[texhax] Calling an external command from source

[ap]

Hi,

I was wondering if there's any way of executing a system command from  
withing a TeX source. I want to create a LaTeX command that can  
automatically have a given file be processed and then use the result.  
I couldn't figure any way of doing that. Of course, I could  
externally call the system command before calling the compiler  
itself, but for various reasons I would much more prefer this being  
done transparently.

I tried to look a bit at graphics.sty and its DeclareGraphicsRule  
command, which asks for a command as forth argument, put I can't  
understand at which point the command is used and how.

Any ideas or suggestions?

Thanks.