专家级Shell脚本编程技巧

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"Apress.Expert.Shell.Scripting" 是一本关于高级Shell脚本编程的书籍，由Ron Peters撰写，涵盖了基本的脚本技术、标准函数库、日期处理以及变量测试与赋值等多个主题。在《基本脚本技术》章节中，作者探讨了Shell脚本调试的方法，包括Shell的跟踪选项，用于控制脚本运行过程中的输出信息。简单输出语句允许开发者清晰地了解脚本执行的状态。通过设置调试级别，可以精细化地管理脚本的输出信息，以适应不同的调试需求。此外，书中还介绍了如何使用函数简化错误检查，以便更高效地定位和解决问题。手动步进调试是另一种有效的方式，允许开发者逐步执行脚本，观察每一步的结果。《标准函数库》章节中，作者讲解了如何创建和使用一个自定义的Shell函数库。这个库包含了一些实用的函数，可以帮助提升脚本的可重用性和效率。读者将学习如何编写和导入这些函数到自己的脚本中，以实现功能的模块化。在《日期在天数中的表示》和《日期和时间操作》章节中，作者深入讨论了如何处理和计算日期和时间。如何获取从纪元至今的天数，以及找到日期对应的秒数有多种替代方法。同时，书中还教读者如何针对当前日期和时间进行比较和测试，这是编写时间相关脚本的关键技能。在《测试变量和赋默认值》章节中，作者详细介绍了如何在Shell脚本中测试变量是否存在并为其设置默认值。这一章涵盖了几种不同的变量赋值语法，如`:=`、`= `、`:-`、`-`、`:?`、`?`、`:+`和`+`，它们在不同场景下有不同的用途，能帮助开发者在不明确变量是否已定义的情况下安全地使用变量。《接受命令行选项、开关和参数》章节则关注如何在Shell脚本中处理命令行输入。理解如何解析这些参数对于编写能够接收用户输入的实用脚本至关重要。读者将学会如何优雅地处理命令行选项和参数，使脚本更加灵活和用户友好。这本书全面覆盖了Shell脚本编写中的关键技巧和最佳实践，是希望提升Shell脚本编程能力的IT专业人士的重要参考资料。无论是调试技巧、函数库的构建，还是日期时间操作和命令行交互，都能帮助读者提升工作效率，编写出更高效、更可靠的脚本。

xxi

Introduction

I learned the basics of programming when I was in school; I learned how to shell-script by

example.

I’ve met and worked with many system administrators and other *NIX folks, each of

whom has their own bag of tricks when it comes to managing a system, interacting with

their environment, or coding a script. It’s always very useful to have conversations and

interact with people like this because you invariably gain some tidbits that you can

throw into your own collection of tricks. I decided to collect all the useful shell-scripting

and interaction techniques I have learned through the years and combine them into one

beneficial reference guide. In fact, I used some of my own notes about those techniques

while writing this book. Since I haven’t memorized everything present in this book, I

would periodically look up items when I was working on various tasks. I want this book

to be the beginning of a higher-level reference library that can be added to and can grow

continually.

You might be aware of the large number of shell-scripting books and online resources

aiding in the mastery of shell scripting. Many are excellent and cover a wide range of topics.

The main purpose of this book is to combine some of the most unique tools, code snippets,

and scripts that go beyond the level of basic scripts. I wanted to create a cookbook of sorts—

lesser-known recipes and fairly advanced algorithms that have proved useful to me.

I have included scripts you can use as is, and sample scripts illustrating a specific algo-

rithm. I also demonstrate a few complex commands that may be useful on the command

line. I have tried to tailor the scripts to be useful at multiple levels. Most times, however,

there is little or no error-checking since that is not necessarily the point of a specific script.

You must be prepared to make modifications to fit your local environment.

How This Book Came About

My friend Brian Culp and I have worked together as UNIX system administrators for many

years. Periodically, Brian or I will be working on some script and run into a problem. One

of us will stop, walk over to the other, and say something like, “Do you have any code that

does X?” The answer may be no, in which case we’ll launch into a discussion on how we

might tackle the problem, or come up with a few different solutions. However, many times

it might be something like, “Hmm, yeah, I think I remember doing something like that in

a script that does X on system Y. Let me look for a minute.” A few carefully chosen grep

commands, and the solution is at hand.

xxii

■INTRODUCTION

Although finding the solution we want is great, it’s not the most efficient use of our

time. To go from having a place to store and organize all of our (and, of course, other

programmers’) gems and having them in a heavily documented form, to writing a book

on the subject was only a short step. Even though it is possible to search online references

quickly for specific code, there were many occasions when I just wanted to pull a book off

my shelf. It’s not always obvious what to search for when you have a specific itch that

needs scratching, so you’re not sure exactly to what search for online.

This is to some extent an expression of my own limitations: my family and friends think

I’m a computer guru, but rest assured, I know better. There are many programmers out

there who are much more adept at shell coding than I am. I mainly intended to collect,

order, and explain code that I have found to be highly useful in my professional experi-

ence as a system administrator, and share that information with others.

Who Should Read This Book

The book is meant for the intermediate shell coder up to the advanced shell-code hacker,

because I don’t explain many basic programming structures. If you’re looking for that

type of book, you should look to the resources mentioned in Appendix C.

This is not to say that the beginner won’t find this book useful; it may work well as a supple-

mentary reference to a more traditional shell-scripting training guide. But there is a difference

between learning English as a second language and learning how to apply sarcasm. This book

is like sarcasm in that example; it assumes some basic shell-code literacy.

I go into great detail about how and why the scripts were written in their present form,

and I include some explanation of how to avoid certain problems. Much of my learning

came from sources heavy in obfuscation and light on clarity, so I tried to be as explicit as

possible, and favored explaining too much rather than too little. You can think of many

chapters I included as shell scripts with extremely detailed commentary.

The book is divided into three parts: “Basic Scripting Techniques,” “System Interaction

and Advanced Techniques,” and “Useful Scripts.” Most chapters serve as stand-alone

discussions, although they may refer to other chapters on some minor points.

Issues and Ideas

I have made every effort to test the code that I included in this book to validate that it

works. With a project of this size, however, even with the number of eyes that have

reviewed it, there may be mistakes. I would like to know about the mistakes as well as, and

more importantly, any other ideas and scripts that could be used for future revisions of

this book. Please drop me a note at rbpeters@peterro.com.

■ ■ ■

CHAPTER 1

Shell Script Debugging

Even though this book isn’t a “how to script” manual, some concepts that are funda-

mental to writing successful scripts should be discussed. Debugging is one of them.

Debugging code is a significant part of writing code. No matter how disciplined you are

or how skilled you become at coding, you will have bugs in your code, in the form of either

syntax or logic errors. The syntactical problems tend to be simpler to resolve since many

times they show up when the code throws an error when it is run. The logical bugs, on the

other hand, may be more difficult to track down since the code may run without error, but

the resulting output does not match the design of the program. The more complex your

code becomes as your skill increases, the more difficult these types of problems will be to

detect.

Since writing bug-free code is nearly impossible, you need a few techniques up your

sleeve that will help you finish, diagnose, repair, and clean up your code. This chapter pre-

sents a few ways to debug code that I have used consistently and that help me extract

details from the inner workings of my scripts. These techniques validate that the code is

living up to my expectations and demonstrate where the code needs more work to per-

form the intended task.

Shell Trace Options

The first technique—using the set command—is the simplest to implement and can give

you great amounts of detail about how the logic is progressing and the values of variables

internal to your script. Using the set command is really just using shell options to display

verbose output when the script is running. One of the functions of the set command is to

turn on and off the various options that are available in the shell. In this case, the option

being set is -x, or xtrace. This is where the running script will, in addition to any normal

output, display the expanded commands and variables of a given line of code before the

code is run. With this increased output, you can easily view what is happening in the run-

ning script and possibly determine where your problem lies.

When you put the instruction set -x into your script, each of the commands that exe-

cute after that set instruction will be displayed, together with any arguments that were

supplied to the command, including variables and their values. Each line of output will be

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