Do I need to buy the same steam game twice for two different operating systems?

I want to buy Rome Total War 2 before it's released because it's currently 50% off.

However, at the screen, it only shows a Windows symbol for the OS.

If I buy it now, will it only be for Windows? Or can I purchase the game once and use it on multiple operating systems?

Please provide a link that describes this policy.

Here is the announcement that it's coming to linux. Does that help?

If it becomes available for Linux, then you won't have to purchase it a second time; there is no guarantee that it will come to Linux, though, which means you'll only be ably to play it on Windows (or possibly with WINE).

Steam Play is how valve refers to cross platform games. It looks like every cross platform game that is available is a Steam Play game, which means that if you buy it on one platform, you can play the game on any platform that it's available for. But as I said, the game needs to be released for Linux for that to happen.

Steam's preffered set-up is that you buy a game once and you get it for every platform it is available on. When a game gets a new port it is normally just added to the existing game listing and existing owners can just install it.

However ultimately the publisher of the game is the one in control. Some games have Mac and Linux ports but steam only carries the windows version. At least one game has it's MAC version listed on steam as a seperate game that must be purchased seperately.

Given that Valve are promoting this game's Linux port I think it is extremely likely that if you buy the Windows version now then if/when the Linux port is released you will be able to download it at no extra charge but it cannot be gauranteed.