Book Image

Linux Shell Scripting Cookbook, Second Edition - Second Edition

Book Image

Linux Shell Scripting Cookbook, Second Edition - Second Edition

Overview of this book

The shell remains one of the most powerful tools on a computer system — yet a large number of users are unaware of how much one can accomplish with it. Using a combination of simple commands, we will see how to solve complex problems in day to day computer usage.Linux Shell Scripting Cookbook, Second Edition will take you through useful real-world recipes designed to make your daily life easy when working with the shell. The book shows the reader how to effectively use the shell to accomplish complex tasks with ease.The book discusses basics of using the shell, general commands and proceeds to show the reader how to use them to perform complex tasks with ease.Starting with the basics of the shell, we will learn simple commands with their usages allowing us to perform operations on files of different kind. The book then proceeds to explain text processing, web interaction and concludes with backups, monitoring and other sysadmin tasks.Linux Shell Scripting Cookbook, Second Edition serves as an excellent guide to solving day to day problems using the shell and few powerful commands together to create solutions.
Table of Contents (16 chapters)
Linux Shell Scripting Cookbook
Credits
About the Authors
About the Reviewers
www.PacktPub.com
Preface
Index

Printing lines in the reverse order


This is a very simple recipe. It may not seem very useful, but it can be used to emulate the stack datastructure in Bash. This is something interesting. Let's print the lines of text in a file in reverse order.

Getting ready

A little hack with awk can do the task. However, there is a direct command, tac , to do the same as well. tac is the reverse of cat.

How to do it...

We will first see how to do this with tac.

  1. The tac syntax is as follows:

    tac file1 file2 …
    

    It can also read from stdin, as follows:

    $ seq 5 | tac
    5 
    4 
    3 
    2 
    1
    

    In tac, \n is the line separator. But, we can also specify our own separator by using the -s "separator" option.

  2. We can do it in awk as follows:

    $ seq 9 | \
    awk '{ lifo[NR]=$0 } 
    END{ for(lno=NR;lno>-1;lno--){ print lifo[lno]; } 
    }'
    

    \ in the shell script is used to conveniently break a single line command sequence into multiple lines.

How it works...

The awk script is very simple. We store each of the lines into an associative array...