Declaring variables

Just like in any programming language, you can declare variables in bash scripts. So, what are these variables and what are the benefits of using them?

Well, a variable is like a placeholder where you store some value for later use in your code.

There are two kinds of variables you can declare in your script:

• User-defined variables
• Environment variables

User-defined variables

To declare a variable, just type the name you want and set its value using the equals sign (=).

Check out this example:

#!/bin/bash 
name="Mokhtar" 
age=35 
total=16.5 
echo $name  #prints Mokhtar 
echo $age   #prints 35 
echo $total #prints 16.5 

As you can see, to print the variable's value, you should use the dollar sign ($) before it.

Note that there are no spaces between the variable name and the equals sign, or between the equals sign and the value.

If you forget and type a space in between, the shell will treat the variable as if it were a command, and, since there is no such command, it will show an error.

All of the following examples are incorrect declarations:

# Don't declare variables like this: 
name = "Mokhtar" 
age =35 
total= 16.5 

Another useful type of user-defined variable is the array. An array can hold multiple values. So, if you have tens of values you want to use, you should use arrays instead of filling your script with variables.

To declare an array, just enclose its elements between brackets, like this:

#!/bin/bash 
myarr=(one two three four five) 

To access a specific array element, you can specify its index like this:

#!/bin/bash 
myarr=(one two three four five) 
echo ${myarr[1]} #prints two which is the second element 

The index is zero based.

To print the array elements, you can use an asterisk, like this:

#!/bin/bash 
myarr=(one two three four five) 
echo ${myarr[*]} 

To remove a specific element from the array, you can use the unset command:

#!/bin/bash 
myarr=(one two three four five) 
unset myarr[1] #This will remove the second element 
unset myarr    #This will remove all elements 

Environment variables

So far, we have used variables that we didn't define, such as $BASH_VERSION, $HOME, $PATH, and $USER. You might wonder, as we didn't declare these variables, where did they come from?

These variables are defined by the shell for your use and they are called environment variables.

There are many environment variables. If you want to list them, you can use the printenv command.

Also, you can print a specific environment variable by specifying it to the printenv command:

$ printenv HOME

We can use any of these variables in our bash scripts.

Note that all environment variables are written in capital letters, so you can declare your variables as lower case to make it easy to differentiate your variables from environment variables. This is not required, but is preferable.