- Each line represents a key-value pair. Values can optionally be quoted.
  - `VAR=VAL` -> `VAL`
  - `VAR="VAL"` -> `VAL`
  - `VAR='VAL'` -> `VAL`
- Inline comments for unquoted values must be preceded with a space.
  - `VAR=VAL # comment` -> `VAL`
  - `VAR=VAL# not a comment` -> `VAL# not a comment`
- Inline comments for quoted values must follow the closing quote.
  - `VAR="VAL # not a comment"` -> `VAL # not a comment`
  - `VAR="VAL" # comment` -> `VAL`
- Single-quoted (`'`) values are used literally.
  - `VAR='$OTHER'` -> `$OTHER`
  - `VAR='${OTHER}'` -> `${OTHER}`
- Quotes can be escaped with `\`.
  - `VAR='Let\'s go!'` -> `Let's go!`
  - `VAR="{\"hello\": \"json\"}"` -> `{"hello": "json"}`
- Common shell escape sequences including `\n`, `\r`, `\t`, and `\\` are supported in double-quoted values.
  - `VAR="some\tvalue"` -> `some  value`
  - `VAR='some\tvalue'` -> `some\tvalue`
  - `VAR=some\tvalue` -> `some\tvalue`

`VAL` may be omitted, in such cases the variable value is an empty string.
`=VAL` may be omitted, in such cases the variable is unset.

```bash
# Set Rails/Rack environment
RACK_ENV=development
VAR="quoted"
```

### `environment`

{{% include "compose/services-environment.md" %}}

Environment variables can be declared by a single key (no value to equals sign). In this case Compose
relies on you to resolve the value. If the value is not resolved, the variable
is unset and is removed from the service container environment.

Map syntax:

```yml
environment:
  RACK_ENV: development
  SHOW: "true"
  USER_INPUT:
```

Array syntax:

```yml
environment:
  - RACK_ENV=development
  - SHOW=true
  - USER_INPUT
```

When both `env_file` and `environment` are set for a service, values set by `environment` have precedence.

### `expose`

`expose` defines the (incoming) port or a range of ports that Compose exposes from the container. These ports must be
accessible to linked services and should not be published to the host machine. Only the internal container
ports can be specified.

Syntax is `<portnum>/[<proto>]` or `<startport-endport>/[<proto>]` for a port range.
When not explicitly set, `tcp` protocol is used.

```yml
expose:
  - "3000"
  - "8000"
  - "8080-8085/tcp"
```

> [!NOTE]
>
> If the Dockerfile for the image already exposes ports, it is visible to other containers on the network even if `expose` is not set in your Compose file. 

### `extends`

`extends` lets you share common configurations among different files, or even different projects entirely. With `extends` you can define a common set of service options in one place and refer to it from anywhere. You can refer to another Compose file and select a service you want to also use in your own application, with the ability to override some attributes for your own needs.

You can use `extends` on any service together with other configuration keys. The `extends` value must be a mapping
defined with a required `service` and an optional `file` key.

```yaml
extends:
  file: common.yml
  service: webapp
```

- `service`: Defines the name of the service being referenced as a base, for example `web` or `database`.
- `file`: The location of a Compose configuration file defining that service.

#### Restrictions 

When a service is referenced using `extends`, it can declare dependencies on other resources. These dependencies may be explicitly defined through attributes like `volumes`, `networks`, `configs`, `secrets`, `links`, `volumes_from`, or `depends_on`. Alternatively, dependencies can reference another service using the `service:{name}` syntax in namespace declarations such as `ipc`, `pid`, or `network_mode`.

Compose does not automatically import these referenced resources into the extended model. It is your responsibility to ensure all required resources are explicitly declared in the model that relies on extends.

Circular references with `extends` are not supported, Compose returns an error when one is detected.

#### Finding referenced service

`file` value can be: