docker issue-Cannot connect to the Docker daemon. Is 'docker -d' running on this host?

Here is my docker version when i run docker version :

Client:
 Version:      1.8.1
 API version:  1.20
 Go version:   go1.4.2
 Git commit:   d12ea79
 Built:        Thu Aug 13 02:49:29 UTC 2015
 OS/Arch:      darwin/amd64
Cannot connect to the Docker daemon. Is 'docker -d' running on this host?

My os is Centos7
I according to https://docs.docker.com/linux/step_one/ but when I type command:
$ docker run hello-world
Cannot connect to the Docker daemon. Is the docker daemon running on this host?
so I come here to find answer.
At last,I started the service docker,and fix this question
$ sudo service docker start
$ docker run hello-world
Unable to find image 'hello-world:latest' locally
latest: Pulling from library/hello-world
03f4658f8b78: Pull complete

执行 docker images 后就能看到hellow-world镜像了。

执行  sudo service docker status 如果是启动状态，

那么就可以直接启动 docker run hello-world

完美！