- 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`

### Substitute with `--env-file`

You can set default values for multiple environment variables, in an `.env` file and then pass the file as an argument in the CLI.

The advantage of this method is that you can store the file anywhere and name it appropriately, for example, 
This file path is relative to the current working directory where the Docker Compose command is executed. Passing the file path is done using the `--env-file` option:

```console
$ docker compose --env-file ./config/.env.dev up
```

#### Additional information 

- This method is useful if you want to temporarily override an `.env` file that is already referenced in your `compose.yaml` file. For example you may have different `.env` files for production ( `.env.prod`) and testing (`.env.test`).
  In the following example, there are two environment files, `.env` and `.env.dev`. Both have different values set for `TAG`. 
  ```console
  $ cat .env
  TAG=v1.5
  $ cat ./config/.env.dev
  TAG=v1.6
  $ cat compose.yaml
  services:
    web:
      image: "webapp:${TAG}"
  ```
  If the `--env-file` is not used in the command line, the `.env` file is loaded by default:
  ```console
  $ docker compose config
  services:
    web:
      image: 'webapp:v1.5'
  ```
  Passing the `--env-file` argument overrides the default file path:
  ```console
  $ docker compose --env-file ./config/.env.dev config
  services:
    web:
      image: 'webapp:v1.6'
  ```
  When an invalid file path is being passed as an `--env-file` argument, Compose returns an error:
  ```console
  $ docker compose --env-file ./doesnotexist/.env.dev  config
  ERROR: Couldn't find env file: /home/user/./doesnotexist/.env.dev
  ```
- You can use multiple `--env-file` options to specify multiple environment files, and Docker Compose reads them in order. Later files can override variables from earlier files.
  ```console
  $ docker compose --env-file .env --env-file .env.override up
  ```
- You can override specific environment variables from the command line when starting containers.