This assumes that the installer is already in /Applications, and that the USB drive is called HighSierra
sudo /Applications/Install\ macOS\ High\ Sierra.app/Contents/Resources/createinstallmedia --volume /Volumes/HighSierra --applicationpath /Applications/Install\ macOS\ High\ Sierra.app --nointeraction &&say Done
This device can be used to upgrade existing installations, do clean installations, and various repair/admin tasks.