I have been playing direwolf20's mod following season9. Is it possible to convert my single player worldmap on (Windows10) to run on Debian 9 linux dedicated server for multiplayer?
Assuming the modpack will stay the same, yes. It's actually pretty easy, as you just need to copy over the singleplayer world and specify the world name in the server.properties. This should let it load your singleplayer world map in multiplayer.
