If you use Docker on Mac OS X then it is likely you are using boot2docker.

To autoload boot2docker’s environment variables (which is necessary to connect to your local docker daemon) you need to run boot2docker shellinit. If you add the following line to your ~/.zshrc or ~/.bashrc.

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eval $(boot2docker shellinit 2>/dev/null)

This will initialize the boot2docker environment variables and copy the output to null (so it doesn’t display anything when you open up your terminal.