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

Returning information from functions


You have learned to pass command-line parameters to functions. Similarly, the function can return integers as a return value. Normally, functions return either TRUE or FALSE. In certain cases, the function can return integer values, such as 5 or 10, as well.

The syntax is:

return N

When the function calls the command return, the function exits with the value specified by N.

If the function does not call the command return, then the exit status returned is that of the last command executed in the function. If what we need is the status of the last command executed in the function, then we need not return any value from the function. This is illustrated in the following script function_14.sh:

#!/bin/bash
is_user_root() { [ $(id -u) -eq 0 ]; }
is_user_root && echo "You are root user, you can go ahead."\
|| echo "You need to be administrator to run this script"

Test the script as follows:

$ chmod +x function_14.sh
$ ./function_14.sh

If you are a root...