Book Image

Linux Shell Scripting Bootcamp

By : James K Lewis
Book Image

Linux Shell Scripting Bootcamp

By: James K Lewis

Overview of this book

Linux Shell Scripting Bootcamp is all about learning the essentials of script creation, validating parameters, and checking for the existence of files and other items needed by the script. We will use scripts to explore iterative operations using loops and learn different types of loop statements, with their differences. Along with this, we will also create a numbered backup script for backup files. Further, you will get well-versed with how variables work on a Linux system and how they relate to scripts. You’ll also learn how to create and call subroutines in a script and create interactive scripts. The most important archive commands, zip and tar, are also discussed for performing backups. Later, you will dive deeper by understanding the use of wget and curl scripts and the use of checksum and file encryption in further chapters. Finally, you will learn how to debug scripts and scripting best practices that will enable you to write a great code every time! By the end of the book, you will be able to write shell scripts that can dig data from the web and process it efficiently.
Table of Contents (19 chapters)
Linux Shell Scripting Bootcamp
About the Author
About the Reviewer
Customer Feedback

Reading files

Now let's look again at the method the backup scripts in the last chapter used to get the value from a file:

Chapter 7 - Script 2

# 6/2/2017
echo "Chapter 7 - Script 2"

FN=filenum1.txt              # input/output filename
MAXFILES=5                   # maximum number before going back to 1

if [ ! -f $FN ] ; then
  echo 1 > $FN               # create the file if it does not exist

echo -n "Contents of $FN: "
cat $FN                      # display the contents

count=`cat $FN`              # put the output of cat into variable count
echo "Initial value of count from $FN: $count"

let count++
if [ $count -gt $MAXFILES ] ; then

echo "New value of count: $count"
echo $count > $FN

echo -n "New contents of $FN: "
cat $FN

echo "End of script2"
exit 0

Here is the screenshot for Script 2:

We start by setting the FN variable to the name of the file (filenum1.txt). It is displayed by the cat command and then the contents of the file are assigned to the...