In early February Microsoft announced their roadmap for .NET Core 2.1 during the first half of 2018. At the time, a firm date for the first preview was not available, but the team has just released the first preview.
The preview of 2.1 is designed to be installed side-by-side with previous versions of the .NET Core SDK, which allows you to indicate which installed version is to be used with your projects. Once installed, the preview will become the default on your system, but this behavior can be overridden by setting the version string on the 'sdk' property in a project’s global.json. Of course this is still a preview, and so it should not be installed on critical systems.
Two new command line options are part of this preview, to make managing different versions of .NET Core easier. The first, dotnet --list-sdks will provide a listing of all of the .NET Core SDKs installed on your system and their location on disk. The second, dotnet --listruntimes provides a list of all of the installed runtimes and their location on disk. Interestingly, once the preview SDK has been installed, both commands will remain as part of the .NET Core host and will remain even if the preview is uninstalled.
To use the .NET Core 2.1 SDK with Visual Studio 2017, you will need 15.6 Preview 6 or newer. Installation packages are available for macOS, Linux, and Windows platforms. Once installed, the dotnet --version command should list 2.1.300-preview1-008174 if the SDK is properly installed. (Be sure to verify the version of .NET Core versus the version of the .NET CLI Tools.) Full release notes are provided as is the current list of known issues.