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NAME | IMPLEMENTING OFFLINE SYSTEM UPDATES | RECOMMENDATIONS | SEE ALSO | NOTES | COLOPHON |
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SYSTEMD....E-UPDATES(7) systemd.offline-updates SYSTEMD....E-UPDATES(7)
systemd.offline-updates - Implementation of offline updates in
systemd
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 logic:
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 or
/etc/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 available
yet.
3. Very early in the new boot systemd-system-update-generator(8)
checks whether /system-update or /etc/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 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 or /etc/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 in parallel.
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 symlink).
7. The update scripts should exit only after the update is
finished. It is expected that the service which performs the
update 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 or
/etc/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 and
/etc/system-update symlink is gone, the generator will not
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
/usr/lib/systemd/system/system-update.target.wants/foobar.service
→ ../foobar.service to your package.
2. Make sure to remove the /system-update and /etc/system-update
symlinks as early as possible in the update script to avoid
reboot loops in case the update fails.
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 org.freedesktop.login1(5) for details
about the logind D-Bus API.
4. The update service should declare DefaultDependencies=no,
Requires=sysinit.target, After=sysinit.target,
After=system-update-pre.target, Before=system-update.target
and explicitly pull in any other services it requires.
5. It may be desirable to always run an auxiliary unit when
booting into offline-updates mode, which itself does not
install updates. To do this create a .service file with
Wants=system-update-pre.target and
Before=system-update-pre.target and add a symlink to that file
under /usr/lib/systemd/system-update.target.wants .
systemd(1), systemd.generator(7),
systemd-system-update-generator(8), dnf.plugin.system-upgrade(8)
1. GNOME design whiteboard
https://wiki.gnome.org/Design/OS/SoftwareUpdates
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
⟨http://www.freedesktop.org/wiki/Software/systemd/#bugreports⟩.
This page was obtained from the project's upstream Git repository
⟨https://github.com/systemd/systemd.git⟩ on 2025-08-11. (At that
time, the date of the most recent commit that was found in the
repository was 2025-08-11.) If you discover any rendering
problems in this HTML version of the page, or you believe there is
a better or more up-to-date source for the page, or you have
corrections or improvements to the information in this COLOPHON
(which is not part of the original manual page), send a mail to
[email protected]
systemd 258~rc2 SYSTEMD....E-UPDATES(7)
Pages that refer to this page: systemd.directives(7), systemd.generator(7), systemd.index(7), systemd.special(7), systemd-system-update-generator(8)
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HTML rendering created 2025-09-06 by Michael Kerrisk, author of The Linux Programming Interface. For details of in-depth Linux/UNIX system programming training courses that I teach, look here. Hosting by jambit GmbH. |
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