深入探索Unix & Linux Shell脚本编程指南

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"Advanced Bash-Scripting Guide - An in-depth exploration of the art of shell scripting by Mendel Cooper" 在Unix和Linux操作系统中，Shell脚本是一种强大的工具，它允许用户通过编写命令序列来自动化日常任务。本资源是Mendel Cooper编写的《Advanced Bash-Scripting Guide》，是一本深入探讨Shell脚本艺术的英文原版教材。这本书不仅适合初学者，也适用于希望提升技能的中级和高级用户。书中包含了各种Shell命令的详尽介绍，可以作为一本实用的字典查阅。该书按照版本进行修订，最新版本为‘RASPBERRY’，发布于2005年3月20日，作者mc对书中的错误进行了修正并添加了重要的更新。它从零基础开始，逐步引导读者进入中级到高级的脚本编写水平，同时穿插讲解UNIX智慧和传统知识。书中的核心内容包括： 1. **基础概念**：解释Shell的基础知识，如变量、条件语句、循环、函数等。 2. **命令行接口**：详细解析各种Unix/Linux命令，如grep、sed、awk、find等，以及如何在脚本中使用它们。 3. **流程控制**：涵盖if-then-else、case语句，以及for、while和until循环的使用。 4. **函数和脚本设计**：教导如何编写可重用的函数，以及良好的脚本设计原则。 5. **输入/输出重定向和管道**：介绍如何利用重定向和管道将命令的输出作为其他命令的输入，实现数据处理的串联。 6. **正则表达式**：讲解如何使用正则表达式进行模式匹配和文本操作。 7. **错误处理和调试**：提供关于如何捕获和处理错误，以及调试脚本的指导。 8. **脚本实例**：包含大量带有详细注释的示例脚本，鼓励读者积极参与编写和修改，实践是学习的最佳方式。此外，本书还适合用作编程概念的入门教材，不仅教授Shell脚本，还涉及更广泛的编程思想。最新版文档可以从作者的个人网站下载，包括SGML源文件和渲染后的HTML版本，方便用户阅读和进一步学习。通过学习这本《Advanced Bash-Scripting Guide》，读者不仅可以掌握Unix/Linux Shell脚本编写的基本技能，还能深入了解系统管理、自动化任务和脚本设计的高级技巧，从而提升工作效率。无论是系统管理员、开发者还是对自动化感兴趣的用户，都能从中受益。

: ${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 12−14.

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 be 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. 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 I). 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.

test operator. Within certain expressions, the ? indicates a test for a condition.

Advanced Bash−Scripting Guide

Chapter 3. Special Characters 11

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