Is docker-machine required on linux?
Docker-machine is a tool that simplifies running docker on VMs, but it's not required in any environment if you want to manage the VM yourself. Since docker runs natively on Linux, there's no need to docker-machine to perform that install, you just install docker directly. This is what most people do with a Linux host.
If you need multiple docker hosts on Linux for some reason, you can create and manage your own VMs, e.g. virtualbox and vagrant, or you can use docker-machine as a front end on that which also handles the machine image and gives a convenient command line to switch the docker CLI between environments.