Docker command can't connect to Docker daemon

You need to add your current user to the docker group as follows:

sudo usermod -aG docker $(whoami)

then logout & login again into the system or restart the system. test by docker version

for further info how to install docker-engine follow docker documentation

Add the user to the docker group

• Add the docker group if it doesn't already exist:

  sudo groupadd docker

• Add the connected user "${USER}" to the docker group:

  sudo gpasswd -a ${USER} docker

• Restart the Docker daemon:

  sudo service docker restart

• Either do a newgrp docker or log out/in to activate the changes to groups.

Usually, the following command does the trick:

sudo service docker restart

This, instead of docker start for the cases where Docker seems to already be running.

If that works then, as suggested and in another answer and on this GitHub issue, if you haven't added yourself in the docker group do it by running:

sudo usermod -aG docker <your-username> 

And you're most likely good to go.

As for anybody else bumping into this, in some OS's docker doesn't start right after you install it and, as a result, the same can't connect to daemon message appears. In this case you can first verify that Docker is indeed not running by checking the status of your docker service by executing:

sudo service docker status

If the output looks something like: docker stop/waiting instead of docker start/running, process 15378 then it obviously means Docker is not active. In this case make sure you start it with:

sudo service docker start

And, as before, you'll most likely be good to go.

note to self: I get the error from the question's title when I forget to run docker command with sudo:

sudo docker run ...

^{[Ubuntu 15.10]}

Had the same issue and what worked for me was:
Checking the ownership of /var/run/docker.sock

ls -l /var/run/docker.sock

If you're not the owner then change ownership with the command

sudo chown *your-username* /var/run/docker.sock

Then you can go ahead and try executing the docker commands hassle-free :D