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---
layout: page
category-page: scripts
category:title Scripting
tags: loop while do script scripting read
author: Matteo Omenetti
title: While Loop
---
<!-- Introduction -->
Loops are an important concept in programming and therefore also in scripting. Thanks to loops you are able to repeat an instruction
automatically several times, until a certain condition turns false. <br>
Two are the main types of loops: while and for. They both generate a repeating piece of code, but with some key differences
that make them suitable for some different needs while programming. <br>
While loops take this form:
<pre>
while [condition]
do
command1
command2
command3
...
done
</pre>
<!-- End of Introduction -->
<!-- First Example -->
Here is a first simple example:
<pre>
i=0;
while [$i -lt 4]
do
echo $i
i=$((i + 1))
done
</pre>
In this first example, you simply create a variable called i and evaluate it to 0.
Then you acces the while loop: the condition <code> [$i -lt 4] </code> means while the varibale
i is less than 4. every cycle of this loop, you print out the value of variable i with <code> echo $i </code>
and finally you increase its value by 1 with <code> i=$((i + 1)) </code>.
Therefore in 4 cycles the value of i will be 4. This will make the condition of the while loop false.
The output of this piece of code is:
<pre>
0
1
2
3
</pre>
<!-- End of First Example -->
<!-- Second Example -->
Sometimes it required to decleare infinite loops for various programming purposes. <br>
Here is an example:
<pre>
i=1
while :
do
printf "i=$i\i: Hello World"
if [ $i == 3 ]
then
echo "I love programming"
elif [ $i == 5]
then
echo "I love Bash"
elif [ $i == 7 ]
then
echo "I love this website"
elif [ $i == 9 ]
then
exit 0
i=$((i + 1))
done
</pre>
No termination codinition is set for this loop in this example. This type of loop is called infinite loop.
The <code> exit </code> statement is used to quit the loop. This loop will iterate for 9 times, then
as soon as i becomes equal to 0, the condtition of the last if statement will evaluate to true and the
loop will be terminated. <br>
The output of this piece of code is:
<pre>
1: Hello World
2: Hello World
3: Hello World
I Love programming
4: Hello World
5: Hello World
I love Bash
6: Hello World
7: Hello World
I love this website
8: Hello World
9: Hello World
</pre>
<!-- End of Second Example -->
If you want your shell to hang forever doing nothing you can write out the following infinite loop:
<pre>
while :
do
:
done
</pre>
In scripting, while loops, are often used to process files line by line. <br>
Here is an examaple:
<pre>
while read -r first_name last_name phone;
do
printf '%s\n' "$last_name"
done < "$file"
</pre>
The <code> read </code> command is used to read file line by line. The flag <code> -r </code> is used to tell the
command read to intepret backslashes (/) literally, instead as escape characters. This command, expect for some few
rare cases, should always be used with this flag.
In this example, <code> < "$file" </code> redirects the loop's input from a file whose name is stored in a variable.
This file has 3 colums, <code> first_name last_name phone </code>, separated by blank space (or a tab).
This piece of code only prints out the second column.