This man page describes how to implement "offline" system updates
with systemd. By "offline" OS updates we mean package installations
and updates that are run with the system booted into a special system
update mode, in order to avoid problems related to conflicts of
libraries and services that are currently running with those on disk.
This document is inspired by this GNOME design whiteboard.
1. The package manager prepares system updates by downloading all
(RPM or DEB or whatever) packages to update off-line in a special
directory /var/lib/system-update (or another directory of the
package/upgrade manager's choice).
2. When the user OK'ed the update, the symlink /system-update is
created that points to /var/lib/system-update (or wherever the
directory with the upgrade files is located) and the system is
rebooted. This symlink is in the root directory, since we need to
check for it very early at boot, at a time where /var is not
3. Very early in the new boot systemd-system-update-generator(8)
checks whether /system-update exists. If so, it (temporarily and
for this boot only) redirects (i.e. symlinks) default.target to
system-update.target, a special target that is pulls in the base
system (i.e. sysinit.target, so that all file systems are
mounted but little else) and the system update units.
4. The system now continues to boot into default.target, and thus
into system-update.target. This target pulls in all system update
units. Only one service should perform an update (see the next
point), and all the other ones should exit cleanly with a
"success" return code and without doing anything. Update services
should be ordered after sysinit.target so that the update starts
after all file systems have been mounted.
5. As the first step, an update service should check if the
/system-update symlink points to the location used by that update
service. In case it does not exist or points to a different
location, the service must exit without error. It is possible for
multiple update services to be installed, and for multiple update
services to be launched in parallel, and only the one that
corresponds to the tool that created the symlink before reboot
should perform any actions. It is unsafe to run multiple updates
6. The update service should now do its job. If applicable and
possible, it should create a file system snapshot, then install
all packages. After completion (regardless whether the update
succeeded or failed) the machine must be rebooted, for example by
calling systemctl reboot. In addition, on failure the script
should revert to the old file system snapshot (without the
7. The upgrade scripts should exit only after the update is
finished. It is expected that the service which performs the
upgrade will cause the machine to reboot after it is done. If the
system-update.target is successfully reached, i.e. all update
services have run, and the /system-update symlink still exists,
it will be removed and the machine rebooted as a safety measure.
8. After a reboot, now that the /system-update symlink is gone, the
generator won't redirect default.target anymore and the system
now boots into the default target again.
1. To make things a bit more robust we recommend hooking the update
script into system-update.target via a .wants/ symlink in the
distribution package, rather than depending on systemctl enable
in the postinst scriptlets of your package. More specifically,
for your update script create a .service file, without [Install]
section, and then add a symlink like
../foobar.service to your package.
2. Make sure to remove the /system-update symlink as early as
possible in the update script to avoid reboot loops in case the
3. Use FailureAction=reboot in the service file for your update
script to ensure that a reboot is automatically triggered if the
update fails. FailureAction= makes sure that the specified unit
is activated if your script exits uncleanly (by non-zero error
code, or signal/coredump). If your script succeeds you should
trigger the reboot in your own code, for example by invoking
logind's Reboot() call or calling systemctl reboot. See loginddbus API for details.
4. The update service should declare DefaultDependencies=false,
Requires=sysinit.target, After=sysinit.target, and explicitly
pull in any other services it requires.
This page is part of the systemd (systemd system and service manager)
project. Information about the project can be found at
⟨http://www.freedesktop.org/wiki/Software/systemd⟩. If you have a bug
report for this manual page, see
page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2017-03-13. If you dis‐
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systemd 233 SYSTEMD.OFFLINE-UPDATES(7)