How to fix Cannot connect to the Docker daemon at unix:///var/run/docker.sock. Is the docker daemon running? in Docker

First, verify the status of the Docker service:

sudo systemctl status docker

If the output shows "inactive (dead)" or "failed", the daemon is not running. On older systems without systemd:

sudo service docker status

If the Docker daemon is not running, start it with:

sudo systemctl start docker

Or on older init systems:

sudo service docker start

Verify it's running:

docker info

To prevent this issue after reboots, enable the Docker service to start automatically:

sudo systemctl enable docker.service
sudo systemctl enable containerd.service

If Docker works with sudo but not without, add your user to the docker group:

sudo usermod -aG docker $USER

Important: You must log out and log back in for the group changes to take effect. Alternatively, run:

newgrp docker

Verify your group membership:

groups

On Windows or macOS, the Docker daemon runs inside Docker Desktop. Start the Docker Desktop application:

- Windows: Search for "Docker Desktop" in the Start menu and launch it
- macOS: Open Docker from Applications or use Spotlight (Cmd + Space, type "Docker")

Wait for the Docker icon in the system tray/menu bar to show "Docker Desktop is running".

If using Docker Desktop with WSL 2 and the error appears in your WSL terminal:

1. Open Docker Desktop settings (click the gear icon)
2. Go to Resources > WSL Integration
3. Ensure "Enable integration with my default WSL distro" is checked
4. Enable integration for your specific distro (e.g., Ubuntu)
5. Click Apply & Restart

Also check Settings > General > "Use the WSL 2 based engine" is enabled.

An incorrectly set DOCKER_HOST variable can cause connection issues:

echo $DOCKER_HOST

If it returns a value (and you're not intentionally using a remote Docker host), unset it:

unset DOCKER_HOST

To make this permanent, check and remove any DOCKER_HOST lines from ~/.bashrc, ~/.zshrc, or ~/.profile.

Sometimes the Docker socket needs to be restarted separately:

sudo systemctl restart docker.socket
sudo systemctl restart docker

This has resolved issues for users on Ubuntu 22.04 and later.

If none of the above solutions work, consider reinstalling Docker:

# Remove existing installation
sudo apt-get purge docker-ce docker-ce-cli containerd.io docker-buildx-plugin docker-compose-plugin

# Remove leftover files
sudo rm -rf /var/lib/docker
sudo rm -rf /var/lib/containerd

# Reinstall following official docs
# https://docs.docker.com/engine/install/

Warning: This will remove all containers, images, and volumes. Back up important data first.