.NET core doesn't exist anymore. It's now just .NET. It is the successor to the old windows only .NET. It is very much not windows only.
.NET core is .NET nowadays. The old Windows only .NET framework isn't being developed anymore (other than fixes).
> My environment:
> [...]
> - Ubuntu 14.04 x64
> [...]
Mono can run on nix targets but .NET itself (not .NET core) is still very much windows only.