How to recover with a full backup?

[Volker Wysk]

Am Samstag, 9. Dezember 2017, 16:05:39 CET schrieb Xen:
> That's really all you need to do. Linux has no "special files" that are 
> hardcoded on your partition; which means you can just do a *file 
> restore* with the one exception that you need to install grub.

That's what I wanted do know.

> > Could I just replace the top level directories with ones from the
> > backup?
> 
> Better not do this in the *running* system.
> 
> You can do the exact same thing in a live session without interfering 
> with any files.

I've tried to access my root file system from a live session (an USB stick). The live session was plain, without LVM or encryption. The root file system is on encrypted LVM, created by the "Guided - use entire disk and set up encrypted LVM" choice in the "Disk Setup" step of the Kubuntu 16.04.3 installer. I wasn'a able to activate the volume group. Regardless of what I tried - vgchange -a y, vgchange, lvs, vgs, vgdisplay, pvs, pvdisplay - all the commands finished with no output. 

So, again: How to access the root file system of the system which is to be recovered from backup?

Volker