GNU Awk编程指南：Arnold Robbins的有效实践

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"Effective AWK Programming: A User's Guide for GNU Awk" 是一本由Arnold D. Robbins撰写的权威指南，他是一位经验丰富的程序员和技术作家，尤其在Unix系统领域有着深厚背景。自1987年起，他便与GNU项目的AWK版本——gawk结缘，并且作为POSIX 1003.2标准制定组的一员，他在规范标准方面发挥了关键作用。Robbins不仅是gawk的维护者，也是其文档的主要贡献者，他还参与了第六版《Learning the vi Editor》的编写。本书针对的是GNU Awk 3.0.3（或更高版本）进行讲解，旨在帮助用户深入了解和高效地使用这个强大的文本处理工具。作为1.0.3版，它涵盖了AWK语言的基础知识、高级特性、模式匹配、流程控制、正则表达式、函数编程以及实用技巧等内容。书中详细解释了如何利用AWK进行数据解析、文本转换和数据处理，强调了其在脚本编写中的灵活性和效率。读者可以从中学习到如何编写清晰、可维护的AWK程序，如何处理大型数据集，以及如何遵循最佳实践以提高代码质量。此外，由于版权许可，用户可以根据GNU通用公共许可证，自由地复制和修改这份手册，只要保留原始版权信息和许可声明即可。对于那些希望精通AWK语言，尤其是GNU Awk的开发人员和系统管理员来说，《Effective AWK Programming》是一本不可或缺的参考书籍，它不仅提供了技术指导，还展示了如何在实际项目中灵活应用AWK的强大功能。无论你是初学者还是高级用户，这本书都能帮助你提升AWK编程的技能和效率。

12 Eﬀective AWK Programming

Sometimes your awk programs can be very long. In this case it is more

convenient to put the program into a separate ﬁle. To tell awk to use that

ﬁle for its program, you type:

awk -f source-ﬁle input-ﬁle1 input-ﬁle2 ...

The ‘-f’ instructs the awk utility to get the awk program from the ﬁle

source-ﬁle. Any ﬁle name can be used for source-ﬁle. For example, you could

put the program:

BEGIN { print "Don’t Panic!" }

into the ﬁle ‘advice’. Then this command:

awk -f advice

does the same thing as this one:

awk "BEGIN { print \"Don’t Panic!\" }"

which was explained earlier (see Section 2.2.2 [Running awk without Input

Files], page 11). Note that you don’t usually need single quotes around

the ﬁle name that you specify with ‘-f’, because most ﬁle names don’t

contain any of the shell’s special characters. Notice that in ‘advice’, the

awk program did not have single quotes around it. The quotes are only

needed for programs that are provided on the awk command line.

If you want to identify your awk program ﬁles clearly as such, you can

add the extension ‘.awk’ to the ﬁle name. This doesn’t aﬀect the execution

of the awk program, but it does make “housekeeping” easier.

2.2.4 Executable awk Programs

Once you have learned awk, you may want to write self-contained awk

scripts, using the ‘#!’ script mechanism. You can do this on many Unix

systems

(and someday on the GNU system).

For example, you could update the ﬁle ‘advice’ to look like this:

#! /bin/awk -f

BEGIN { print "Don’t Panic!" }

After making this ﬁle executable (with the chmod utility), you can simply

type ‘advice’ at the shell, and the system will arrange to run awk

as if you

had typed ‘awk -f advice’.

The ‘#!’ mechanism works on Linux systems, Unix systems derived from

Berkeley Unix, System V Release 4, and some System V Release 3

systems.

The line beginning with ‘#!’ lists the full ﬁle name of an interpreter to

be run, and an optional initial command line argument to pass to that

interpreter. The operating system then runs the interpreter with the

given argument and the full argument list of the executed program. The

Chapter 2: Getting Started with awk 13

$ advice

Don’t Panic!

Self-contained awk scripts are useful when you want to write a program which

users can invoke without their having to know that the program is written

in awk.

Some older systems do not support the ‘#!’ mechanism. You can get a

similar eﬀect using a regular shell script. It would look something like this:

: The colon ensures execution by the standard shell.

awk ’program’ "$@"

Using this technique, it is vital to enclose the program in single quotes

to protect it from interpretation by the shell. If you omit the quotes, only a

shell wizard can predict the results.

The "$@" causes the shell to forward all the command line arguments to

the awk program, without interpretation. The ﬁrst line, which starts with

a colon, is used so that this shell script will work even if invoked by a user

who uses the C shell. (Not all older systems obey this convention, but many

do.)

2.2.5 Comments in awk Programs

A comment is some text that is included in a program for the sake of

human readers; it is not really part of the program. Comments can explain

what the program does, and how it works. Nearly all programming lan-

guages have provisions for comments, because programs are typically hard

to understand without their extra help.

In the awk language, a comment starts with the sharp sign character, ‘#’,

and continues to the end of the line. The ‘#’ does not have to be the ﬁrst

character on the line. The awk language ignores the rest of a line following

a sharp sign. For example, we could have put the following into ‘advice’:

# This program prints a nice friendly message. It helps

# keep novice users from being afraid of the computer.

BEGIN { print "Don’t Panic!" }

You can put comment lines into keyboard-composed throw-away awk pro-

grams also, but this usually isn’t very useful; the purpose of a comment is

to help you or another person understand the program at a later time.

2.3 A Very Simple Example

ﬁrst argument in the list is the full ﬁle name of the awk program. The

rest of the argument list will either be options to awk, or data ﬁles, or

both.

14 Eﬀective AWK Programming

The following command runs a simple awk program that searches the in-

put ﬁle ‘BBS-list’ for the string of characters: ‘foo’. (A string of characters

is usually called a string. The term string is perhaps based on similar usage

in English, such as “a string of pearls,” or, “a string of cars in a train.”)

awk ’/foo/ { print $0 }’ BBS-list

When lines containing ‘foo’ are found, they are printed, because ‘print $0’

means print the current line. (Just ‘print’ by itself means the same thing,

so we could have written that instead.)

You will notice that slashes, ‘/’, surround the string ‘foo’ in the awk

program. The slashes indicate that ‘foo’ is a pattern to search for. This

type of pattern is called a regular expression, and is covered in more detail

later (see Chapter 4 [Regular Expressions], page 23). The pattern is allowed

to match parts of words. There are single-quotes around the awk program

so that the shell won’t interpret any of it as special shell characters.

Here is what this program prints:

$ awk ’/foo/ { print $0 }’ BBS-list

fooey 555-1234 2400/1200/300 B

foot 555-6699 1200/300 B

macfoo 555-6480 1200/300 A

sabafoo 555-2127 1200/300 C

In an awk rule, either the pattern or the action can be omitted, but not

both. If the pattern is omitted, then the action is performed for every input

line. If the action is omitted, the default action is to print all lines that

match the pattern.

Thus, we could leave out the action (the print statement and the curly

braces) in the above example, and the result would be the same: all lines

matching the pattern ‘foo’ would be printed. By comparison, omitting the

print statement but retaining the curly braces makes an empty action that

does nothing; then no lines would be printed.

2.4 An Example with Two Rules

The awk utility reads the input ﬁles one line at a time. For each line, awk

tries the patterns of each of the rules. If several patterns match then several

actions are run, in the order in which they appear in the awk program. If no

patterns match, then no actions are run.

After processing all the rules (perhaps none) that match the line, awk

reads the next line (however, see Section 9.7 [The next Statement], page 111,

and also see Section 9.8 [The nextfile Statement], page 112). This contin-

ues until the end of the ﬁle is reached.

For example, the awk program:

16 Eﬀective AWK Programming

The ‘ls -lg’ part of this example is a system command that gives you a

listing of the ﬁles in a directory, including ﬁle size and the date the ﬁle was

last modiﬁed. Its output looks like this:

-rw-r--r-- 1 arnold user 1933 Nov 7 13:05 Makefile

-rw-r--r-- 1 arnold user 10809 Nov 7 13:03 gawk.h

-rw-r--r-- 1 arnold user 983 Apr 13 12:14 gawk.tab.h

-rw-r--r-- 1 arnold user 31869 Jun 15 12:20 gawk.y

-rw-r--r-- 1 arnold user 22414 Nov 7 13:03 gawk1.c

-rw-r--r-- 1 arnold user 37455 Nov 7 13:03 gawk2.c

-rw-r--r-- 1 arnold user 27511 Dec 9 13:07 gawk3.c

-rw-r--r-- 1 arnold user 7989 Nov 7 13:03 gawk4.c

The ﬁrst ﬁeld contains read-write permissions, the second ﬁeld contains the

number of links to the ﬁle, and the third ﬁeld identiﬁes the owner of the

ﬁle. The fourth ﬁeld identiﬁes the group of the ﬁle. The ﬁfth ﬁeld contains

the size of the ﬁle in bytes. The sixth, seventh and eighth ﬁelds contain the

month, day, and time, respectively, that the ﬁle was last modiﬁed. Finally,

the ninth ﬁeld contains the name of the ﬁle.

The ‘$6 == "Nov"’ in our awk program is an expression that tests whether

the sixth ﬁeld of the output from ‘ls -lg’ matches the string ‘Nov’. Each

time a line has the string ‘Nov’ for its sixth ﬁeld, the action ‘sum += $5’ is

performed. This adds the ﬁfth ﬁeld (the ﬁle size) to the variable sum. As a

result, when awk has ﬁnished reading all the input lines, sum is the sum of

the sizes of ﬁles whose lines matched the pattern. (This works because awk

variables are automatically initialized to zero.)

After the last line of output from ls has been processed, the END rule is

executed, and the value of sum is printed. In this example, the value of sum

would be 80600.

These more advanced awk techniques are covered in later sections (see

Section 8.2 [Overview of Actions], page 102). Before you can move on to

more advanced awk programming, you have to know how awk interprets

your input and displays your output. By manipulating ﬁelds and using

print statements, you can produce some very useful and impressive looking

reports.

2.6 awk Statements Versus Lines

Most often, each line in an awk program is a separate statement or sepa-

rate rule, like this:

awk ’/12/ { print $0 }

/21/ { print $0 }’ BBS-list inventory-shipped

However, gawk will ignore newlines after any of the following:

, { ? : || && do else

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