Book Image

Learn Linux Shell Scripting – Fundamentals of Bash 4.4

By : Sebastiaan Tammer
Book Image

Learn Linux Shell Scripting – Fundamentals of Bash 4.4

By: Sebastiaan Tammer

Overview of this book

Shell scripts allow us to program commands in chains and have the system execute them as a scripted event, just like batch files. This book will start with an overview of Linux and Bash shell scripting, and then quickly deep dive into helping you set up your local environment, before introducing you to tools that are used to write shell scripts. The next set of chapters will focus on helping you understand Linux under the hood and what Bash provides the user. Soon, you will have embarked on your journey along the command line. You will now begin writing actual scripts instead of commands, and will be introduced to practical applications for scripts. The final set of chapters will deep dive into the more advanced topics in shell scripting. These advanced topics will take you from simple scripts to reusable, valuable programs that exist in the real world. The final chapter will leave you with some handy tips and tricks and, as regards the most frequently used commands, a cheat sheet containing the most interesting flags and options will also be provided. After completing this book, you should feel confident about starting your own shell scripting projects, no matter how simple or complex the task previously seemed. We aim to teach you how to script and what to consider, to complement the clear-cut patterns that you can use in your daily scripting challenges.
Table of Contents (24 chapters)
Title Page
About Packt
Contributors
Preface
Free Chapter
1
Introduction
Index

Here documents


The final concept we'll introduce in this chapter is the here document. Here documents, also called heredocs, are used to supply input to certain commands, slightly different to stdin redirection. Notably, it is an easy way to give multiline input to a command. It works with the following syntax:

cat << EOF
input
more input
the last input
EOF

If you run this in your Terminal, you'll see the following:

reader@ubuntu:~/scripts/chapter_12$ cat << EOF
> input
> more input
> the last input
> EOF
input
more input
the last input

The << syntax lets Bash know you want to use a heredoc. Right after that, you're supplying a delimiting identifier. This might seem complicated, but it really means that you supply a string that will terminate the input. So, in our example, we supplied the commonly used EOF (short for end of file).

Now, if the heredoc encounters a line in the input that exactly matches the delimiting identifier, it stops listening for further input...