Parameters as Environment Variables

In real life, a container’s inputs and outputs are likely to vary according to the container’s environment. For instance, if you run a web application, it is likely to connect to a database and listen for incoming requests on a given DNS. The database connection details and DNS will have different values on a development machine, on the test server, and the production server.

Reading a value

Whatever the technology you use inside your container, you can access environment variables. For instance, if you set a name environment variable, you may access it with:

Technology	Access
Linux shell	$name
.NET Core	.AddEnvironmentVariables();
Java	System.getenv(“name”)
Node.JS	process.env.name
PHP	.$_ENV[“name”]
Python	os.environ.get(‘name’)

Providing a value

On a real machine, environment variables are set on your system. Inside a container, they can be set from several sources, which make them appropriate for parameterizing your containers.

In order to provide an environment variable’s value at runtime, you simply use the -e name=value parameter on the docker run command.

A special use case is when the system that runs the container has the name environment variable defined, and you want to reuse it, then you can simply use the -e name parameter without specifying a value.

Default value

You may also want to define a default value for an environment variable, in case it isn’t provided when a container is created; this may be done in the Dockerfile file, using the ENV instruction. For instance, the following makes sure that if ...