Linux - Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

Problem

Problem with docker:

[root@localhost ~]# docker ps
Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?

Solution

Fix for this problem:

Execute below command to start docker after you rebooted your linux.

$ systemctl start docker

Practical example:

[root@localhost ~]# systemctl start docker
[root@localhost ~]# docker ps
CONTAINER ID        IMAGE                        COMMAND                CREATED              STATUS              PORTS               NAMES
4c01db0b339c        ubuntu:12.04                 bash                   28 seconds ago       Up 27 seconds       3300-3310/tcp       webapp
[root@localhost ~]#