systemd-update-done.service, systemd-update-done - Mark /etc and /var fully
systemd-update-done.service is a service that is invoked as part of the first
boot after the vendor operating system resources in /usr have been updated.
This is useful to implement offline updates of /usr which might require
updates to /etc or /var on the following boot.
systemd-update-done.service updates the file modification time (mtime) of the
stamp files /etc/.updated and /var/.updated to the modification time of the
/usr directory, unless the stamp files are already newer.
Services that shall run after offline upgrades of /usr should order themselves
before systemd-update-done.service, and use the ConditionNeedsUpdate=
) condition to make sure to run when /etc or /var
are older than /usr according to the modification times of the files described
above. This requires that updates to /usr are always followed by an update of
the modification time of /usr, for example by invoking touch(1)