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Operators

What we have looked at so far is tinkering with values returned from variable expansions and descriptors used in a tricky way. So, something nice, but we could not do much more, since we do not have a way to actually relate values, compare or even modify at our will.

Here is where the operators come in to play, and we will see how to modify the value of a variable so that it will hold a value and, over time, modify to gather new information. So, let's start from something simple, from basic math then move on to something more complex.

One last thing we have to bear in mind before proceeding is that the operators follow an order of precedence:

The compound logical operators -a, -o, and && have a low precedence
The arithmetic operators have the following precedence:
- Multiply
- Divide
- Add
- Subtract
The evaluation of operators with equal precedence is from...

Key benefits

From roots to leaves, learn how to program in Bash and automate daily tasks, pouring some spice in your scripts

Daemonize a script and make a real service of it, ensuring it’s available at any time to process user-fed data or commands

This book provides functional examples that show you practical applications of commands

Description

System administration is an everyday effort that involves a lot of tedious tasks, and devious pits. Knowing your environment is the key to unleashing the most powerful solution that will make your life easy as an administrator, and show you the path to new heights. Bash is your Swiss army knife to set up your working or home environment as you want, when you want. This book will enable you to customize your system step by step, making your own real, virtual, home out of it. The journey will take you swiftly through the basis of the shell programming in Bash to more interesting and challenging tasks. You will be introduced to one of the most famous open source monitoring systems—Nagios, and write complex programs with it in any languages. You’ll see how to perform checks on your sites and applications. Moving on, you’ll discover how to write your own daemons so you can create your services and take advantage of inter-process communication to let your scripts talk to each other. So, despite these being everyday tasks, you’ll have a lot of fun on the way. By the end of the book, you will have gained advanced knowledge of Bash that will help you automate routine tasks and manage your systems.

What you will learn

Understand Bash right from the basics and progress to an advanced level

Customise your environment and automate system routine tasks

Write structured scripts and create a command-line interface for your scripts

Understand arrays, menus, and functions

Securely execute remote commands using ssh

Write Nagios plugins to automate your infrastructure checks

Interact with web services, and a Slack notification script

Find out how to execute subshells and take advantage of parallelism

Explore inter-process communication and write your own daemon

What do you get with Print?

Instant access to your digital eBook copy whilst your Print order is Shipped

Paperback book shipped to your preferred address

Download this book in EPUB and PDF formats

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Dragon Nov 18, 2022

A lot of information. Very well put together, now I have to apply what I've learned , I have a lot of highlighted parts I keep for reference, great book !!! I would recommend it to any one with a computer

Amazon Verified review

skeptic Aug 17, 2017

The books is about scripting in bash, which makes is a welcome addition of "bash bookshelf" of system administrators. Only very few books are concentrated on this topic. And it provides some pretty elaborate, practical examples of programs in bash language.It consists of two parts: overview of the language and a set chapters devoted to selected topics.Overview of the "Bash language" provided in the Part I of the book is so-so. It does not cover modern capabilities of Bash 4.2 and later well, although some information provided is valuable for Unix sysadmins. But multiple other, better books exists on this topic.But some chapters in Part II of the book are really interesting and contain valuable information. Two of them really stand out:1. A chapter on how to writs a simple Nagios probes. It is written well and allow you immediately write you own simple probe. which for any sysadmin alone justifies the price of the book.2. The second chapter which strikes me as very useful is based on pretty slick idea that you can reuse some chat services for monitoring of your infrastructure. This is probably unacceptable for multinationals, but might be very useful for cash strapped start-ups and non-profits.The author uses Slack messaging service as an example of service that can be used this way. Slack has so called Webhooks that you can use for sending messages via HTTP put:== quote ==Slack has some endpoints, sensitive URLs; and when you post something through HTTP to endpoints, you actually communicate with Slack. What makes these WebHook interesting is that they are stateless, since they do not rely on a continuously open connection to the service; and you just ping Slack whenever you need to post or retrieve some pieces of information. Slack supports two different kinds of WebHooks...== end of quote ==As for any book from this publisher (Packt Publishing ), if you buy paper version of the book you are eligible to get ebook which is another important plus. We need to support publishers that use this, more modern, method of book production and distribution.== end of quote ==

Gregory Levin Apr 03, 2019

Easy book for shell

Decano Feb 10, 2018

I have been writing shell scripts "since the beginning", but things have changed. Bash, in particular, has evolved (in a very positive way) over the years, and is a far cry from the old sh and csh. The more recent versions (bash 3.x and 4.x) have features that are new to those of us who are "old timers". Yes, I could look up the new features on the Net, but I wanted a "real book" that I could (a) browse through to see what I am missing and (b) use as a desktop reference.Unfortunately this is not an acceptable book, or at least is not acceptable to me.A short list of defects:Examples, even of very simple concepts, are poorly written. The author seems to hide the one relevant line in a dozen or more lines of completely unrelated cruft. I could figure it all out, since I am not a "bash newbie". But it was wasting my time. And for a newcomer, I am afraid that they would just get lost.Attempts to be "cute", like the obscured examples, were just wasting my time. Move on.Finally, I found some of the grammar questionable, but that is so subjective that I will just say that the author's style is not mine!I'll look for a better book.

Operator	Evaluation order
++ --	Unary operators for incrementing/decrementing, evaluated from left to right
+- !~	Unary plus and minus, evaluated from right to left
* / %	Multiplication, division, modulo, are evaluated from left to right and are evaluated after
+ -	Addition and subtraction are evaluated from left to right
<<>>	Bitwise shift are evaluated from left to right
<= =><>	Comparison operators, from left to right
== !=	Equality operators, from left to right
&	Bitwise AND...

0	Success
1	Failure
2	Misuse of builtin
126	Command not executable
127	Command not found
128	Invalid argument
128+x	Fatal error exit with signal x
130	Execution terminated by Ctrl +C
255	Exit state out of boundary (0-255)

Mastering Bash: A Step-by-Step Guide to working with Bash Programming and Shell Scripting

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Mastering Bash

Operators

Arithmetic operators

The + operator

Assignment operators

The += operator

Bitwise operators

Left shift (<<)

Logical operators

Logical NOT (!)

Comma operator (,)

Operators evaluation order and precedence in decreasing relevance

Exit codes

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Mastering Bash: A Step-by-Step Guide to working with Bash Programming and Shell Scripting

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Key benefits

Description

Who is this book for?

What you will learn

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Product Details

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