全面学习：awk编程语言英文教程

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"这是一份关于awk编程语言的全面学习教程，由Alfred V. Aho、Brian W. Kernighan和Peter J. Weinberger共同编写，他们都是AT&T Bell Laboratories的研究员。这本书是Addison-Wesley Series in Computer Science系列的一部分，旨在深入介绍awk编程语言，包括其语法、功能和应用。" awk是一种强大的文本分析工具，广泛用于处理和解析结构化数据，如日志文件、CSV或以制表符分隔的数据。它的名字来源于三位作者的姓氏首字母：Alfred Aho、Brian Kernighan和Peter Weinberger。这本书详细介绍了awk语言的基础和高级概念，包括模式匹配、变量、函数、输入/输出操作以及控制结构。在awk中，程序通常由一系列规则组成，每个规则包含一个模式和一个动作。模式定义了要匹配的文本模式，而动作则是在匹配时执行的代码块。awk会逐行读取输入文件，对每一行进行模式匹配，并执行相应的动作。这种工作方式使得awk非常适合于数据分析和报告生成。书中可能涵盖了以下知识点： 1. **基础语法**：awk的语句结构，变量类型（数字和字符串），常量，以及赋值操作。 2. **模式匹配**：使用正则表达式进行文本匹配，如基本字符类、重复量词、位置锚点等。 3. **内置变量**：预定义的awk变量，如$0表示整个记录，$1至$NF表示字段值。 4. **控制流**：条件语句（if-else）和循环语句（for和while）。 5. **函数**：awk内置的数学、字符串和数组函数，以及自定义函数的创建和使用。 6. **输入/输出**：如何读取和写入文件，重定向和管道操作。 7. **数组**：awk中的数组处理，包括关联数组和索引数组。 8. **模式和动作**：如何定义和使用模式匹配规则，以及规则间的优先级。 9. **命令行选项**：使用awk命令行参数来定制其行为。 10. **脚本编写和调试**：编写awk脚本的技巧，以及如何调试awk程序。通过这份全面的教程，读者可以学习到如何利用awk的强大功能进行文本处理，实现自动化任务，提高数据处理效率。书中还可能包含丰富的实例和练习，帮助读者巩固理论知识并提升实践能力。

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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