docker-machine/docs/reference/active.md

active

See which machine is "active" (a machine is considered active if the DOCKER_HOST environment variable points to it).

$ docker-machine ls
NAME      ACTIVE   DRIVER         STATE     URL
dev       -        virtualbox     Running   tcp://192.168.99.103:2376
staging   *        digitalocean   Running   tcp://104.236.50.118:2376
$ echo $DOCKER_HOST
tcp://104.236.50.118:2376
$ docker-machine active
staging