systemd.offline-updates(7) — Linux manual page


SYSTEMD....E-UPDATES(7)  systemd.offline-updates SYSTEMD....E-UPDATES(7)

NAME         top

       systemd.offline-updates - Implementation of offline updates in


       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

        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) to, a
           special target that pulls in the base system (i.e.
 , so that all file systems are mounted but
           little else) and the system update units.

        4. The system now continues to boot into, and
           thus into 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
  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

        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 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 won't
           redirect 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 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 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,
           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
  and add a symlink to that
           file under /usr/lib/systemd/ .

SEE ALSO         top

       systemd(1), systemd.generator(7),
       systemd-system-update-generator(8), dnf.plugin.system-upgrade(8)

NOTES         top

        1. GNOME design whiteboard

COLOPHON         top

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systemd 255                                      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)