systemd-halt.service, systemd-poweroff.service, systemd-reboot.service, systemd-kexec.service, systemd-shutdown — System shutdown logic
systemd-halt.service is a
system service that is pulled in by
halt.target and is responsible
for the actual system halt. Similarly,
pulled in by
kexec.target to execute the
When these services are run, they ensure that PID
1 is replaced by the
tool which is then responsible for the actual
shutdown. Before shutting down, this binary will try to
unmount all remaining file systems, disable all
remaining swap devices, detach all remaining storage
devices and kill all remaining processes.
It is necessary to have this code in a separate binary because otherwise rebooting after an upgrade might be broken — the running PID 1 could still depend on libraries which are not available any more, thus keeping the file system busy, which then cannot be re-mounted read-only.
Immediately before executing the actual system
systemd-shutdown will run all
and pass one arguments to them: either
kexec", depending on the chosen
action. All executables in this directory are executed
in parallel, and execution of the action is not
continued before all executables finished.
systemd-halt.service (and the
related units) should never be executed
directly. Instead, trigger system shutdown with a
command such as "
systemctl halt" or