Chapter 1, Let's Start Programming, is our first brush with the magic of Bash. We will use basic shell programming bits to write easy code that will forecast all the benefits of more advanced scripts.
Chapter 2, Operators, is where we perform some simple operations, such as checking whether something is greater, equal to, or less than something else and how to add, subtract, and fiddle with numbers. This is the first step toward imposing conditions on events dealt with in our scripts.
Chapter 3, Testing, explains how checking whether something fits into boundaries and certain conditions are met or not is fundamental to making our scripts able to react to events and to decide what to do based on real-time indicators coming from the system or from other programs.
Chapter 4, Quoting and Escaping, tells you how the shell has its own reserved words, which cannot be used without knowing exactly what they do. Furthermore, the variables hold values that must be preserved while we are working on them. This is where we'll learn to be cautions about what we are going to write.
Chapter 5, Menus, Arrays, and Functions, explores how to make the script interact with the user, for example, giving the user the chance to answer some questions and deal with the options highlighted. This involves the ability to create a command-line interface for the program itself and a way to store the data in a structure that will make it easy to retrieve that data. And that is what arrays are all about.
Chapter 6, Iterations, explains how iterations are fundamental to going over data and extracting and processing them based on some conditions while they last, for instance, or for some values we use as counters. We will learn how to use while and for loops.
Chapter 7, Plug into the Real World, introduces one of the most famous open source monitoring system, Nagios, which is all about plugins. You can write complex programs in any language to perform whichever checks you want on your sites and applications. But some of the most tricky plugins I have used have been written using Bash, and nothing else.
Chapter 8, We Want to Chat, is about Slack, currently one of the most widely used messaging systems. Why not write a small fragment of code to send our thoughts over a Slack channel and, maybe, make a communication plugin out of it, enabling other scripts to send messages through the messaging system?
Chapter 9, Subshells, Signals, and Job Controls, discusses how sometimes a single process is not enough. Our script has to do many things at once, using a sort of raw parallelism to get to the desired outcome. Well, it's time to see what we can spawn inside a shell, how to control our jobs, and send signals.
Chapter 10, Let's Make a Process Chat, explores the topic of processes talking to each other, feeding each other data and sharing the burden of data elaboration. Pipes, redirections, process substitution, and a bit of netcat--this could open up new scenarios, and we'll see how.
Chapter 11, Living as a Daemon, explains how sometimes sending a script into the background is not enough. It will not survive long, but you can use some tricks such as double forking, setsid, and disowning to make it a bit devilish and survive until process death. Make it a daemon and let it wait for your orders.
Chapter 12, Remote Connections over SSH, tells you how scripts can be run locally, but they can do much more for you. They can log in remotely over a secure channel and issue commands on your behalf without you inputting any further instructions. Everything is stored in a key, which unlocks a whole bunch of new possibilities.
Chapter 13, It's Time for a Timer, discusses how to fully automate routine tasks. We have to have a method to run our scripts based on some conditions. The most common is based on time, such as hourly, daily, weekly, or monthly repetitions. Just think about a simple log rotation triggered on certain conditions, the most common being on a daily schedule.
Chapter 14, Time for Safety, explains how safety is a must in your working environment. Scripting often means access to remote servers and interacting with them, so learning some tricks to keep your server more secure will help you prevent intrusions and keep your job away from unwanted eyes.
