Unique id for a linux system

system_uuid.sh

#!/bin/bash

cat /var/lib/dbus/machine-id && exit
# Provided by dbus, hence available on all systemd systems.
# Any user can read it and it is persistent accross boots.
# It is unique per installation, and works well in VMs.
# Not all systems (i.e. stage3 gentoo/handbook install)
# have dbus installed by default.

cat /sys/class/dmi/id/product_uuid && exit
# Which should be the same as
#     dmidecode -s system-uuid
# Requires root
# Persistent accross re-installs
# Depends on x86 hardware
# Not available on all kernels, may be module dependent.

# Lennert Poettering has a good analysis
# http://0pointer.de/blog/projects/ids.html

# FreeBSD has
sysctl kern.hostuuid
# Which is persistent accross reboots

What about /etc/machine-id ?

When I wrote this, I don't think systemd was doing that much with machine-id, and the file didn't exist on my system

That isnt true any more.