Docker-Docs/_includes/content/compose-var-sub.md

Your configuration options can contain environment variables. Compose uses the variable values from the shell environment in which docker-compose is run. For example, suppose the shell contains POSTGRES_VERSION=9.3 and you supply this configuration:

db:
  image: "postgres:${POSTGRES_VERSION}"

When you run docker-compose up with this configuration, Compose looks for the POSTGRES_VERSION environment variable in the shell and substitutes its value in. For this example, Compose resolves the image to postgres:9.3 before running the configuration.

If an environment variable is not set, Compose substitutes with an empty string. In the example above, if POSTGRES_VERSION is not set, the value for the image option is postgres:.

You can set default values for environment variables using a .env file, which Compose automatically looks for. Values set in the shell environment override those set in the .env file.

Note when using docker stack deploy

The .env file feature only works when you use the docker-compose up command and does not work with docker stack deploy. {: .important }

Both $VARIABLE and ${VARIABLE} syntax are supported. Additionally when using the 2.1 file format, it is possible to provide inline default values using typical shell syntax:

• ${VARIABLE:-default} evaluates to default if VARIABLE is unset or empty in the environment.
• ${VARIABLE-default} evaluates to default only if VARIABLE is unset in the environment.

Similarly, the following syntax allows you to specify mandatory variables:

• ${VARIABLE:?err} exits with an error message containing err if VARIABLE is unset or empty in the environment.
• ${VARIABLE?err} exits with an error message containing err if VARIABLE is unset in the environment.

Other extended shell-style features, such as ${VARIABLE/foo/bar}, are not supported.

You can use a $$ (double-dollar sign) when your configuration needs a literal dollar sign. This also prevents Compose from interpolating a value, so a $$ allows you to refer to environment variables that you don't want processed by Compose.

web:
  build: .
  command: "$$VAR_NOT_INTERPOLATED_BY_COMPOSE"

If you forget and use a single dollar sign ($), Compose interprets the value as an environment variable and warns you:

The VAR_NOT_INTERPOLATED_BY_COMPOSE is not set. Substituting an empty string.