docker-docs/machine/reference/active.md

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Identify active machines	machine, active, subcommand	docker-machine active	true

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://203.0.113.81:2376

$ echo $DOCKER_HOST
tcp://203.0.113.81:2376

$ docker-machine active
staging