[luatex] First question: Retrieve *family* name of a font / the three main families

[Urs Liska]

Am 21.01.2018 um 21:29 schrieb Ulrike Fischer:
> Am Sat, 20 Jan 2018 14:29:04 -0500 schrieb Br. Samuel Springuel:
>
>> When Urs says he wants the font family, then, he's looking for that
>> primary name which would allow a "dumb" program to select the normal,
>> bold, italic, etc. font varieties from that same family.
> Then he should probably inspect the definition of
> \csname TU/\rmdefault/m/n\endcsname
>
> and extract the font name "Arial" from the output:
>
>> \TU/Arial(0)/m/n=macro:
> ->\<->"Arial:mode=node;script=latn;language=DFLT;+tlig;" .
>

I was just about to test what you had suggested earlier (didn't have 
access to my LaTeX environment over the weekend :-( )

I have a hard time to understand your code examples. As far as I can see 
your original code "shows" the definition of \rmdefault in certain 
variants, and it puts in in the log output. If that's correct how would 
I get it not to print to the console but be stored somewhere I can 
process it further, and how would I process it in a way that LaTeX or 
Lua's objects can tell me the "family"?

Sorry if this sounds confusing, but it is because I *am* confused. I 
admit this kind of TeX is an area where I'm not as up to the tasks as 
with other languages ...

Best
Urs