Mastering Bash: Automate Daily Tasks with Bash

496 Pages·2017·53.762 MB·English

by  Giorgio Zarrelli

Mastering Bash

Automate daily tasks with Bash
Giorgio Zarrelli

BIRMINGHAM - MUMBAI
Mastering Bash
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First published: June 2017
Production reference: 1190617
Published by Packt Publishing Ltd.
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ISBN 978-1-78439-687-9
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Credits
Author Copy Editors
Giorgio Zarrelli Dipti Mankame
Yesha Gangani
Reviewer Project Coordinator
Sebastian F. Colomar Judie Jose
Commissioning Editor Proofreader
Kartikey Pandey Safis Editing
Acquisition Editor Indexer
Rahul Nair Rekha Nair
Content Development Editor Graphics
Abhishek Jadhav Kirk D'Penha
Technical Editor Production Coordinator
Aditya Khadye Melwyn Dsa
About the Author
Giorgio Zarrelli is a passionate GNU/Linux system administrator and Debian user, but has
worked over the years with Windows, Mac, and OpenBSD, writing scripts, programming,
installing and configuring services--whatever is required from an IT guy. He started
tinkering seriously with servers back in his university days, when he took part in the
Computational Philosophy Laboratory and was introduced to the Prolog language. As a
young guy, he had fun being paid for playing games and write about them in video game
magazines. Then he grew up and worked as an IT journalist and Nagios architect, and
recently moved over to the threat intelligence field, where a lot of interesting stuff is
happening nowadays.
Over the years, he has worked for start-ups and well-established companies, among them
In3 incubator and Onebip as a database and systems administrator, IBM as QRadar support,
and Anomali as CSO, trying to find the best ways to help companies make the best out of
IT.
Giorgio has written several books in Italian on different topics related to IT, from Windows
security to Linux system administration, covering MySQL DB administration and Bash
scripting.
At last, some acknowledgments since we cannot do much without the help of the people
who make our lives better. Firstly, Ilaria, who had to go through all the weekends and the
mornings I spent writing instead of strolling downtown. Then, mum and dad and my
brother, Maurizio. Being Italian, my mum would kill me if I did not acknowledge her--and,
by the way, they are such an important part of my life. Let’s keep it short, since I cannot
thank all the people who enrich my life and have put some flourishes into this book. So let
me thank my bosses at Anomali, Gabe and Mitul, for supporting me and letting me use a
Mac (I do whatever needed to write a book, even if it is crazy) when my laptop broke and
the replacement was stuck somewhere around the globe. Thanks to my editor, Abhishek, for
being supportive, professional, and patient during the writing of this book. Finally, thank
you, dear reader, for having a look at this book--sometimes IT can be boring; I've tried to
make it fun.
About the Reviewer
Sebastian F. Colomar is a GNU/Linux system engineer specializing in the scripting,
installation, configuration, and maintenance of Linux servers for better security and
performance.
He is currently an infrastructure architect at Hanscan, having been a consultant for scripting
and Linux administration for many companies, such as IBM, Indra, Thales, Accelya,
Accenture, AXA, Cetelem, RTVCM, EMT, and ESA.
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Table of Contents
Preface
1
Chapter 1: Let's Start Programming
8
I/O redirection 12
Messing around with stdin, stdout, and stderr 17
Time for the interpreter: the sha-bang 20
Calling your script 22
Something went wrong, let's trace it 28
Variables 31
Assigning a variable 32
Keep the variable name safe 33
Variables with limited scope 34
Environment variables 36
Variable expansion 44
Pattern matching against variables 49
Special variables 54
Summary 59
Chapter 2: Operators
60
Arithmetic operators 60
The + operator 61
The - operator 62
The * operator 62
The / operator 62
The % operator 62
The ** operator 63
Assignment operators 63
The += operator 63
The -= operator 64
The *= operator 64
The /= operator 65
The %= operator 65
The ++ or -- operators 66
Bitwise operators 67
Left shift (<>) 69
Bitwise AND(cid:3)(cid:11)(cid:9)(cid:12) 70
Bitwise OR (|) 71
Bitwise XOR (^) 71
Bitwise NOT (~) 71
Logical operators 73
Logical NOT (!) 73
Logical AND(cid:3)(cid:11)(cid:9)(cid:9)(cid:12) 74
Logical OR (||) 75
Comma operator (,) 76
Operators evaluation order and precedence in decreasing relevance 77
Exit codes 77
Exiting a script 80
Summary 83
Chapter 3: Testing
84
What if...else 84
Test command recap 92
Testing files 93
Testing integers 111
Testing strings 118
More on tests 124
Summary 126
Chapter 4: Quoting and Escaping
127
Special characters 128
The hash character (#) 128
The semicolon character (;) 129
The double semicolon character (;;) 130
The case terminator (;;&) and (;(cid:9)(cid:12)(cid:12) 131
The dot character (.) 131
The double quotes ((cid:5)(cid:17)(cid:17)(cid:5)(cid:12) 137
The single quotes ('...') 137
The comma character (,) 137
The ,, and , () case modificators 138
The ^^ and ^ () case modificators 138
The backslash (\) 140
The forward slash (/) 140
'...' 140
The colon character (:) 140
The exclamation (!) 141
Keywords 141
[ ii ]