When the reports that MicroSoft would port SQL Server to linux, I had to check my calendar. Was it already 1 April? Remember, that story has appeared countless times over that last couple of decades on that day. Had the usual suspects gotten confused by leap day?
No, it turns out to be true this time. And long overdue, in a way. While I don't have any experience writing the SS engine, I'm willing to guess that it's almost wholly pure C++, that is not many calls to windows apis. It really shouldn't be a huge deal to compile to a linux target. And MicroSoft has had a couple of decades to pick away at the problem.
In the end, MicroSoft isn't likely to dent Oracle much, but it will kill off DB2 on linux. Why? Simple: if you're on linux and want a Big Deal database that isn't Oracle, you had to swallow hard and use DB2/LUW. It is, by the way, a fine RDBMS, treated as a red headed step child by IBM. Now, you can have the familiar SS. Also a fine database, by the way. Windows is horrid, but SS is just fine.