AWK编程语言经典教程

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"《AWK Programming Language》是经典AWK编程语言教程的英文版，由Alfred V. Aho、Brian W. Kernighan和Peter J. Weinberger合著，出版于AT&T Bell Laboratories。该书是Addison-Wesley计算机科学系列的一部分，由Michael A. Harrison担任咨询编辑。" 在计算机领域，AWK是一种强大的文本处理语言，常用于数据提取、报告生成以及模式匹配等任务。这个教程深入浅出地介绍了AWK语言的核心概念和语法，适合对shell脚本有基础了解的读者进一步学习AWK。 AWK的基本工作方式是在输入文件中逐行扫描，对每一行进行模式匹配和操作。其名称来源于三位作者的首字母：Alfred V. Aho、Brian W. Kernighan和Peter J. Weinberger。这三位都是计算机科学领域的知名专家，其中Brian W. Kernighan也是C语言的主要开发者之一。本书包含的内容可能包括但不限于以下几个方面： 1. **基本语法**：介绍AWK的语法结构，如变量定义、赋值语句、条件语句（if-else）、循环控制（for和while）等。 2. **模式匹配**：讲解如何使用正则表达式进行模式匹配，这是AWK最强大的特性之一。 3. **内置函数**：AWK提供了许多内置函数，用于字符串处理、数值计算和文件操作等，书中会详细阐述这些函数的用法。 4. **文件操作**：如何读取、处理和输出文件，包括处理标准输入和重定向。 5. **数组**：AWK中的数组可以用来存储和操作任意数量的关联数据。 6. **程序结构**：函数定义和调用，以及如何组织AWK程序以提高代码的可读性和复用性。 7. **awk编程实践**：通过实例展示如何解决实际问题，比如数据分析、日志分析等。 8. **扩展和优化**：可能还会涉及AWK的高级特性，如自定义函数和多文件处理，以及如何通过gawk（GNU Awk）实现更复杂的功能。 9. **索引和参考**：书籍通常会提供一个详细的索引，方便读者查找特定的主题或函数。学习这本书将帮助读者掌握AWK语言，从而能更有效地处理和解析文本数据，编写出简洁而高效的脚本。对于那些经常需要处理日志文件、文本分析或者需要快速实现简单数据处理任务的IT专业人员来说，AWK是一门非常实用的技能。

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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