Book Image

Learning Linux Shell Scripting

By : Ganesh Sanjiv Naik
Book Image

Learning Linux Shell Scripting

By: Ganesh Sanjiv Naik

Overview of this book

Linux is the one of the most powerful and universally adopted OSes. 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 can read this file and act on the commands as if they were typed on the keyboard. Shell scripting is used to automate day-to-day administration, and for testing or product development tasks. This book covers Bash, GNU Bourne Again SHell, preparing you to work in the exciting world of Linux shell scripting. We start with an introduction to the Shell environment and explain basic commands used in Shell. Next we move on to check, kill, and control the execution of processes in Linux OS. Further, we teach you about the filter tools available in Linux and explain standard output and standard errors devices. Then we will ensure you understand Shell’s interpretation of commands and get a firmer grasp so you use them in practice. Next, you’ll experience some real-world essentials such as debugging and perform Shell arithmetic fluently. Then you’ll take a step ahead and learn new and advanced topics in Shell scripting, such as starting up a system and customizing a Linux system. Finally, you’ll get to understand the capabilities of scripting and learn about Grep, Stream Editor, and Awk.
Table of Contents (20 chapters)
Learning Linux Shell Scripting
Credits
About the Author
Acknowledgments
About the Reviewers
www.PacktPub.com
Preface
Index

Working with the do while loop


Similar to the for command, while is also the command for loop operations. The command next to while is evaluated. If it is successful or 0 then the commands inside do and done are executed.

The purpose of a loop is to test a certain condition or expression and execute a given command while the condition is true (while loop) or until the condition becomes true (until loop):

while condition

do

commands

done

until condition

do

commands

done

The following is the while_01.sh script, in which we read a file and display its it's content:

#!/bin/bash
file=/etc/resolv.conf
while IFS= read -r line    # IFS : inter field separator
do
   # echo line is stored in $line
  echo $line
done < "$file"

Let's test the program:

$ chmod +x while_01.sh
$ ./while_01.sh

The following will be the output after executing the preceding commands:

nameserver 192.168.168.2
search localdomain

In the following script while_02.sh, we are printing number 1–10 on the screen using the while...