Request for explanation of error message

[Paul Smith]

On Mon, 2019-07-29 at 03:19 +0800, Bret Busby wrote:
> And, after having run
> sudo rm /var/log/lightdm/x-0.log
> whilst the file no longer shows as being present, the space has not
> been freed; still no free space in the / partition.

See my other email.  Try logging out and back in.  If that doesn't help
then you'll have to kill the lightdm process by hand.  Or you can try
rebooting: normally that's what I would suggest as the simplest
solution but if your root partition is full then when you reboot it may
(probably not, but may) have a problem.
 
> In going through the folders displayed by caja, in the / partition, I
> note that proc shows as having about 875,000 items, "totalling 140.7TB
> (0 bytes on disk)" (magic?).

/proc is not a real directory: it's exported directly from the kernel. 
It doesn't actually take up any space on any disk partition and its
size is not real.  You should ignore it for the purposes of finding
space usage.

Same with /sys.

If you want to be able to see all files using space you will, of
course, have to be root else you won't be able to view any files and
directories that only root can view (such as the root user's home
directory, /root).