Book Image

Learning Linux Shell Scripting - Second Edition

By : Ganesh Sanjiv Naik
Book Image

Learning Linux Shell Scripting - Second Edition

By: Ganesh Sanjiv Naik

Overview of this book

Linux is the most powerful and universally adopted OS. Shell is a program that gives the user direct interaction with the operating system. Scripts are collections of commands that are stored in a file. The shell reads this file and acts on commands as if they were typed on the keyboard. Learning Linux Shell Scripting covers Bash, GNU Bourne Again Shell, preparing you to work in the exciting world of Linux shell scripting. CentOS is a popular rpm-based stable and secured Linux distribution. Therefore, we have used CentOS distribution instead of Ubuntu distribution. Linux Shell Scripting is independent of Linux distributions, but we have covered both types of distros. We start with an introduction to the Shell environment and basic commands used. Next, we explore process management in Linux OS, real-world essentials such as debugging and perform Shell arithmetic fluently. You'll then take a step ahead and learn new and advanced topics in Shell scripting, such as decision making, starting up a system, and customizing a Linux environment. You will also learn about grep, stream editor, and AWK, which are very powerful text filters and editors. Finally, you'll get to grips with taking backup, using other language scripts in Shell Scripts as well as automating database administration tasks for MySQL and Oracle. By the end of this book, you will be able to confidently use your own shell scripts in the real world.
Table of Contents (17 chapters)

The IFS and loops

The shell has one environment variable, which is named the Internal Field Separator (IFS). This variable indicates how the words are separated on the command line. The IFS variable is, normally or by default, a whitespace (''). The IFS variable is used as a word separator (token) for the for command. In many documents, IFS can be any one of the white spaces, :, |, :, or any other desired character. This will be useful while using commands such as read, set, and for. If we are going to change the default IFS, then it is a good practice to store the original IFS in a variable.

Later on, when we have done our required tasks, then we can assign the original character back to IFS.

In the following for_16.sh script, we are using : as the IFS character:

#/bin/bash 
cities=Delhi:Chennai:Bangaluru:Kolkata 
old_ifs="$IFS"           # Saving original...