How to manage permissions of files owned by user "_unknown" in Mac OS 10.13.4?
I have found that 'chmod' is broken on 10.13.4, but 'cp', 'rm' and 'mv' are not. 

That is, as a sudoer I can:


use 'cp' to copy the file (I now own the copy)
use 'rm' to remove the file with corrupted permissions
use 'mv' to rename my copy as the original
use 'chmod' and 'chown' to adjust ownership and permissions on the file.


It's a bit clumsy, but I can use a simple BASH script to fix one of these files, when I find it.