Learn Linux Shell Scripting – Fundamentals of Bash 4.4: A comprehensive guide to automating administrative tasks with the Bash shell

440 Pages·2018·5.807 MB·English

by  Sebastiaan Tammer

Learn Linux Shell Scripting –
Fundamentals of Bash 4.4
A comprehensive guide to automating administrative tasks
with the Bash shell
Sebastiaan Tammer
BIRMINGHAM - MUMBAI
Learn Linux Shell Scripting – Fundamentals
of Bash 4.4
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First published: December 2018
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ISBN 978-1-78899-559-7
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Contributors
About the author
Sebastiaan Tammer is a Linux enthusiast from the Netherlands. After attaining his BSc in
information sciences, he graduated with an MSc in business informatics, both from Utrecht
University. His professional career started in Java development, before he pivoted into a
Linux opportunity. Because of this dual background, he feels most at home in a DevOps
environment.
Besides working extensively with technologies such as Puppet, Chef, Docker, and
Kubernetes, he has also attained the RHCE and OSCP certificates. He spends a lot of time in
and around Bash. Whether it is creating complex scripting solutions or just automating
simple tasks, there is hardly anything he hasn't done with Bash!
I would like to thank my girlfriend, Sanne, for all the help and support she has given me
throughout the years. She has had to endure the late nights studying, me fixing stuff
(which I had inevitably broken only hours earlier), and my endless storytelling about all
those exciting new technologies. Thanks for the enormous amount of patience and love. I
could not have done it without you!
About the reviewer
Heathe Kyle Yeakley holds degrees in technical communications and network
management. He began his IT career in 1999 doing entry-level help desk support. In 2003,
he began his first enterprise data center job performing tape backup and recovery for the
United States Federal Aviation Administration. He worked in the aerospace sector for
several years, during which time he worked on a wide range of products, including HP
Tru64 Unix, Red Hat Enterprise Linux, Solaris 10, Legato Networker, Symantec NetBackup,
HP and NetApp storage arrays, Spectra Logic and ADIC tape libraries, VMware, and HP
Blade servers.
He currently works for Agio, where he and his coworkers deliver managed IT services to
some of the world's most prestigious companies.
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Table of Contents
Preface 1
Chapter 1: Introduction 7
What is Linux? 7
What is Bash? 8
Summary 10
Chapter 2: Setting Up Your Local Environment 11
Technical requirements 11
Choosing between a virtual machine and a physical installation 12
Setting up VirtualBox 13
Creating an Ubuntu virtual machine 14
Creating the virtual machine in VirtualBox 14
Installing Ubuntu on the virtual machine 21
Accessing the virtual machine via SSH 29
Summary 30
Questions 30
Further reading 31
Chapter 3: Choosing the Right Tools 32
Technical requirements 32
Using graphical editors for shell scripting 33
Atom 33
Atom installation and configuration 34
Notepad++ 37
Using command-line editors 38
Vim 38
Vim summary 41
.vimrc 41
Vim cheat sheet 42
nano 43
Combining graphical editors with command-line editors when
writing shell scripts 45
Summary 46
Questions 47
Further reading 47
Chapter 4: The Linux Filesystem 48
Technical requirements 48
The Linux filesystem explained 49
Table of Contents
What is a filesystem? 49
What makes the Linux filesystem unique? 50
Structure of the Linux filesystem 52
Tree structure 52
Overview of top-level directories 57
What about multiple partitions? 58
/bin/, /sbin/, and /usr/ 60
/etc/ 62
/opt/, /tmp/, and /var/ 63
Everything is a file 65
Different types of files 65
Summary 68
Questions 69
Further reading 69
Chapter 5: Understanding the Linux Permissions Scheme 70
Technical requirements 70
Read, write, and execute 71
RWX 71
Users, groups, and others 72
Manipulating file permissions and ownership 75
chmod, umask 75
sudo, chown, and chgrp 81
sudo 81
chown, chgrp 83
Working with multiple users 85
Advanced permissions 89
File attributes 89
Special file permissions 90
Access Control Lists (ACLs) 90
Summary 91
Questions 92
Further reading 92
Chapter 6: File Manipulation 93
Technical requirements 93
Common file operations 93
Copying 94
Removing 96
Renaming, moving, and linking 97
Archiving 100
Finding files 104
locate 104
find 105
Summary 109
[ ii ]
Table of Contents
Questions 109
Further reading 110
Chapter 7: Hello World! 111
Technical requirements 111
First steps 111
The shebang 113
Running scripts 113
Readability 114
Comments 114
Script header 115
Verbosity 118
Verbosity in comments 118
Verbosity of commands 119
Verbosity of command output 120
Keep It Simple, Stupid (KISS) 121
Summary 122
Questions 123
Further reading 123
Chapter 8: Variables and User Input 124
Technical requirements 124
What is a variable? 125
Why do we need variables? 127
Variables or constants? 129
Variable naming 129
Dealing with user input 131
Basic input 131
Parameters and arguments 134
Interactive versus non-interactive scripts 135
Combining positional arguments and read 137
Summary 141
Questions 142
Further reading 143
Chapter 9: Error Checking and Handling 144
Technical requirements 144
Error checking 145
Exit status 145
Functional checks 147
Test shorthand 149
Variable refresher 151
Bash debugging 152
Error handling 152
if-then-exit 153
if-then-else 157
[ iii ]
Table of Contents
Shorthand syntax 161
Error prevention 163
Checking arguments 163
Managing absolute and relative paths 167
Dealing with y/n 169
Summary 172
Questions 173
Further reading 174
Chapter 10: Regular Expressions 175
Technical requirements 175
Introducing regular expressions 175
What is a regular expression? 176
grep 176
Greediness 178
Character matching 179
Line anchors 183
Character classes 186
Globbing 188
What is globbing? 188
Similarities with regular expressions 189
More globbing 190
Advanced globbing 192
Disabling globbing, and other options 193
Using regular expressions with egrep and sed 194
Advanced grep 194
Introducing egrep 198
sed, the stream editor 202
Stream editing 203
In-place editing 204
Line manipulation 206
Final remarks 209
Summary 210
Questions 211
Further reading 211
Chapter 11: Conditional Testing and Scripting Loops 212
Technical requirements 212
Advanced if-then-else 213
A recap on if-then-else 213
Using regular expressions in tests 214
The elif condition 217
Nesting 220
Getting help 222
The while loop 223
The until loop 225
[ iv ]