Chapter 3. Using Loops and the sleep Command

This chapter shows how to use loops to perform iterative operations. It also shows how to create a delay in a script. The reader will learn how to use loops and the sleep command in a script.

Topics covered in this chapter are as follows:

Standard for, while, and until loops.
Nesting of loops, and how not to get confused.
Introduce the sleep command and how it is used to cause a delay in a script.
Go over a common pitfall of using sleep.

Using loops

One of the most important features of any programming language is the ability to perform a task, or tasks, a number of times and then stop when an ending condition is met. This is accomplished by using a loop.

The next section shows an example of a very simple while loop:

Chapter 3 - Script 1

#!/bin/sh
#
# 5/2/2017
#
echo "script1 - Linux Scripting Book"
x=1
while [ $x -le 10 ]
do
 echo x: $x
 let x++
done

echo "End of script1"

exit 0

And here is the output:

We start by setting variable x to 1. The while statement checks to see if x is less than or equal to 10 and if so, runs the commands between the do and done statements. It will continue to do this until x equals 11, in which case the lines after the done statement are then run.

Run this on your system. It is very important to understand this script so that we can move on to more advanced loops.

Let's look at another script in the next section—see if you can determine what is wrong with it.

Chapter 3 - Script 2

#!/bin/sh
#
# 5/2/2017
#
echo "script2 - Linux Scripting Book"

x=1
while [ $x -ge 0 ]
do
 echo x: $x
 let x++
done

echo "End of script2"

exit 0

Feel free to skip the running of this one unless you really want to. Look carefully at the while test. It says while x is greater than or equal to 0, run the commands inside the loop. Is x ever going to not meet this condition? No, it is not, and this is what is known as an infinite loop. Don't worry; you can still end the script by pressing Ctrl + C (hold down the Ctrl key and press C). This will terminate the script.

I wanted to cover infinite loops right away as you will almost certainly do this from time to time, and I wanted you to know how to terminate the script when it happens. I certainly did this a few times when I was first starting out.

Okay let's do something more useful. Suppose you are starting a new project and need to create some directories on your system. You could do it one command at a time, or use a loop in a script.

We'll a look at this in Script 3.

Chapter 3 - Script 3

#!/bin/sh
#
# 5/2/2017
#
echo "script3 - Linux Scripting Book"

x=1
while [ $x -le 10 ]
do
 echo x=$x
 mkdir chapter$x
 let x++
done
echo "End of script3"

exit 0

This simple script assumes you are starting at the base directory. When run it will create directories chapter 1 through chapter 10 and then proceed to the end.

When running scripts that make changes to your computer, it is a good idea to make sure the logic is correct before running it for real. For example, before running this I commented out the mkdir line. I then ran the script to make sure it stopped after it displayed that x was equal to 10. I then uncommented the line and ran it for real.

Table of Contents for Linux Shell Scripting Bootcamp

Chapter 3. Using Loops and the sleep Command

Using loops

Chapter 3 - Script 1

Chapter 3 - Script 2

Chapter 3 - Script 3

Table of Contents for
Linux Shell Scripting Bootcamp