Difference between single and double quotes for parallel?

[Maciej Pilichowski]

Hi,

  Ole, thank you for fixing the issue with white space, I will check it 
as soon it will be released (other issues -- during weekend), but now I 
have another problem -- related to it.

  In order to preserve the space within some argument, normally you 
would enclose it in double quotes.

  However, when you use composed commands for parallel you have to put 
those in some quotes -- if your command has already double quotes, you 
cannot enclose the entire line with them. But if you use single quotes, 
the commands will be passed as-is, and I only guess parallel handles 
parsing such argument.

  Ok, I tried single quotes and some of my variables were not resolved, 
let's say:

FN=something
'.... "${FN}" .... '

  Since in documentation to parallel, single and double quotes are just 
used, without going into details, I still have difficulties grasping how 
I _should_ use quotes for parallel.

  Ok, it is already too long post -- in short, could I ask for little 
explanation how to use single and double quotes for parallel + composed 
commands + arguments with (possible) whitespaces?
  Thank you in advance.

Kind regards,