Question: I learned the following command-

`$[variable]-If set, access the variable`

In this command what type of **variable** it is talking about?

Answer: 
You can use variables to store and manipulate numbers or strings. **`bash`** is the standard shell in Ubuntu, and it uses variables like the following examples with a variable with the name `var1`:

Set the variable (give it a value)

```
var1="Hello World"

```

Print the variable to the screen

```
echo "$var1"

```

There are several useful tutorials, that you find if you search the internet, for example with the search string **bash variable tutorial**, for example

[ryanstutorials.net/bash-scripting-tutorial/bash-variables.php](https://ryanstutorials.net/bash-scripting-tutorial/bash-variables.php)

---

The manual `man bash` writes the following about **Arithmetic Expansion**,

> 
> Arithmetic expansion allows the evaluation of an arithmetic expression
>  and the substitution of the result. The format for arithmetic expansion is:
> 
> 
> 
> ```
>           $((expression))
> 
> ```
> 
> The old format `$[expression]` is deprecated and will be removed in upcoming versions of bash.
> 
> 
> 

Evaluating an expression and printing to the screen,

```
$ echo $((7*8))  # recommended
56
$ echo $[6*9]    # works now, but deprecated
54

```
`$[variable]` is referring to a variable that you set with the following command:

`VARIABLE=<path>`

`<path>` can be anything. It can be a path to another drive, a file server, a document, an application, and even a shortcut.