dockerコマンドでエラー docker.sock: no such file or directory

docker --version
は動いたけど、
docker ps
でエラー。

#docker ps
Get http:///var/run/docker.sock/v1.19/containers/json: dial unix /var/run/docker.sock: no such file or directory. Are you trying to connect to a TLS-enabled daemon without TLS?

service docker start

で解決。