SYSTEMD_UNIT_PATH=foobar: systemd-analyze verify barbar/unit.service will load units from barbar/, foobar/, /etc/systemd/system/, etc. SYSTEMD_UNIT_PATH= systemd-analyze verify barbar/unit.service will load units only from barbar/, which is useful e.g. when testing systemd's own units on a system with an older version of systemd installed.