅出的教程，适合所有需要处理文本数据的程序员和系统管理员阅读，无论你是初学者还是经验丰富的开发者，都能从中受益匪浅。

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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