### Example .env File Structure

Source: https://github.com/dotenv-rs/dotenv/blob/master/README.md

Illustrates the basic structure of a .env file, including comments and key-value pairs. This file defines environment variables that can be loaded by the dotenv library.

```sh
# a comment, will be ignored
REDIS_ADDRESS=localhost:6379
MEANING_OF_LIFE=42
```

--------------------------------

### Variable Substitution in .env Files

Source: https://github.com/dotenv-rs/dotenv/blob/master/README.md

Explains how variables can be reused within a .env file using `$VARIABLE` or `${VARIABLE}` syntax. This example covers various scenarios including non-existing values, alphanumeric names, curly braces for complex names, escaping, and the precedence of system environment variables.

```sh
VAR=one
VAR_2=two

# Non-existing values are replaced with an empty string
RESULT=$NOPE #value: '' (empty string)

# All the letters after $ symbol are treated as the variable name to replace
RESULT=$VAR #value: 'one'

# Double quotes do not affect the substitution
RESULT="$VAR" #value: 'one'

# Different syntax, same result 
RESULT=${VAR} #value: 'one'

# Curly braces are useful in cases when we need to use a variable with non-alphanumeric name
RESULT=$VAR_2 #value: 'one_2' since $ with no curly braces stops after first non-alphanumeric symbol 
RESULT=${VAR_2} #value: 'two'

# The replacement can be escaped with either single quotes or a backslash:
RESULT='$VAR' #value: '$VAR'
RESULT=\$VAR #value: '$VAR'

# Environment variables are used in the substutution and always override the local variables
RESULT=$PATH #value: the contents of the $PATH environment variable
PATH="My local variable value"
RESULT=$PATH #value: the contents of the $PATH environment variable, even though the local variable is defined
```

--------------------------------

### Basic Rust Application Using dotenv

Source: https://github.com/dotenv-rs/dotenv/blob/master/README.md

Demonstrates how to integrate `dotenv` into a Rust application. It shows calling `dotenv().ok()` to load variables from a '.env' file and then iterating through all environment variables using `std::env::vars()` to print them.

```rust
extern crate dotenv;

use dotenv::dotenv;
use std::env;

fn main() {
    dotenv().ok();

    for (key, value) in env::vars() {
        println!("{}: {}", key, value);
    }
}
```

--------------------------------

### Using the dotenv! Compile-Time Macro

Source: https://github.com/dotenv-rs/dotenv/blob/master/README.md

Demonstrates how to use the `dotenv!` macro provided by the `dotenv_codegen` crate. This macro allows accessing environment variables defined in a '.env' file at compile time, similar to Rust's built-in `env!` macro.

```rust
#[macro_use]
extern crate dotenv_codegen;
```

```rust
fn main() {
  println!("{}", dotenv!("MEANING_OF_LIFE"));
}
```

=== COMPLETE CONTENT === This response contains all available snippets from this library. No additional content exists. Do not make further requests.