"System restart required" flag, who sets it and why? How to read it? [duplicate]

Solution 1:

Short version:

cat /var/run/reboot-required.pkgs

Explanation:

Looks like there is an easy way to automatically extract the requested information.

Inside .deb files there are control files for installation, including postinst (run after installation).

For example, in linux-image-2.6.35-25-generic_2.6.35-25.44_amd64.deb,
postinst includes

my $notifier          = "/usr/share/update-notifier/notify-reboot-required";

my $warn_reboot     = 'Yes';     # Warn that we are installing a version of
                                 # the kernel we are running

and

# Warn of a reboot
if (-x $notifier) {
 system($notifier);
}

The shell script /usr/share/update-notifier/notify-reboot-required updates
/var/run/reboot-required and /var/run/reboot-required.pkgs.

The latter file contains a list of packages requesting a reboot.

Solution 2:

Reboot is recommended by the unattended-upgrades when it sees that a /var/run/reboot-required exists. The file is created by postinst (post-installation) scripts in some packages, it looks something like this:

[ -x /usr/share/update-notifier/notify-reboot-required ] && \
/usr/share/update-notifier/notify-reboot-required || true

If you want to see which packages triggered this, you can have a look at the contents of the /var/run/reboot-required.pkgs file.

For more info also see this thread.