深入探索高级Shell脚本艺术：实战指南与修订历史

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《高级Bash脚本指南》(Advanced Bash Scripting Guide)是一本深入探讨Shell脚本艺术的详尽教程，由Mendel Cooper撰写，电子邮件地址为thegrendel@theriver.com。该书自2000年6月14日发布首个版本0.1以来，历经多次修订，不断修复错误、添加新材料和示例脚本，旨在提升读者对Unix Shell编程的理解和实践能力。在各个版本中，作者持续关注改进和扩展，如： - Revision 0.2 (2000年10月30日)：主要修复了错误，并增加了大量额外的内容和更多示例脚本，体现了作者对细节的严谨处理和对教学内容的不断完善。 - Revision 0.3 (2001年2月12日)：进行了又一次重大更新，表明了作者对新技术和脚本实践的持续学习和整合。 - Revision 0.4 (2001年7月8日)：进一步的bug修复，更多的材料和脚本，标志着这本书经历了全面的修订和扩展，成为一本更为成熟和丰富的教材。 - Revision 0.5 (2001年9月3日)：进行了一次大规模的更新，包括bug修复、新内容添加以及章节和部分结构的重新组织，显示了作者对教材布局优化的重视。特别值得注意的是，从Revision 1.0（2001年10月14日）开始，标志着本书进入稳定发行阶段，之后的修订1.1到1.4分别在2002年进行，除了继续修复错误，还增添了更多材料和脚本，这些版本被命名为“TANGERINE”和“MANGO”，表明每一次迭代都带来了实质性的进步和内容扩充。《高级Bash脚本指南》不仅教授基础概念，还涵盖了高级技巧，适合于想要提高Shell脚本编写技能的开发者和系统管理员。书中不仅提供理论知识，而且包含了大量的实战示例，使读者能够通过实际操作来掌握和应用所学知识。这本指南对于理解和应对Unix/Linux环境下的自动化任务具有很高的实用价值。

Evaluate string of variables using parameter substitution (as in Example 9−13).

: ${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 12−11.

In combination with the >> redirection operator, updates a file access/modification time (: >>

new_file). If the file did not previously exist, creates it. This is equivalent to touch.

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 F). 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

Advanced Bash−Scripting Guide

Chapter 3. Special Characters 10

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