---
layout: page
category-page: scripts
category-title: Scripting
tags: loop while do script scripting read
author: Matteo Omenetti
title: While Loop
---
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.
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.
While loops take this form:
{% highlight bash %}
while [condition]
do
command1
command2
command3
...
done
{% endhighlight %}
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 acces the while loop: the condition [$i -lt 4]
means that this while
loop will run until the i
varibale is less than 4.
Every cycle of this loop, you print out the value of variable i with echo $i
and finally you increase its value by 1 with i=$((i + 1))
.
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:
0 1 2 3Sometimes it is required to decleare infinite loops for various programming purposes.
exit
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. 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 WorldIf you want your shell to hang forever doing nothing you can write out the following infinite loop: {% highlight bash %} while : do : done {% endhighlight %} In scripting, while loops are often used to process files line by line.
read
command is used to read a file line by line.
The flag -r
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, < "$file"
redirects the loop's input from a file
whose name is stored in a variable.
This file has 3 colums, first_name last_name phone
, separated by
blank space (or a tab).
This piece of code only prints out the second column.