---
description: Declare default environment variables in a file
keywords: fig, composition, compose, docker, orchestration, environment, env file
title: Declare default environment variables in file
---

Compose supports declaring environment variables in an environment file.

## Syntax
The following syntax rules apply to environment files:

- Each line represents a key-value pair. Values can optionally be quoted.
  - `VAR=VAL` -> `VAL`
  - `VAR="VAL"` -> `VAL`
  - `VAR='VAL'` -> `VAL`
- Lines beginning with `#` are processed as comments and ignored.
- 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`
- Blank lines are ignored.
- Unquoted and double-quoted (`"`) values have [parameter expansion](#parameter-expansion) applied.
- 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`

### Parameter Expansion
Compose supports parameter expansion in environment files.
Parameter expansion is applied for unquoted and double-quoted values.
Both braced (`${VAR}`) and unbraced (`$VAR`) expressions are supported.

For braced expressions, the following formats are supported:
- Direct substitution
  - `${VAR}` -> value of `VAR`
- Default value
  - `${VAR:-default}` -> value of `VAR` if set and non-empty, otherwise `default`
  - `${VAR-default}` -> value of `VAR` if set, otherwise `default`
- Required value
  - `${VAR:?error}` -> value of `VAR` if set and non-empty, otherwise exit with error
  - `${VAR?error}` -> value of `VAR` if set, otherwise exit with error
- Alternative value
  - `${VAR:+replacement}` -> `replacement` if `VAR` is set and non-empty, otherwise empty
  - `${VAR+replacement}` -> `replacement` if `VAR` is set, otherwise empty

## Precedence
Environment variables from an environment file have lower precedence than those passed via the command-line or via the `environment` section in project YAML.
Refer to [Environment Variables Precedence](./envvars-precedence.md) for details.

## Compose file and CLI variables

The environment variables you define here are used for
[variable substitution](compose-file/compose-file-v3.md#variable-substitution)
in your Compose file, and can also be used to define the following
[CLI variables](reference/envvars.md):

- `COMPOSE_API_VERSION`
- `COMPOSE_CONVERT_WINDOWS_PATHS`
- `COMPOSE_FILE`
- `COMPOSE_HTTP_TIMEOUT`
- `COMPOSE_PROFILES`
- `COMPOSE_PROJECT_NAME`
- `COMPOSE_TLS_VERSION`
- `DOCKER_CERT_PATH`
- `DOCKER_HOST`
- `DOCKER_TLS_VERIFY`

> **Notes**
>
> * Values present in the environment at runtime always override those defined
>   inside the `.env` file. Similarly, values passed via command-line arguments
>   take precedence as well.
> * Environment variables defined in the `.env` file are not automatically
>   visible inside containers. To set container-applicable environment variables,
>   follow the guidelines in the topic
>   [Environment variables in Compose](environment-variables.md), which
>   describes how to pass shell environment variables through to containers,
>   define environment variables in Compose files, and more.

## More Compose documentation

- [User guide](index.md)
- [Installing Compose](install/index.md)
- [Getting Started](gettingstarted.md)
- [Command line reference](reference/index.md)
- [Compose file reference](compose-file/index.md)
- [Sample apps with Compose](samples-for-compose.md)