mirror of https://github.com/docker/docs.git
65 lines
2.9 KiB
Markdown
65 lines
2.9 KiB
Markdown
---
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description: Understand the accepted syntax for declaring environment variables with an environment file
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keywords: fig, composition, compose, docker, orchestration, environment, env file
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title: Syntax for environment files in Docker Compose
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aliases:
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- /compose/env-file/
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---
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This page provides information on the syntax rules and guidelines when using an `.env` file. It defines the rules for commenting, and explains how values are processed. Additionally, it introduces the concept of interpolation, which allows the use of variables within environment files.
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> **Important**
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>
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> Environment variables from an environment file have lower precedence than those from any other [method](set-environment-variables.md).
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For more information, see [Environment variables precedence](envvars-precedence.md).
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{ .important }
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## Syntax
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The following syntax rules apply to environment files:
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- Lines beginning with `#` are processed as comments and ignored.
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- Blank lines are ignored.
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- Unquoted and double-quoted (`"`) values have [interpolation](#interpolation) applied.
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- Each line represents a key-value pair. Values can optionally be quoted.
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- `VAR=VAL` -> `VAL`
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- `VAR="VAL"` -> `VAL`
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- `VAR='VAL'` -> `VAL`
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- Inline comments for unquoted values must be preceded with a space.
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- `VAR=VAL # comment` -> `VAL`
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- `VAR=VAL# not a comment` -> `VAL# not a comment`
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- Inline comments for quoted values must follow the closing quote.
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- `VAR="VAL # not a comment"` -> `VAL # not a comment`
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- `VAR="VAL" # comment` -> `VAL`
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- Single-quoted (`'`) values are used literally.
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- `VAR='$OTHER'` -> `$OTHER`
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- `VAR='${OTHER}'` -> `${OTHER}`
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- Quotes can be escaped with `\`.
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- `VAR='Let\'s go!'` -> `Let's go!`
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- `VAR="{\"hello\": \"json\"}"` -> `{"hello": "json"}`
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- Common shell escape sequences including `\n`, `\r`, `\t`, and `\\` are supported in double-quoted values.
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- `VAR="some\tvalue"` -> `some value`
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- `VAR='some\tvalue'` -> `some\tvalue`
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- `VAR=some\tvalue` -> `some\tvalue`
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### Interpolation
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Compose supports interpolation in environment files.
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Interpolation is applied for unquoted and double-quoted values.
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Both braced (`${VAR}`) and unbraced (`$VAR`) expressions are supported.
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For braced expressions, the following formats are supported:
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- Direct substitution
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- `${VAR}` -> value of `VAR`
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- Default value
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- `${VAR:-default}` -> value of `VAR` if set and non-empty, otherwise `default`
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- `${VAR-default}` -> value of `VAR` if set, otherwise `default`
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- Required value
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- `${VAR:?error}` -> value of `VAR` if set and non-empty, otherwise exit with error
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- `${VAR?error}` -> value of `VAR` if set, otherwise exit with error
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- Alternative value
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- `${VAR:+replacement}` -> `replacement` if `VAR` is set and non-empty, otherwise empty
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- `${VAR+replacement}` -> `replacement` if `VAR` is set, otherwise empty
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For more information, see [Interpolation](../compose-file/12-interpolation.md) in the Compose Specification.
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