Privacy concerns aside, using the boot device serial number might be a better solution as it can't be deleted or modified and survives reboots and reinstalls.

This line will find it easily. Not mine, I simply put together the work of others adding only very small modifications. Needs smartctl (smartmontools package on Debian) which can be run only by root.

smartctl -i `df -P / | tail -n 1 | awk '/.*/ { print $1 }'` | egrep ^"Serial Number:" | awk '{print $3}'