Docker: 'Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running?' - how do I fix it on Linux?

Cause: the docker CLI is only a client. It talks to the Docker daemon (dockerd) over a socket. The error means either the daemon isn't running, or the client is pointed at a different daemon (for example a leftover DOCKER_HOST variable) that can't be reached. A related but different error, 'permission denied while trying to connect to the Docker daemon socket', means the daemon is up but your user may not use the socket. Fix (best first): 1. Check whether the daemon is up: docker info, or systemctl is-active docker, or look for a dockerd process. 2. If it's stopped, start it: sudo systemctl start docker. To have it start on boot on systemd distributions other than Debian/Ubuntu (where it is already the default), run sudo systemctl enable docker.service and sudo systemctl enable containerd.service. If it fails to start, read sudo journalctl -u docker for the real reason. A common cause is a systemd override of ExecStart that also leaves hosts set in daemon.json, which makes the daemon refuse to start; use one or the other. 3. Check env | grep DOCKER_HOST. If it's set by mistake, run unset DOCKER_HOST and remove it from ~/.bashrc or ~/.profile. If it's intentional, confirm the remote daemon is running and reachable. 4. For 'permission denied' on the socket: the socket is owned by root by default. Add your user to the docker group (sudo usermod -aG docker $USER), then log out and in again or run newgrp docker. Be aware that docker group membership is effectively root-level access on that host; for stronger isolation consider rootless mode instead. 5. On Docker Desktop (Mac/Windows), start the app and wait until it reports running; there is no systemd unit to manage. Verify: docker info prints both Client and Server sections without an error, and docker run --rm hello-world completes. Sources: https://docs.docker.com/engine/daemon/troubleshoot/ https://docs.docker.com/engine/install/linux-postinstall/