system-shutdown is the interface to the system-shutdown-daemon.
system-shutdown -h | -p | -r
DIRECTIVE in the envdir at the
shutdown-directive subdirectory of the
system runtime directory.
-h: at the end of the shutdown sequence, halt the system.
-p: at the end of the shutdown sequence, poweroff the system.
-r: at the end of the shutdown sequence, reboot the system.
There is a reason this program exists as part of (multiple) goetia-init packages and not as part of goetia-base. It is that some supported init systems, like s6-linux-init (through goetia-s6-linux-init) provide their own shutdown procedure and interface.