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
is responsible for the actual system halt. Similarly,
systemd-poweroff.service is pulled in by
kexec.target to execute the respective
When these services are run, they ensure that PID 1 is
replaced by the
/usr/lib/systemd/systemd-shutdown 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
will run all executables in
/usr/lib/systemd/system-shutdown/ 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
systemd-halt.service (and the
related units) should never be executed directly. Instead, trigger
system shutdown with a command such as "
halt" or suchlike.