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

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"这是一份关于Advanced Bash-Scripting Guide的高级shell_bash指南，由Mendel Cooper撰写，旨在深入探索shell脚本的艺术。该文档涵盖了从基础到高级的shell脚本编写技术，并在讲解过程中穿插了UNIX智慧和传统的知识。它适用于没有编程背景的初学者，同时也能满足中级至高级水平的学习需求。教程包含练习和大量注释的例子，鼓励读者通过实际编写脚本来学习。此外，这本书也适合作为编程概念的课堂教学材料。" 本文档的修订历史表明，自2007年起，经过多次更新（如'INKBERRY', 'SERVICEBERRY', 和 'LINGONBERRY'版本），内容不断得到完善和升级。在学习bash shell脚本时，以下是一些关键知识点： 1. **变量**：在bash中，变量用于存储数据，可以是字符串、数字或布尔值。声明变量无需类型定义，直接赋值即可。 2. **命令行参数**：脚本可以通过位置参数（$1, $2等）访问命令行传递的参数，$0代表脚本本身的名称。 3. **流程控制**：包括条件语句（if-then-else-finally）、循环（for, while, until）以及case语句，用于根据不同的条件执行不同的代码块。 4. **函数**：在bash中定义函数，可以封装重复使用的代码。 5. **数组**：bash从4.0版本开始支持数组，允许存储多个值在一个变量中。 6. **输入/输出重定向**：使用`<`进行输入重定向，将文件内容作为命令的输入；使用`>`和`>>`进行输出重定向，分别覆盖和追加到文件。 7. **管道（Pipeline）**：通过`|`符号，可以将一个命令的输出作为另一个命令的输入，实现命令间的连接。 8. **正则表达式**：在bash中，可以使用正则表达式进行模式匹配和字符串操作。 9. **控制结构**：如背景执行（&）、作业控制（fg, bg, jobs）和子shell。 10. **文件测试运算符**：如`-e`, `-d`, `-f`等，用于检查文件是否存在、是否为目录或普通文件。 11. **错误处理**：通过设置trap命令，可以在脚本遇到错误时执行特定的操作。 12. **函数库与模块化**：通过source命令（`. filename.sh`）可以引入其他脚本文件，实现代码的复用和模块化。 13. **环境变量**：全局变量，对所有子进程可见，用于设置系统环境。 14. **shell别名**：可以创建命令别名，简化常用命令的输入。 15. **shell脚本调试**：使用`set -x`开启命令行打印，便于查看脚本执行过程。通过这份高级bash脚本指南，读者不仅可以掌握bash的基本语法，还能了解到更高级的脚本技巧，从而编写出高效、可维护的shell脚本。此外，书中提供的练习和注释丰富的示例将有助于加深理解和提高实践能力。

comma operator. The comma operator [12] links together a series of arithmetic operations. All are

evaluated, but only the last one is returned.

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`}

Advanced Bash-Scripting Guide

Chapter 3. Special Characters 10

# ${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 9-16).

: ${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.

Advanced Bash-Scripting Guide

Chapter 3. Special Characters 11

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