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Better variable substitution example 

Signed-off-by: Aanand Prasad <aanand.prasad@gmail.com>
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Aanand Prasad 2016-03-31 15:45:14 +01:00
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@ -1089,21 +1089,24 @@ It's more complicated if you're using particular configuration features:
## Variable substitution ## Variable substitution
Your configuration options can contain environment variables. Compose uses the Your configuration options can contain environment variables. Compose uses the
variable values from the shell environment in which `docker-compose` is run. For variable values from the shell environment in which `docker-compose` is run.
example, suppose the shell contains `POSTGRES_VERSION=9.3` and you supply this For example, suppose the shell contains `EXTERNAL_PORT=8000` and you supply
configuration: this configuration:
db: web:
image: "postgres:${POSTGRES_VERSION}" build: .
ports:
- "${EXTERNAL_PORT}:5000"
When you run `docker-compose up` with this configuration, Compose looks for the When you run `docker-compose up` with this configuration, Compose looks for
`POSTGRES_VERSION` environment variable in the shell and substitutes its value the `EXTERNAL_PORT` environment variable in the shell and substitutes its
in. For this example, Compose resolves the `image` to `postgres:9.3` before value in. In this example, Compose resolves the port mapping to `"8000:5000"`
running the configuration. before creating the `web` container.
If an environment variable is not set, Compose substitutes with an empty 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 string. In the example above, if `EXTERNAL_PORT` is not set, the value for the
the `image` option is `postgres:`. port mapping is `:5000` (which is of course an invalid port mapping, and will
result in an error when attempting to create the container).
Both `$VARIABLE` and `${VARIABLE}` syntax are supported. Extended shell-style Both `$VARIABLE` and `${VARIABLE}` syntax are supported. Extended shell-style
features, such as `${VARIABLE-default}` and `${VARIABLE/foo/bar}`, are not features, such as `${VARIABLE-default}` and `${VARIABLE/foo/bar}`, are not