# Code - Expansion

Expansion is just a transformation (replacement, …) applied to a syntax.

An expansion is a rule or pattern that specifies how a certain input sequence (often a sequence of characters) should be mapped to a replacement output sequence (also often a sequence of characters) according to a defined procedure.

The mapping process that transforms the input as into the output is known as expansion.

This is often used in template.

## Example

### Expansion of the environment variable

With the following USER environment variable

````export USER=Nico`
```
• The below env file
````USER=\$USER`
```
• would become at runtime
````USER=Nico`
```