Cannot connect to the doc daemon appears after the doc is installed.

  docker, question

System: ubuntu16.04
I installed it according to docker’s documentation.https://docs.docker.com/linux/step_one/
After the installation is completed, doc’s pspullsearun command will report this error.Cannot connect to the Docker daemon. Is the docker daemon running on this host?

See on the document let me checkDOCKER_HOSTThis environmental variable, however, I do not know where to check it
图片描述

I have made similar mistakes.
Because, I forgot to start the docker service.
service docker start