theshell.ch/site/pages/scripts/while-loop.html

---
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 different needs while programming.<br>

While loops take this form:
{% highlight bash %}
while [condition]
do
    command1
    command2
    command3
    ...
done
{% endhighlight %}
<!-- End of Introduction -->

<!-- First Example -->
Here is a first simple example:
{% highlight bash %}
i=0;

while [$i -lt 4]
do
    echo $i
    i=$((i + 1))
done
{% endhighlight %}

In this first example, you simply create a variable called i and evaluate it to 0.
Then you access the while loop: the condition <code> [$i -lt 4]</code> means that this while
loop will run until the <code> i</code> variable 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 -->


<!-- Infinite Loop First Example -->
Sometimes it is required to declare infinite loops for various programming purposes. <br>
Here is an example:
{% highlight bash %}
i=1;

while :
do
    printf "i=$i\i: Hello World"
    if [ $i == 3 ]; then
        echo "I love DrRacket"
    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
{% endhighlight %}

No termination condition is set for this loop in this example. This type of loop
is called infinite loop.<br>
The <code> exit</code> statement is used to quit the loop. This loop will
iterate for 9 times, then
as soon as <code> i</code> becomes equal to 0, the condition 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 Infinite Loop First Example  -->

<!-- Infinite Loop Second Example  -->
If you want your shell to hang forever doing nothing you can write out the following infinite loop:
{% highlight bash %}
while :
do
    :
done
{% endhighlight %}
<!-- End of Infinite Loop Second Example  -->

<!-- Read  -->
In scripting, while loops are often used to process files line by line. <br>
Here is an example:
{% highlight bash %}
while read -r first_name last_name phone;
do
    printf '%s\n' "$last_name"
done &lt; "$file"
{% endhighlight %}

The <code> read </code> command is used to read a file line by line.
The flag <code> -r</code> is used to tell the
command read to interpret 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> &lt; "$file"</code> redirects the loop's input from a file
whose name is stored in a variable.
This file has 3 columns, <code> first_name last_name phone </code>, separated by
blank space (or a tab).
This piece of code only prints out the second column.
<!-- End of Read -->