The Ultimate Linux Shell Scripting Guide: Automate, Optimize, and Empower tasks with Linux Shell Scripting

The Ultimate Linux Shell Scripting Guide

Automate, Optimize, and Empower tasks with Linux Shell Scripting

Donald A. Tevault

The Ultimate Linux Shell Scripting Guide

Copyright © 2024 Packt Publishing

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First published: October 2024

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ISBN 978-1-83546-357-4

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To my loving friends and family

– Donald A. Tevault

Contributors

About the author

Donald A. Tevault, but you can call him Donnie. He started with Linux in 2006, and has been working with it ever since. In that time, Donnie has created training documentation for Linux administration, bash scripting, and Nagios administration. He has served as the Linux consultant for an Internet of Things security firm, and operates the BeginLinux Guru channel on YouTube. Donnie’s other books include Mastering Linux Security and Hardening and Linux Service Management Made Easy with systemd.

I’d like to thank the team at Packt Publishing for guiding this book through to completion, and my tech reviewer Jason for his invaluable suggestions.

About the reviewer

Jason Willson has been working in the Tech industry for over 20 years since his first job at the help desk at his alma mater, Grove City College. He was first introduced to Linux in 2007 at a startup in Boston and has worked with it professionally and personally ever since. He’s used command line and shell scripting techniques for a variety of tasks relating to Data Analysis, Systems Administration, and DevOps. He currently works as a DevOps Engineer at Carnegie Mellon University. In addition to reviewing this book, he has also reviewed another book published by Packt titled Linux Command Line and Shell Scripting Techniques by Vedran Dakic.

I’d like to thank the incredible Linkedin community for making this connection possible with Packt Publishing. I’d also like to thank all the coworkers, classmates, and mentors (personal,professional, and academic) who helped to shape me into who I am today. And last but not least,I’d like to thank my amazing wife Eva, who has been a constant support to me in reviewing this book despite such a hectic work schedule.

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Contents

Preface

Who this book is for

What this book covers

To get the most out of this book

Get in touch

Getting Started with the Shell

Understanding Shells

Finding Help with Shell Commands

Understanding Manual Pages

Understanding Info Pages

Getting to Know the Linux Documentation Project

Using Your Favorite Search Engine

Using a Text Editor to Create Shell Scripts

Text-mode Editors

GUI Text Editors

Understanding Compiled versus Interpreted Programming

Understanding root and sudo Privileges

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Interpreting Commands

Understanding the Structure of a Command

Using Command Options

Hands-on Lab – Practice With Command Options

Using Command Arguments

Executing Multiple Commands at Once

Running Commands Interactively

Using Command Sequences

Chaining Commands with a Semi-Colon

Conditional Command Execution with Double Ampersands

Conditional Command Execution with Double Pipes

Using the find Utility

Performing Multiple Actions with find

Hands-on Lab – Using find to Perform Other Commands

Running Commands Recursively

Hands-on Lab – Using Commands with Recursion

Understanding the Command History

Escaping and Quoting

Escaping Metacharacters

Quoting

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Understanding Variables and Pipelines

Understanding Environmental Variables

Understanding Programming Variables

Understanding Pipelines

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Understanding Input/Output Redirection

Introduction to Input/Output Redirection

Understanding stdout

Preventing File Overwrites

Using the File Descriptor

Understanding stdin

Understanding stderr

Understanding tee

Hands-on Lab – Pipes, Redirectors, and find

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Customizing the Environment

Technical Requirements

Reviewing the Environmental Variables

Understanding Shell Sessions

Understanding the Configuration Files

bash Global Configuration Files on Fedora

Users’ Configuration Files on Fedora

bash Global Configuration Files on Debian

Users’ Configuration Files on Debian

Setting the Default Editor on Debian

Setting Shell Options from the Command-line

Understanding Aliases

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Text-Stream Filters – Part 1

Technical Requirements

Introduction to Text-Stream Filters

Using cat

Using tac

Using cut

Using paste

Using join

Using sort

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Text Stream Filters – Part 2

Technical Requirements

Using expand

Using unexpand

Using nl

Using head

Using tail

Using Head And Tail Together

Using od

Using uniq

Using wc

Using fmt

Using split

Using tr

Using xargs

Using pr

Printing from the Command-line

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Basic Shell Script Construction

Technical Requirements

Understanding Basic Shell Script Construction

Hands-on Lab – Counting Logged-in Users

Performing Tests

Using the test Keyword

Enclosing a test Condition Within Square Brackets

Using an if. . .then Construct

Using Other Types of Tests

Understanding Subshells

Hands-on Lab – Testing Conditions

Understanding Scripting Variables

Creating and Deleting Variables

Understanding Variables and Shell Levels

Understanding Case Sensitivity

Understanding Read-Only Variables

Understanding Array Variables

Hands-on Lab – Using Arrays

Understanding Variable Expansion

Substituting a Value for an Unset Variable

Substituting a Value for a Set Variable

Assigning a Value to a Variable

Displaying an Error Message

Using Variable Offsets

Matching Patterns

Understanding Command Substitution

Understanding Decisions and Loops

The if. .then Construct

The do. . while construct

The for..in Construct

The for Construct

Using break

Using continue

The until Construct

The case Construct

Using Positional Parameters

Understanding Exit Codes

Standard Shell Exit Codes

User-defined Exit Codes

More Information About echo

Looking at Some Real-World Examples

Hands-on Lab: Using if..then

Hands-on Lab – Parsing an Apache Access Log

Hands-on Lab – Beta Testing a Hard Drive

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Filtering Text with grep, sed, and Regular Expressions

Technical Requirements

Understanding Regular Expressions

Literals and Metacharacters

Understanding sed

Understanding sed Portability Issues

Installing gsed on FreeBSD

Installing gsed on macOS

Installing gsed on OpenIndiana

Substitution with sed

Example 1: Modifying an Office Memo

Example 2: Modifying a List of Hollywood Actors

Example 3: Modifying Lists of Cars

Example 4: Performing a Whole-Word Substitution

Deletion with sed

Example 1: Deleting Items from a List

Example 2: Deleting Blank Lines

Appending and Inserting with sed

Example 1: Appending Lines of Text

Example 2: Performing Multiple Operations at Once

Example 3: Inserting Lines of Text

Changing with sed

Example 1: Changing Edsel to Studebaker

Example 2: Changing Entire Lines of Text

Other Miscellaneous sed tricks

Example 1: Using the q Command

Example 2: Using the w Command

Example 3: Using the r Command

Using sed program files

Example 1: Appending Lines in a Text File

Example 2: Changing Lines in a Text File

Example 3: Substituting Text

Example 4: Copying Lines from One File to Another

Compound Scripts in sed Program Files

Using sed in Shell Scripts

Understanding grep

Basic Searches with grep

More Advanced Searches with grep

Example 1: Searching for Whole Words

Even More Advanced Searches with grep

Example 1: Auditing Source Code Files

Example 2: Searching for Social Security Numbers

Example 3: Using the ^ Metacharacter

Using Extended Regular Expressions with grep

Example 1: Basic Search with Extended Syntax

Example 2: Searching for Consecutive Duplicate Words

Example 3: Searching for Words that Begin with a Certain Letter

Example 4: Searching for Words with Digits

Using Fixed-strings Regular Expressions with grep

Using RegEx Helper Programs

RegexBuddy and RegexMagic

Regex101

Looking at Some Real-World Examples

Modifying Multiple Files at Once

Searching Through Apache Webserver Logs for Cross-site Scripting Attacks

Automating Third-party Repository Installations

Filling Empty Fields in a .csv File

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Understanding Functions

Technical Requirements

Introduction to Functions

Defining a Function

Using Functions in Shell Scripts

Creating and Calling Functions

Passing Positional Parameters to Functions

Passing Values from a Function

Creating Function Libraries

Looking at Some Real-World Examples

Checking Network Connectivity

Using the CoinGecko API

Hands-on Lab – Creating the coingecko.sh Script

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Performing Mathematical Operations

Technical Requirements

Performing Integer Math with Expressions

Using the expr Command

Using echo with Math Expressions

Performing Integer Math with Integer Variables

Performing Floating Point Math with bc

Using bc in Interactive Mode

Using bc Program Files

Using bc in Shell Scripts

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Automating Scripts with here Documents and expect

Technical Requirements

Using here Documents

Creating here Documents with Static Data

Creating here documents with Dynamic Data

Using Functions in here Documents

Automating Responses with expect

Security Implications with expect

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Scripting with ImageMagick

Technical Requirements

Converting Non-standard Filename Extensions

Installing ImageMagick

Displaying Images

Viewing Image Properties

Resizing and Customizing Images

Batch-processing Image Files

Using Fred’s ImageMagick Scripts

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Using awk – Part 1

Introducing awk

Understanding Patterns and Actions

Obtaining Input from Text Files

Looking for Human Users

Parsing Webserver Access Logs

Using Regular Expressions

Obtaining Input from Commands

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Using awk – Part 2

Technical Requirements

Basic awk Script Construction

Using Conditional Statements

Using a while Construct and Setting Variables

Summing Numbers in a Line

Finding the CPU Generation

Using for loops and Arrays

Using Floating Point Math and printf

Working with Multi-Line Records

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Creating User Interfaces with yad, dialog, and xdialog

Technical Requirements

Creating a Graphical User Interface with yad

The yad Basics

Creating Data Entry Forms

Creating a Drop-down List

Using the yad File Manager

Creating a File Checksum Utility

Creating a GUI Front-end for ImageMagick

Programming Form Buttons

Some Final Thoughts about yad

Creating User Interfaces with dialog and xdialog

The dialog Basics

The xdialog Basics

Automatically Choosing Either dialog or xdialog

Adding Widgets

Creating an SSH Login Interface

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Using Shell Script Options with getops

Technical Requirements

Understanding the Need for getopts

Understanding getopt versus getopts

Using getopts

Looking at Real-world Examples

The Modified Coingecko Script

The Tecmint Monitor Script

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Shell Scripting for Security Professionals

Technical Requirements

Simple Scripts for Auditing

Identifying an Operating System

A Simple Port-scanning Script

Auditing the root User Account

Creating the root Account Auditing Script for Linux and OpenIndiana

Modifying the root Account Auditing Script for Use on FreeBSD

Creating a User Activity Monitoring Script

Creating Simple Firewall Scripts

Creating an IP Address Blocking Script for Red Hat Distros

Hands-on Lab: Create the Script with an Array and a for loop

Hands-on Lab: Creating the Script with xargs

Searching for Existing Security-related Scripts

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Shell Script Portability

Technical Requirements

Running bash on Non-Linux Systems

Using env to Set the bash Environment

Creating a Symbolic Link to bash

Understanding POSIX compliance

Understanding the Differences Between Shells

Understanding Bashisms

Using Portable Tests

Making Portable Arrays

Understanding Portability Problems with echo

Testing Scripts for POSIX Compliance

Creating Scripts on a POSIX-compliant Shell

Using checkbashisms

Using shellcheck

Specifying a Shell with the -s Option

Hands-on Lab – Using -s to Scan Function Libraries

Using shall

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Shell Script Security

Technical Requirements

Controlling Access to Your Scripts

Assigning sudo Privileges

Hands-on Lab ­– Configuring sudo

Using an Access Control List

Hands-on Lab – Setting an ACL for Horatio on Linux

Hands-on Lab – Setting an ACL for Horatio on FreeBSD 14

Hands-on Lab – Setting an ACL for Horatio on OpenIndiana

Obfuscating Plain-Text Scripts

Installing shc

Hands-on Lab – Using shc

Hands-on Lab – Creating Untraceable Executables

Decrypting shc Binaries

Understanding SUID and SGID Considerations

Avoiding Sensitive Data Leakage

Securing Temporary Files

Understanding the /tmp/ Directory

The Wrong Way to Create Temporary Files

The Right Way to Create Temporary Files

Using Passwords in Shell Scripts

Hands-on Lab – Encrypting Passwords

Understanding Command Injection with eval

Using eval on the Command-line

Using eval Safely

Using eval Dangerously

Using Alternatives to eval

Using Command Substitution

Evaluating if eval is Necessary

Understanding Path Security

Attack Scenario 1: Compromising the User’s Account

Attack Scenario 2: Social Engineering

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Debugging Shell Scripts

Technical Requirements

Understanding Common Scripting Errors

Not Enough Quoting

Filenames with Blank Spaces

Problems with Unset Variables

Creating a Wild Loop

Using Shell Script Debugging Tools and Techniques

Using echo Statements

Using xtrace for Debugging

Checking for Undefined Variables

Checking for Errors with the -e Option

Understanding the Problems with set -e and -e

Using bash Debugger

Installing bashdb on Linux

Installing bashdb on FreeBSD

Installing on macOS

Debugging a Script with bashdb

Getting Help with bashdb

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Introduction to Z Shell Scripting

Technical Requirements

Introducing zsh

Installing zsh

Understanding the Unique Features of zsh Scripting

Differences in Variable Expansion

Substituting Values

Substituting Substrings

Translating Between Upper and Lower Case

Extended File Globbing

Understanding zsh Arrays

Enhanced Math Capabilities

Using zsh Modules

Using the mathfunc Module

The datetime Module

Summary

Questions

Further Reading

Answers

Join our community on Discord!

Using PowerShell on Linux

Technical Requirements

Installing PowerShell on Linux and macOS

Installing PowerShell on Linux via a snap Package

Installing PowerShell on Fedora

Installing PowerShell on macOS

Invoking PowerShell

Reasons for Linux and Mac Admins to Learn PowerShell

Working with Mixed Operating System Environments

PowerShell Commands Can Be Simpler

Enhanced Builtin Math Capabilities

Differences Between PowerShell Scripting and Traditional Linux/Unix Scripting

Using Filename Extensions and the Executable Permission

PowerShell is Object-oriented

PowerShell Uses Cmdlets

Using Aliases on PowerShell

Viewing the Available PowerShell Commands

Getting Help with PowerShell Commands

Real-World Cross-Platform PowerShell Scripts

The write-marquee.ps1 Script

The check-cpu.ps1 Script

Summary

Further Reading

Other Books You May Enjoy

Index

Landmarks

Cover

Index

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 most out of this book

Since the book begins with the very basics of Linux and Unix command-line usage, the reader really just needs to be comfortable with the idea of setting up VirtualBox and installing Linux, FreeBSD, and OpenIndiana virtual machines.

VirtualBox is a free download that you can get from here: https://www.virtualbox.org/

To run VirtualBox, you’ll need a machine with a CPU that is capable of virtualization. Most modern CPUs have that capability, with the exception of certain Intel Core i3 and Core i5 models. (That’s because they lack the hardware acceleration that’s required for virtualization.) Also, you’ll have to ensure that virtualization is enabled in your computer’s BIOS.

For the demos, we’ll be using Fedora, Debian, Ubuntu, FreeBSD, and OpenIndiana virtual machines. Here’s where you can download the installation images:

Fedora: https://fedoraproject.org/

Debian: https://www.debian.org/

Ubuntu: https://ubuntu.com/

FreeBSD: https://www.freebsd.org/

OpenIndiana: https://openindiana.org/

In all cases, you’ll need to create a normal user account that has full sudo privileges. That happens automatically with Ubuntu and OpenIndiana during installation. With Debian and Fedora, that will happen automatically if you omit creating a root user password during installation.

For FreeBSD, things are a bit different. That’s because the FreeBSD installer will have you create a password for the root user, and

sudo

won’t be installed. So, here’s the procedure for installing FreeBSD.

When you get to the installer section that has you create your own user account, you’ll see:

Login group is

your_username

. Invite

your_username

into other groups.

Respond by typing

wheel

, in order to add yourself to the

wheel

group.

After the installation has completed, log into the root user account, using the password that you created during installation.

Install the

sudo

package by doing:

pkg install sudo

Configure

sudo

so that members of the wheel group have full

sudo

privileges. Begin by entering the command:

visudo

Scroll down to where you see this line:

# %wheel ALL=(ALL:ALL) ALL

Remove the # and the leading blank space from in front of this line.

Save the file and exit.

Log out from the root user's account, and log back in with your own account.

When you need to perform an administrative command, you can now use

sudo

, as you would on any Linux distro.

Next, you’ll need to install

bash

on FreeBSD.

Since bash doesn’t come installed on FreeBSD by default, you’ll need to install it yourself. Here’s the procedure:

Install bash with this command:

sudo pkg install bash

Create a symbolic line to the bash executable, like this:

sudo ln -s /usr/local/bin/bash /bin/bash

Download the example code files

The code bundle for the book is hosted on GitHub at https://github.com/PacktPublishing/The-Ultimate-Linux-Shell-Scripting-Guide.git. We also have other code bundles from our rich catalog of books and videos available at https://github.com/PacktPublishing/. Check them out!

Download the color images

We also provide a PDF file that has color images of the screenshots/diagrams used in this book. You can download it here: https://packt.link/gbp/9781835463574.

Conventions used

There are a number of text conventions used throughout this book.

CodeInText

: Indicates code words in text, database table names, folder names, filenames, file extensions, pathnames, dummy URLs, user input, and Twitter handles. For example: "Add the new functions to the

/etc/bashrc

file."

donnie@opensuse:~> git clone https://github.com/PacktPublishing/The-Ultimate-Linux-Shell-Scripting-Guide.git

Bold: Indicates a new term, an important word, or words that you see on the screen. For instance, words in menus or dialog boxes appear in the text like this. For example: " First, let’s see how many processes are in either the Running state or the Zombie state."

Warnings or important notes appear like this.

Tips and tricks appear like this.

Get in touch

Feedback from our readers is always welcome.

General feedback: Email

[email protected]

and mention the book’s title in the subject of your message. If you have questions about any aspect of this book, please email us at

[email protected]

.

Errata: Although we have taken every care to ensure the accuracy of our content, mistakes do happen. If you have found a mistake in this book, we would be grateful if you reported this to us. Please visit http://www.packtpub.com/submit-errata, click Submit Errata, and fill in the form.

Piracy: If you come across any illegal copies of our works in any form on the internet, we would be grateful if you would provide us with the location address or website name. Please contact us at

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1

Getting Started with the Shell

Before we can talk about shell scripting, we need to know what a shell is and what kinds of shells are available for Linux, Unix, and Unix-like operating systems. We’ll also talk about other important topics that will help get you started in the wide, wonderful world of shell scripting.

Topics in this chapter include:

Understanding shells

Finding help with shell commands

Using a text editor

Understanding compiled versus interpreted programming

Understanding

root

and

sudo

privileges

If you’re ready, let’s get started on this important journey. And, always remember to have some fun along the way.

Understanding Shells

So, you’re scratching your head and saying, What is a shell, and why should I care? Well, a shell is a program that acts as an intermediary between the user and the operating system kernel. A user types commands into the shell, which passes them into the kernel for processing. The output is then presented to the user via the computer terminal, which can also referred to as the screen. The most common shell on Linux systems is bash, but the Z shell (zsh) has been gaining popularity in recent years. (I’ll explain why in Chapter 22, Using the Z Shell.) You’ll find

bash

as the default shell on most Linux distros and certain Unix-like distros such as OpenIndiana, and

zsh

as the default on Kali Linux.

If you are brand new to the wild, wonderful world of Linux and its Unix or Unix-like cousins, you might be wondering what a distro is. Well, unlike Windows and macOS, which are proprietary and controlled by a single company, Linux and its cousins are primarily open source software, which means that anyone can take the source code and create their own implementations, or distributions. Red Hat Enterprise Linux, Fedora, and Ubuntu are examples of Linux distributions, and OpenIndiana and FreeBSD are examples of Unix-like distributions. But, we hard-core geeks rarely utter the word distribution, and instead just say distro, for short.

Also, the reason that I differentiate between Unix and Unix-like distros has to do with legal reasons that date back to the 1980s. This involves a rather complicated mess that I would rather not go into here. Suffice it to say that the creators of distros such as FreeBSD are not allowed to refer to their creations as Unix, even though they are mostly functionally equivalent. But, they can say that their creations are Unix-like.

The newest versions of macOS also have

zsh

set as the default shell. Fortunately, much of what you’ll learn about

bash

also works on

zsh

. The main difference is that

zsh

has a few cool features that

bash

doesn’t have. (Again, I’ll explain all about that in Chapter 22.) PowerShell, which originally was only available for Microsoft Windows operating systems, has also been available for Linux and macOS since 2016. PowerShell is a whole different animal, but you might find it quite useful, as you should see when we get to Chapter 23, Using PowerShell on Linux.

It’s common to hear people refer to

bash

as the bash shell. But,

bash

is short for Bourne Again Shell. So, when you say bash shell, you’re really saying Bourne Again Shell Shell, which is a bit awkward. This is the same as when people talk about going to the ATM machine to withdraw some money. What they’re really saying is that they’re going to the Automatic Teller Machine Machine, which is also awkward.

And, don’t even get me started on the people who talk about hot water heaters. I mean, if the water is already hot, why heat it?

On the other hand, if you find that you still need to say bash shell so that people will know what you’re talking about, I’ll understand and won’t condemn you for it. In fact, you might even see me do that on occasion.

The coolest thing about modern operating system shells is that they’re much more than just an interface tool. They’re also full-blown programming environments with many of the same programming constructs as more complex programming languages, such as Pascal, C, or Java. Systems administrators can make their jobs much easier by using shell scripts to automate complex, repetitive tasks.

When you log into a text-mode Linux or Unix server, you’ll be presented with a black screen and some text, which looks like this:

Figure 1.1: Plain bash on a text-mode Debian Linux machine

This is the unadorned, plain-jane shell. Machines with desktop environments installed will interface with the shell via a terminal emulator, which will look something like this:

Figure 1.2: A terminal emulator that interfaces with bash on an OpenIndiana machine

The name of the terminal emulator will differ from one desktop environment to the next, but all do the same job. The advantage of using a terminal emulator is that you’ll have the luxury of using scroll bars, customizing the display, and using copy-and-paste for the command-line.

In any case, you can see which shell you’re using by typing:

donnie@fedora:~$ echo $SHELL /bin/bash donnie@fedora:~$

In this case, you see that you’re using

bash

.

Finding Help with Shell Commands

It doesn’t matter how much of an expert you think you are, there will still be times when you’ll need to look up some bit of information. With Linux, Unix, and Unix-like operating systems, there are several options for that.

Understanding Manual Pages

Manual pages, or man pages for short, have been built into Unix-like operating systems since almost forever. To use a man page, just enter

man

, followed by the name of the command, configuration file, or system component for which you seek information. For example, you could find out how to use the

ls

command like this:

man ls

Most of the time, the

man

command will open a man page in the

less

pager. (Some Unix implementations might use the

more

pager instead, but I haven’t found any recent ones that do.) Either way, you’ll be able to scroll through the man page or perform key word searches within the page to find the information that you seek.

The man pages are divided into sections that each correspond to a different category. On most Unix-like and Linux systems, there are eight main categories, more commonly referred to as sections, which are as follows:

Table 1.1: Describing the man page sections

You’ll see the subdirectories that contain these man page files in the

/usr/share/man/

directory. You also might see some subdirectories with names like

man0p

,

man5p

, or

man8x

. These subdirectories contain certain special-purpose man pages, which will differ on different Linux distros.

A lot of times, you won’t need to think about these sections, because the

man

command will pull up the proper man page for you. Other times, you will need to pay attention to these sections, because many key words for which you’ll search can be found in multiple sections. For example, here on the Fedora workstation that I’m using to write this, there are two man pages for

printf

. There are two ways to find them. First, you can use the

man -aw

command, like this:

[donnie@fedora ~]$ man -aw printf /usr/share/man/man1/printf.1.gz /usr/share/man/man3/printf.3.gz [donnie@fedora ~]$

You can also use the

whatis

command, like this:

[donnie@fedora ~]$ whatis printf printf (1) - format and print data printf (3) - formatted output conversion [donnie@fedora ~]$

Note that

whatis

is a synonym for

man -f

. You’ll get the same results with either command, but my own preference is to use

whatis

.

So, we have a

printf

man page in Section 1, which means that we have a normal user command that’s called

printf

. We also see a

printf

man page in Section 3, which means that there’s a library function that’s called

printf

. If you enter

man printf

, you’ll see the man page from Section 1. You’ll see that in the first line of the man page, which will look like this:

PRINTF(1) User Commands PRINTF(1)

If you instead want to see the man page from Section 3, you’ll need to specify that in your command, like this:

man 3 printf

To broaden your search for all man pages that contain

printf

in either the title or the description of the man page, even if it’s embedded into another text string, use either

apropos

or

man -k

, like this:

[donnie@fedora ~]$ apropos printf asprintf (3) - print to allocated string BIO_printf (3ossl) - formatted output to a BIO BIO_snprintf (3ossl) - formatted output to a BIO BIO_vprintf (3ossl) - formatted output to a BIO BIO_vsnprintf (3ossl) - formatted output to a BIO curl_mprintf (3) - formatted output conversion dprintf (3) - formatted output conversion tpm2_print (1) - Prints TPM data structures fprintf (3) - formatted output conversion fwprintf (3) - formatted wide-character output conversion printf (1) - format and print data printf (3) - formatted output conversion . . . [donnie@fedora ~]$

Again, either command will give you the same output, but my own preference has always been to use

apropos

.

Most of the time, your Linux system does a good job of keeping the man page index updated. Once in a while though, you’ll need to do it manually, like this:

[donnie@fedora ~]$ sudo mandb [sudo] password for donnie: Purging old database entries in /usr/share/man... Processing manual pages under /usr/share/man... Purging old database entries in /usr/share/man/ca... Processing manual pages under /usr/share/man/ca... . . . . . . Processing manual pages under /usr/local/share/man... 0 man subdirectories contained newer manual pages. 0 manual pages were added. 0 stray cats were added. 0 old database entries were purged. [donnie@fedora ~]$

Okay, that about does it for the man page system. Let’s talk about the info system.

Understanding Info Pages

The info page system is newer, and was invented by Richard M. Stallman as part of the GNU Project. The unique part about it is that each info page contains hyperlinks that can lead you to additional pages of information. For example, to obtain information about the info system, enter

info info

. This info page contains a menu, which looks something like this:

* Menu: * Stand-alone Info:: What is Info? * Invoking Info:: Options you can pass on the command line. * Cursor Commands:: Commands which move the cursor within a node. . . . ., . . * Variables:: How to change the default behavior of Info. * Colors and Styles:: Customize the colors used by Info. * Custom Key Bindings:: How to define your own key-to-command bindings. * Index:: Global index.

Each underlined item you see is a hyperlink to another page. With your cursor keys, move the cursor to the hyperlink that you want to see, and hit the Enter key. To see an info page for a specific command, such as

ls,

just do this:

info ls

If you need help with navigating through the info pages, just hit the

H

key to bring up a navigation menu.

And, that’s about it for the info pages. Let’s talk about on-line documentation.

Getting to Know the Linux Documentation Project

The Linux Documentation Project has been around since almost forever, and is an invaluable resource. The best part about it is the Guides section, where you’ll find free-of-charge, full-length books about Linux and

bash

that you can download in a variety of formats. They’re all quite old, with the newest one having been last updated in 2014. For the Bash Guide for Beginners book and the Advanced Bash-Scripting book that you’ll find there, that doesn’t matter. The concepts in those two books are eternal, and haven’t really changed over the years. To see these books, go to https://tldp.org/guides.html.

Using Your Favorite Search Engine

If all else fails, just use your favorite search engine to find what you need to know about either scripting in general, or scripting on a particular operating system. You’ll find plenty of help, such as blog posts, YouTube videos, and official documentation. There are plenty of Linux-specific websites that offer help on various things, and it’s quite simple to find them.

Next, let’s talk about text editors.

Using a Text Editor to Create Shell Scripts

To create your shell scripts, you’ll need a text editor that’s designed for Linux and Unix systems. You have plenty of choices, and which one you choose will depend upon several criteria:

Are you editing on a text-mode machine or on a desktop machine?

What features do you need?

What is your own personal preference?

Text-mode Editors

Text-mode text editors can be used on machines that don’t have a graphical user interface installed. The two most common text-mode text editors are

nano

and

vim

. The

nano

editor is installed by default on pretty much every Linux distro, and is quite easy to use. To use it, just type

nano

, followed by the name of the file that you want to either edit or create. At the bottom of the screen, you’ll see the list of available commands. To invoke a command, press the CTRL key, followed by the letter key that corresponds to the desired command.

The downside of using

nano

is that it doesn’t have the full range of features that you might want in a programmers’ text editor. You can see here that the implementation of

nano

on my Fedora workstation has color-coding for the syntax, but it doesn’t automatically format the code.

Figure 1.3: The nano text editor on my Fedora workstation

Note that on other Linux distros,

nano

might not even have color-coding.

My favorite text-mode editor is

vim

, which has features that would make almost any programmer happy. Not only does it have color-coded syntax highlighting, but it also automatically formats your code with proper indentations, as you see here:

Figure 1.4: The vim text editor on my Fedora workstation

In reality, indentation isn’t needed for

bash

scripting, because

bash

scripts work fine without it. However, the indentation does make code easier for humans to read, and having an editor that will apply proper indentation automatically is quite handy. Additionally,

vim

comes with a powerful search-and-replace feature, allows you to split the screen so that you can work on two files at once, and can be customized with a fairly wide selection of plug-ins. Even though it’s a text-mode editor, you can use the right-click menu from your mouse to copy and paste text if you’re remotely logged in to your server from a desktop machine or if you’re editing a local file on your desktop machine.

The older

vi

text editor is normally installed on most Linux distros by default, but

vim

often isn’t. On some distros, the

vim

command will work, even if

vim

isn’t actually installed. That’s because the

vim

command on them might be pointing to either

vim-minimal

or even to the old

vi

. At any rate, to install full-fledged

vim

on any Red Hat-type of distro, such as RHEL, Fedora, AlmaLinux, or Rocky Linux, just do:

sudo dnf install vim-enhanced

To install

vim

on Debian or Ubuntu, do:

sudo apt install vim

As much as I like

vim

, I do have to tell you that some users are a bit put off from using it, because they believe that it’s too hard to learn. That’s because the original version of

vi

was created back in the Stone Age of Computing, before computer keyboards had cursor keys, backspace keys, or delete keys. The old

vi

commands that you used to have to use instead of these keys have been carried over to the modern implementations of

vim

.

So, most

vim

tutorials that you’ll find will still try to teach you all of those old keyboard commands.

Figure 1.5: This photo of me was taken during the Stone Age of Computing, before computer keyboards had cursor keys, backspace keys, or delete keys.

However, on the current versions of

vim

that you’ll install on Linux and modern Unix-like distros such as FreeBSD and OpenIndiana, the cursor keys, backspace key, and delete key all work as they do on any other text editor. So, it’s no longer necessary to learn all of those keyboard commands that you would have had to learn years ago. I mean, you’ll still need to learn a few basic keyboard commands, but not as many as you had to before.

GUI Text Editors

If you’re using a desktop machine, you can still use either

nano

or

vim

if you desire. But, there’s also a wide range of GUI-type editors available if you’d rather use one of them. Some sort of no-frills text editor, such as

gedit

or

leafpad

, is probably already installed on your desktop system. Some slightly fancier programmer’s editors, such as

geany

,

kwrite

, and

bluefish

, are available in the normal repositories of most Linux distros and some Unix-like distros. Your best bet is to play around with different editors to see what you like. Here’s an example of

kwrite

with color-coded syntax highlighting enabled:

Figure 1.6: The Kwrite text editor.

If you’re a Windows user, you’ll never want to create or edit a shell script on your Windows machine with a Windows text editor such as Notepad or Wordpad, and then transfer the script to your Linux machine. That’s because Windows text editors insert an invisible carriage return character at the end of each line. You can’t see them, but your Linux shell can, and will refuse to run the script. Having said that, you might at times encounter scripts that someone else created with a Windows text editor, and you’ll need to know how to fix them so that they’ll run on your Linux or Unix machine. That’s easy to do, and we’ll look at that in Chapter 7, Text Stream Filters-Part 2.

That’s about it for our overview of text editors for Linux. Let’s move on and talk about compiled versus interpreted programming languages.

Understanding Compiled versus Interpreted Programming

Compiled programming consists of writing program code in a text editor, and then using a compiler to convert the text file into an executable binary file. Once that’s done, users of the program won’t be able to easily view the source code of the program. With interpreted programming, the program runs directly from a text file, without having to compile it first.

Compiled programming languages, such as C, C++, or Fortran, are good for when you need maximum performance from your programs. However, they can be fairly hard to learn, especially when it comes to the lower-level functions such as working with files. Interpreted languages might not offer quite as high a level of performance, but they are generally quite flexible, and generally easier to learn. Interpreted languages in general also offer a higher degree of portability between different operating systems. Shell scripting falls into the category of interpreted languages.

Here are some reasons why you might consider using an interpreted language:

When you are looking for a simple solution.

When you need a solution that is portable. If you pay attention to portability concerns, you can write one script that will work on different Linux distros, as well as on Unix/Unix-like systems. That can come in handy if you’re working in a large corporation with a large network of mixed operating systems. (You might even find some larger corporations that are still running legacy Unix systems, such as AIX, HPUX, or SUNOS, alongside more modern implementations of Linux, BSD, or macOS.)

And, here are some reasons why you might consider using a compiled language:

When the tasks require intensive use of system resources. This is especially true when speed is extremely important.

When you are using math operations that require heavy number crunching.

When you need complex applications.

When your application has many sub-components with dependencies.

When you want to create proprietary applications, and prevent users from viewing the application source code.

When you think about it, pretty much every example of productivity, server, gaming, or scientific software falls into one or more of these categories, which means that they really should be built with compiled languages for best performance.

Okay, let’s now talk about

sudo

.

Understanding root and sudo Privileges

Some of the things you’ll do in this course will require you to have administrative privileges. While it’s possible and convenient to just log into the root command prompt, that’s something that I like to discourage as much as possible. For best security, and to get used to what you’d be doing in an enterprise setting, your best bet is to use

sudo

.

Modern Linux distros allow you to add yourself to an administrators’ group as you install the operating system. (That’s the

wheel

group on Red Hat-type systems, and the

sudo

group on Debian/Ubuntu-type systems.) To run a command that requires administrative privileges, just do something like this:

sudo nftables list ruleset

You’ll then be asked to enter the password for your own user account, rather than the one for the root user account.

That’s about all we need to say about this topic, so let’s summarize and move on to the next chapter.

Summary

In this chapter, I’ve laid a bit of the groundwork for what’s to come in the following chapters. We looked at what an operating system shell is, and why we would use one. Then, we looked at the various ways to find help, did a high-level overview of Linux text editors, and wrapped up with a discussion of compiled versus interpreted programming and a brief mention of why we want to use

sudo

to run administrative commands.

In the next chapter, we’ll begin looking at the various things that an operating system shell does for us. I’ll see you there.

Questions

What is the most widely-used shell for Linux systems?

zsh

bash

korn

csh

What will happen if you create a Linux shell script on a Windows computer with a Windows text editor, such as Notepad or Wordpad?

The script will run fine on a Linux machine.

Your Windows machine will just shut down in protest of the fact that you’re using it to create Linux scripts.

The script won’t run on a Linux machine, because Windows text editors insert an invisible carriage return character at the end of each line.

Former Microsoft CEO Steve Ballmer will visit you and explain why Linux is a cancer.

3. In which section would you find the man pages for administrative commands?

1

3

5

6

8

Which of the following statements is true?

Interpreted programming languages are good for programs that perform heavy-duty math problems.

Compiled programming languages are generally better than interpreted languages for any large, complex programs.

Examples of interpreted programming languages include C, C++, and Fortran.

There’s no difference in performance between interpreted and compiled programming languages.

True or False: To run administrative commands, it’s best to just log into the root user account.

Further Reading

22 Best Linux Text Editors for Coding: https://phoenixnap.com/kb/best-linux-text-editors-for-coding

Ballmer: Linux is a Cancer: https://www.theregister.com/2001/06/02/ballmer_linux_is_a_cancer/

Microsoft once called Linux a cancer, and that was a big mistake: https://www.zdnet.com/article/microsoft-once-called-linux-a-cancer-and-that-was-a-big-mistake/

VIM tutorial for Beginners: https://linuxconfig.org/vim-tutorial

Distrowatch.com: https://distrowatch.com/

The Linux Documentation Project: https://tldp.org/

LinuxQuestions.org: https://www.linuxquestions.org/

Linux man pages: https://linux.die.net/man/

Answers

b

c

e

b

False. It’s better to use

sudo

from your own user account.

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2

Interpreting Commands

To fulfill its job as the interface between the user and the operating system kernel, a shell has to perform five different functions. These functions include interpreting commands, setting variables, enabling input/output redirection, enabling pipelines, and allowing customization of a user’s working environment. In this chapter, we’ll look at how

bash

and

zsh

interpret commands. As an added bonus, much of what we’ll cover in the next few chapters will also help you prepare for certain Linux certification exams, such as the Linux Professional Institute or CompTIA Linux+ exams.

Topics in this chapter include:

Understanding the structure of a command

Executing multiple commands at once

Running commands recursively

Understanding the command history

Escaping and quoting

To follow along, you can use pretty much any Linux distro that you desire, as long as it’s running with either

bash

or

zsh

. Your best bet is to use a virtual machine instead of your production workstation, in case you accidentally delete or change something that you shouldn’t.

Understanding the Structure of a Command

A handy thing to know for both real-life and any certification exams that you may take, is the structure of a command. Commands can consist of up to three parts, and there’s a certain order for the parts. Here are the parts and the order in which you’ll normally place them:

The command itself

Command