systemd-poweroff.service, systemd-halt.service, systemd-reboot.service, systemd-kexec.service, systemd-shutdown — System shutdown logic
systemd-poweroff.service is a system service that is pulled in by
poweroff.target and is responsible for the actual system power-off
systemd-halt.service is pulled in by
kexec.target to execute the respective actions.
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 (or at least
remount them read-only), 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.
Shortly before executing the actual system power-off/halt/reboot/kexec
systemd-shutdown 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 finished. Note that
these executables are run after all services have been shut down, and after most
mounts have been detached (the root file system as well as
/run/ and various API
file systems are still around though). This means any programs dropped into this directory must be
prepared to run in such a limited execution environment and not rely on external services or hierarchies
/var/ to be around (or writable).
systemd-poweroff.service (and the related units) should never be
executed directly. Instead, trigger system shutdown with a command such as "
Another form of shutdown is provided by the systemd-soft-reboot.service(8) functionality. It reboots only the OS userspace, leaving the kernel, firmware, and hardware as it is.