快速入门Linux Shell编程指南

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《ExpertShellScripting》是一本专为Linux用户设计的入门级Shell脚本教程，由Ron Peters编著，旨在帮助读者快速理解和掌握Shell编程的基础知识。这本书以简洁明了的方式讲解，适合初学者和有一定基础的IT专业人士，无论是希望提升技能还是深入理解系统底层操作，都能从中获益。书中涵盖了广泛的Shell编程主题，包括但不限于基本命令的使用、变量和数据类型、条件语句、循环结构、函数定义、文件和目录操作、错误处理以及高级技巧等。作者通过实例和练习引导读者逐步掌握Shell脚本的编写和调试方法，使读者能够在实际工作中灵活运用所学知识。版权方面，该书享有严格的保护，未经作者及出版社许可，禁止任何形式的复制或传播。本书的电子版和纸质版分别拥有ISBN-13编号，确保了版本的一致性和正规发行。作者尊重并遵守商标法，书中提及的商标名仅用于编辑目的，并非商业宣传。编纂团队包括经验丰富的技术审阅者Brian Culp，以及一支由多名资深编辑、项目经理和校对人员组成的专业团队，他们共同确保了本书内容的专业性和质量。通过Frank Pohlmann担任主编，读者可以期待一个严谨且易于理解的教学过程。学习《ExpertShellScripting》，读者不仅能提升Linux Shell编程能力，还能了解到如何利用Shell脚本来自动化任务、管理文件系统、执行系统维护工作，甚至构建更复杂的系统管理工具。无论你是Linux新手还是想深化Shell技能的开发者，这本书都是一本不可多得的参考资料。通过阅读和实践书中的内容，你将踏上一条高效且充满挑战的IT技能成长之路。

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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