docs/compose/environment-variables/env-file.md

description	keywords	title	redirect_from
Understand the accepted syntax for declaring environment variables.	fig, composition, compose, docker, orchestration, environment, env file	Use an environment file	/compose/env-file/

{% include compose-eol.md %}

Syntax

The following syntax rules apply to environment files:

• Lines beginning with # are processed as comments and ignored.
• Blank lines are ignored.
• Unquoted and double-quoted (") values have parameter expansion applied.
• 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

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 attribute in the docker-compose.yml file. For more information, see Environment variables precedence.