深入探索Bash脚本艺术：Advanced Bash-Scripting Guide

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"Advanced Bash-Scripting Guide" 《高级Bash脚本指南》是一本深入探索Shell脚本艺术的专业书籍，由Mendel Cooper撰写。这本书对于程序员来说是一份宝贵的资源，适合所有级别的Bash脚本学习者，无论你是初学者还是有一定经验的开发者。在内容上，该书从零基础开始，逐步引导读者进入中级至高级的脚本编写水平，同时巧妙地融入了UNIX智慧和传统。它不仅可用作教科书，帮助自学者理解编程概念，还是一本实用的参考手册，提供了丰富的Shell脚本技术知识。书中包含的练习和详尽注释的例子鼓励读者积极参与，因为作者坚信实践是真正掌握脚本编写技巧的唯一途径。该书的更新历史表明，作者持续对内容进行了维护和升级，如“SPICEBERRY”、“WINTERBERRY”和“WAXBERRY”等版本，这些更新可能包括错误修复、新增内容和改进。最新版本可以作为存档的bzip2压缩的“tarball”文件在作者的个人网站上下载，其中包括SGML源代码和渲染后的HTML版本，方便读者在不同环境下阅读和学习。通过学习《高级Bash脚本指南》，读者将能够掌握如何利用Bash shell进行高效和强大的自动化任务处理，如文件操作、流程控制、条件判断、函数定义、环境变量管理以及与系统交互等。此外，书中还涵盖了正则表达式、管道、重定向、进程管理和脚本调试等关键技能，这些都是成为一名熟练的Shell脚本编写者所必需的。《高级Bash脚本指南》是一本全面且深入的教程，对于想要提升Bash脚本能力的人来说，它提供了一个系统的学习路径，有助于你成为UNIX/Linux环境中不可或缺的脚本专家。无论是进行日常系统管理，还是开发复杂的自动化解决方案，这本书都将是你不可或缺的参考资料。

let "t2 = ((a = 9, 15 / 3))" # Set "a = 9" and "t2 = 15 / 3"

escape [backslash]. A quoting mechanism for single characters.

\X "escapes" the character X. This has the effect of "quoting" X, equivalent to 'X'. The \ may be used

to quote " and ', so they are expressed literally.

See Chapter 5 for an in−depth explanation of escaped characters.

Filename path separator [forward slash]. Separates the components of a filename (as in

/home/bozo/projects/Makefile).

This is also the division arithmetic operator.

command substitution. The `command` construct makes available the output of command for

assignment to a variable. This is also known as backquotes or backticks.

null command [colon]. This is the shell equivalent of a "NOP" (no op, a do−nothing operation). It

may be considered a synonym for the shell builtin true. The ":" command is itself a Bash builtin, and

its exit status is "true" (0).

echo $? # 0

Endless loop:

while :

operation−1

operation−2

...

operation−n

done

# Same as:

# while true

# do

# ...

# done

Placeholder in if/then test:

if condition

then : # Do nothing and branch ahead

else

take−some−action

Provide a placeholder where a binary operation is expected, see Example 8−2 and default parameters.

: ${username=`whoami`}

# ${username=`whoami`} Gives an error without the leading :

# unless "username" is a command or builtin...

Provide a placeholder where a command is expected in a here document. See Example 18−10.

Evaluate string of variables using parameter substitution (as in Example 9−15).

Advanced Bash−Scripting Guide

Chapter 3. Special Characters 10

: ${HOSTNAME?} ${USER?} ${MAIL?}

# Prints error message

#+ if one or more of essential environmental variables not set.

Variable expansion / substring replacement.

In combination with the > redirection operator, truncates a file to zero length, without changing its

permissions. If the file did not previously exist, creates it.

: > data.xxx # File "data.xxx" now empty.

# Same effect as cat /dev/null >data.xxx

# However, this does not fork a new process, since ":" is a builtin.

See also Example 15−14.

In combination with the >> redirection operator, has no effect on a pre−existing target file (: >>

target_file). If the file did not previously exist, creates it.

This applies to regular files, not pipes, symlinks, and certain special files.

May be used to begin a comment line, although this is not recommended. Using # for a comment

turns off error checking for the remainder of that line, so almost anything may appear in a comment.

However, this is not the case with :.

: This is a comment that generates an error, ( if [ $x −eq 3] ).

The ":" also serves as a field separator, in /etc/passwd, and in the $PATH variable.

bash$ echo $PATH

/usr/local/bin:/bin:/usr/bin:/usr/X11R6/bin:/sbin:/usr/sbin:/usr/games

reverse (or negate) the sense of a test or exit status [bang]. The ! operator inverts the exit status of

the command to which it is applied (see Example 6−2). It also inverts the meaning of a test operator.

This can, for example, change the sense of "equal" ( = ) to "not−equal" ( != ). The ! operator is a Bash

keyword.

In a different context, the ! also appears in indirect variable references.

In yet another context, from the command line, the ! invokes the Bash history mechanism (see

Appendix J). Note that within a script, the history mechanism is disabled.

wild card [asterisk]. The * character serves as a "wild card" for filename expansion in globbing. By

itself, it matches every filename in a given directory.

bash$ echo *

abs−book.sgml add−drive.sh agram.sh alias.sh

The * also represents any number (or zero) characters in a regular expression.

arithmetic operator. In the context of arithmetic operations, the * denotes multiplication.

A double asterisk, **, is the exponentiation operator.

Advanced Bash−Scripting Guide

Chapter 3. Special Characters 11

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