深入学习Bash脚本编程指南：从入门到高级

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"Advanced Bash Scripting Guide 是一本深入探讨shell脚本编程艺术的详尽教程，由 Mendel Cooper 编写。该书针对初学者设计，但逐步引导读者达到中级到高级的水平，同时融入Unix®的智慧和传统。作者假设读者没有编程背景，但通过丰富的实例、大量注释和实践练习，鼓励读者积极参与，强调通过实际编写脚本来真正掌握技能。本书不仅是教学教材，适合课堂上作为编程概念的入门，还是一本自我学习手册和shell脚本技术的参考书。书中包含了大量的示例和详尽的解释，旨在提供实用的技巧和知识。此外，作者还提供了最新的文档更新，包括SGML源代码和编译后的HTML版本，可以从作者的个人网站上找到并下载一个存档的bzip2压缩的tarball文件包。随着版本的迭代，如从SPICEBERRY（小更新）到WINTERBERRY（大更新），再到WAXBERRY（小更新），可以看出作者对内容的持续改进和完善。4.1版的发布日期是2006年10月8日，这意味着它反映了当时最先进的bash脚本技术和最佳实践。对于希望在Linux或Unix系统环境中提升脚本编写能力的学习者或开发者来说，这是一本不可多得的参考资料和学习工具。"

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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