高级Bash脚本编程指南：英文版解析

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"《高级Bash脚本编程指南》是一本深入探讨Shell脚本艺术的英文教程，由Mendel Cooper编写。该书分为多个修订版本，最新的版本为4.0，更新日期为2006年6月18日。这本书旨在帮助没有编程背景的读者快速进步到中级或高级的Bash脚本编程水平，同时融入了UNIX智慧和传统的知识。" 本书不仅仅是一本教材，也适用于自我学习和作为Shell脚本技术的参考手册。它包含了大量的练习和详尽注释的例子，鼓励读者通过实际编写脚本来学习。作者认为，只有亲自编写脚本，才能真正掌握脚本编程。此外，这本书的内容丰富，适合用作课堂上介绍编程概念的一般性教材。书中涵盖了各种Bash脚本编程的基础和高级主题，如变量、条件语句、循环、函数、数组、错误处理、输入/输出重定向、管道、脚本调试技巧以及与系统交互的方法。读者将学习如何利用Bash的强大功能来自动化日常任务，创建复杂的脚本，并理解如何更有效地在命令行环境中工作。在学习过程中，读者会接触到一些核心的UNIX工具和命令，这些工具是编写高效Bash脚本的关键。例如，读者将学习如何使用grep、sed、awk等工具进行文本处理，以及如何利用find、xargs等工具处理文件系统操作。此外，还会深入理解Bash的特殊变量、命令替换、流程控制结构以及函数的定义和调用。通过阅读和实践《高级Bash脚本编程指南》，读者不仅能够掌握Bash脚本编程的基本技能，还能了解到UNIX环境下的最佳实践和常用工具。这本书对于那些想要提升shell脚本编写能力，或者希望在Linux和Unix系统中提高工作效率的用户来说，是一份宝贵的资源。最新版本的文档可以以存档的bzip2压缩tarball形式从作者的个人网站下载，其中包括SGML源代码和渲染后的HTML版本，方便读者根据需要选择阅读方式。

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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高级Bash脚本编程指南：英文版解析

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