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

Understanding "crontab"


If we need to run a specific task repetitively, then the solution is to use crontab.

The syntax of the command is as follows:

$  crontab    –e

This will open a new editor. The following diagram is the syntax to add tasks. The fields to use for repeating tasks at what time are explained here:

Finally, to save the jobs, use the following:

wq        # save and quite crontab job

The following are a few examples of the crontab command:

  • Use the following command to run a script every 5 minutes, every day:

    5 * * * *      $HOME/bin/daily.job >> $HOME/tmp/out  2>&1
    
  • Use the following command to run 5 minutes after midnight every day:

    5 0 * * *      $HOME/bin/daily.job >> $HOME/tmp/out  2>&1
    
  • Use the following command to run at 2.15 P.M. on the first of every month—the output is mailed to Paul:

    15 14 1 * * *     $HOME/bin/monthly
    
  • Use the following command to run at 10 P.M. on weekdays, send the e-mail to :

    0 22 * *  1-5   sendmail...