gpg trust from command line

[Piotr Bratkowski]

Hello,

I need to invoke trust command but from linux shell. I was thinking that 
this will do:
gpg --edit host_name trust 3

to set my trust marginal for host_name, but it didn't, it took me to the 
gpg command line.

I need this becouse I'm currently writing program that is using gpg. 
It's in C so making it to write to stdin of gpg would be a lot of fuss, 
as a command line I can simply use system function.

So my question is is it possible?? If yes how??

Regards,
Piotr Bratkowski