Hm, it seems I haven't clearly clarified what I want to do. I'm using preseed file (through "auto url=" option) to install Debian Jessie in a VM. I would like to configure preseed in such way that it accomplishes two things.
1. It downloads a script from github (we figured out that part)
2. Starts the downloaded script when I log ("debian login") for the first time
on the freshly installed system.
I have successfully accomplished 1. (download) but not 2. (autorun).
Thanks for your patience.
