Preface

Welcome to The Ultimate Linux Shell Scripting Guide! This book, which is ideal for both Linux beginners and more advanced Linux administrators, will guide you through the shell script creation process. We'll begin with basic command-line usage and will progress through more advanced concepts in every succeeding chapter. You'll see how to build scripts that can help you automate repetitive administrative tasks, as well as many other cool things. We'll primarily concentrate on bash scripting throughout most of the book. Later, we'll show you how to make your scripts portable so that they can run on legacy Unix systems that can't run bash. After chapters on shell script debugging and shell script security, we'll wrap up with introductions to the Z Shell and PowerShell.

Who this book is for

This book is appropriate for anyone who needs to master the concepts of shell scripting. Linux beginners can benefit, because it can help them master the concepts that will be covered on the CompTIA Linux+/Linux Professional Institute exam. More advanced Linux administrators can benefit because it will show them the more advanced concepts that they need to build really useful, practical shell scripts.

What this book covers

Chapter 1, Getting Started with the Shell, this chapter covers the basics of operating system shells that can be found on Linux and Unix-like systems. The reader will need to know these principles in order to understand principles that will be presented in later chapters.

Chapter 2, Interpreting Commands, there are five things that an operating system shell will do for us. These include interpreting commands, setting variables, enabling pipelines, allowing input/output redirection, and allowing customization of the user's working environment. In this chapter, we'll look at how shells interpret a user's commands.

Chapter 3, Understanding Variables and Pipelines, in this chapter, we'll look at the next two things that an operating system shell does for us, which is to allow us to set variables and use command pipelines. There's not that much to say about either of these topics, which is why we're combining them both into one chapter.

Chapter 4, Understanding Input/Output Redirection, in this chapter, we'll look at how to send the text output of a command to somewhere other than the terminal, which is the default output device. We'll then look at how to make a command bring in text from somewhere other than the keyboard, which is the default input device.

Finally, we'll look at how to send error messages to somewhere other than the terminal.

Chapter 5, Customizing the Environment, in this chapter, we'll look at the various configuration files for the various shell environments. We'll look at how to customize these configuration files, and how to set certain environmental options from the command-line.

Chapter 6, Text Stream Filters - Part 1, many times, an administrator will need to write a shell script that will retrieve text information from an external source, format it, and create a report. In this chapter, we'll introduce the concept of text stream filters, which can help with this process. Also, knowing about these text stream filters can help you pass certain Linux certification exams, such as the LPI/Linux+ exam. We will then show you how to use several of these filters.

Chapter 7, Text Stream Filters - Part 2, in this chapter, we'll continue our exploration of text stream filters.

Chapter 8, Basic Shell Script Construction, in this chapter, we'll explain about the basic structure of a shell script, and will use some of the text stream filters from the previous chapters to create simple scripts. We'll also look at some basic programming constructs that are common to all programming languages, and show you how to use them.

Chapter 9, Filtering Text with grep, sed, and Regular Expressions, in this chapter, you'll learn about the concept of regular expressions, and how to use them with grep and sed to filter or manipulate text. These techniques can not only help you find certain text, but can also help automate the creation of reports and the editing of multiple text files at once.

Chapter 10, Understanding Functions, functions are an important part of every programming language, because they make it easy for a programmer to reuse a block of code in numerous programs, or in numerous places within one single program. The programmer can pass parameters to a function, have the function operate on those parameters, and pass back the results to the main program.

Chapter 11, Performing Mathematical Operations, the various operating system shells all have means of performing mathematical operations either from the command-line, or from within a shell script. In this chapter, we'll look at how to perform operations with both integer and floating point math.

Chapter 12, Automating Scripts with here Documents and expect, although it's easy to have a shell script pull data out of a separate text file, it's sometimes handier to store the data within the shell script itself. We'll do that using a "here" document. In this chapter, you'll learn how to create and use "here" documents. You'll also see how to automate certain scripts with the expect utility.

Chapter 13, Scripting with ImageMagick, imageMagick is a text-mode program that is used to edit, manipulate, and view graphical image files. In this chapter, you'll learn how to automate the processing of images by using ImageMagick commands within shell scripts.

Chapter 14, Using awk-Part 1, this chapter covers awk, which is a tool that can extract specific text from text files, and automate the creation of reports and databases. Since awk is a full-blown programming language in its own right, we won't be covering it in depth here. Instead, we'll give you enough information so that you can create awk "one-liners" that can be used within shell scripts.

Chapter 15, Using awk-Part 2, this is a continuation of the previous chapter, in which we'll cover the more advanced concepts of scripting with awk.

Chapter 16, Creating User Interfaces with yad, dialog, and xdialog, so far, we've only looked at shell scripts that run strictly from the command-line. And indeed, that's how most people use them, and is what most people think about when they think about shell scripts. But, it's also possible to create shell scripts that offer a user interface. In this chapter, we'll use yad to create graphical user interfaces, and dialog to create ncurses-style interfaces.

Chapter 17, Using Shell Script Options with getopts, often, an administrator will need to pass both arguments and options to a shell script. Passing arguments, the objects upon which a script will operate, is easy. To also pass options, which modify how the script will operate, requires another type of operator. In this chapter, you'll learn how to use getopts to pass options to a script.

Chapter 18, Shell Scripting for Security Professionals, in this chapter, you'll learn how to either create shell scripts or search for existing shell scripts that can help security administrators perform their jobs. We'll also look at how to modify or improve existing shell scripts to meet specific needs of security administrators.

Chapter 19, Shell Script Portability, large organizations, such as large government agencies or large corporations, might have a diverse mix of Linux, Unix, and Unix-like machines. Sometimes, it's handy to write shell scripts that can automatically detect the type of system on which they're running, and run the appropriate code for each type of system. In this chapter, we'll look at several methods for enhancing script portability.

Chapter 20, Shell Script Security, scripting errors can cause a script to inadvertently cause the exposure of sensitive data, or to allow someone to perform unauthorized activities on a system. In this chapter, we'll look at ways to help the reader write shell scripts that are as secure as they possibly can be.

Chapter 21, Debugging Shell Scripts, shell scripts can have bugs, the same as with any other programming language. Sometimes, the bugs are easy to find, and sometimes they're not. In this chapter, we'll look at various methods that can help a busy administrator debug shell scripts that aren't working properly.

Chapter 22, Introduction to Z Shell Scripting, the Z Shell, or zsh, is an alternate shell that can be used in place of bash. It's mainly used in the same manner as bash, but it also has enhancements that bash doesn't have. In this chapter, we'll look at these enhancements, and also at some scripting tricks that you can't do with bash.

Chapter 23, Using PowerShell on Linux, powerShell was created by Microsoft for use on Windows operating systems back in 2006. In 2016, Microsoft announced that they had open-sourced PowerShell, and were making it available for Linux and macOS, as well as for Windows. In this chapter, we'll look at how PowerShell can be beneficial for Linux administrators, how to install it, and how to use it.

To get the...