theshell.ch/site/pages/scripts/variables.html

---
layout: page
category-page: scripts
category-title: Scripting
tags: variables, defining, deleting, naming
author: Dario Rasic
title: Script Variables
---
<!-- Intro -->
<p>
A variable is simply a string to which we assign a certain type of data,
which could be a text, a number, a filename and other types of data.
<br>
<!-- How to name a variable - text -->
<h3>Naming a variable</h3>
<!-- Explaination -->
To name a variable in Unix we have to use only letters, numbers or
the underscore character (_).<br>
Other characters can't be used because they have a special meaning in Unix Shell.<br>
<br>
<!-- Examples of naming -->
Some simple examples are:

<pre>
  VAR_1
  VAR_2
  NAME_3
  name_4
</pre>

<br>
<!-- How to define a variable - text -->
<h3>Defining a variable</h3>

To define a certain variable, we could use the following basecase:

<pre>
  variable_name=variable_value
</pre>
<!-- Examples of defining -->
Let me show you a simple example:

<pre>
  VAR_1=Strawberry
</pre>
<!-- How to access the variables - text -->
To access a variable we have to use the dollar sign ($). So if I want to
access VAR_1, I have to write:
<!-- Examples of accessing -->
<pre>
  VAR_1=Strawberry
  echo $VAR_1
</pre>
And shell will give us the following result:
<pre>
  Strawberry
</pre>
<br>
<!-- How to delete a variable - text -->
<h3>Deleting a variable</h3>
<!-- Explaination -->
Deleting a variable means that shell will remove a certain variable from the list of
those that it tracks.<br>
To delete a variable we use the following command:
<!-- Examples of deleting -->
<pre>
  unset variable_name
</pre>

which in our case would be:

<pre>
  unset VAR_1
</pre>
<br>
<!-- How to protect  a  variable - text -->
<h3>Protecting variables</h3>
<!-- Explaination -->
To protect a certain variable, we can set them as read-only so that it can't be
changed or deleted.<br>
So, if we try to change the value of VAR_1, the result will be the following:
<!-- Examples of protection -->
<pre>
  VAR_1=Strawberry
  readonly VAR_1
  VAR_1=Blueberry
</pre>

which will give us:

<pre>
  VAR_1: This variable is read only.
</pre>

If we try to delete the variable, shell will give us the following value:

<pre>
  VAR_1=Strawberry
  unset  VAR_1
  echo $VAR_1
</pre>

As VAR_1 is read-only, shell will not give us any output, as you can't use the
unset command with read-only files.
</p>