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

Declaring local variables in functions


Whenever we declare a variable in a script, it is accessible to all functions. The variable is global by default. If the variable is modified by any line of script or any function, it will be modified in global scope. This may create problems in certain situations. We will see this problem in the following script function_12.sh:

#!/bin/bash
name="John"
hello()
{name="Maya"
   echo $name
}
echo $name# name contains John
hello# name contains Maya
echo $name# name contains Maya

Test the script as follows:

$ chmod +x function_12.sh
$ ./function_12.sh

Output:

John
Maya
Maya

To make a variable local, we declare it as follows:

local var=value
local varName

Let's write the script function_13.sh as follows:

#!/bin/bash
name="John"
hello()
{local name="Mary"
echo $name
}
echo $name# name contains John
hello# name contains Mary
echo $name# name contains John

Test the script as follows:

$ chmod +x function_13.sh
$ ./function_13.sh

Output:

John
Mary
John

The command...