This cmd option allows you to pass in a command that will be run within the container. If this value is omitted, then the container's default value will be used. This could be the cmd value, which is supplied in the Dockerfile.

An example within the provision block of your Vagrantfile would be as follows:

dock.run "ubuntu", cmd: "echo $HOME"

In the preceding example, we are referencing the run option and the cmd option. The cmd option will simply run that command within the container. It just accesses the $HOME environment variable, which is the user's home path directory.