This is a short python script that tricks Steam for Linux into downloading non-Linux apps.

Note: Steam will not run apps that don't have Linux support, but it will still download the data.

When you tell Steam to download an app, it first checks whether a Linux version exists. If it doesn't exist, it tells you so ("[App] is not available on your current platform.") and doesn't do anything. If it exists, it creates an appmanifest file (which contains game meta-data: name, size on hard-disk, time of last update, etc), and then proceeds to download it.

I found that if the appmanifest file is created manually, Steam will still download the app regardless of platform. There are a minimum of three variables that have to be set in order for this to work. The first is the AppID -- the ID of the app you're trying to download. The second is the Universe. Refer to the Valve Developer Wiki for more info. The last and the most magical one is the StateFlags. Setting this to 1026 tells Steam that an update is required and that the update has been started previously. More info on StateFlags can be found here. (Thanks to strycore for pointing this out.)

You need Python 3 and Python 3 GObject Bindings for the script to run.

• Debian and Ubuntu (and derivatives) don't have these installed by default. The packages are python3 and python3-gi.
• ArchLinux and derivatives can install python and python-gobject.
• Fedora should have everything installed by default.

After you have installed these, download steam-appmanifest.py. Make the file executable ($ chmod +x steam-appmanifest.py) and start it. A dialog should appear. Type in your Steam Community ID in the top textbox and hit Refresh. Make sure your profile is publicly viewable. A list of titles should appear. Install the apps that you want by clicking the checkbox to the left of the Title (and AppID). After you finish, restart Steam.

I created the python script for ease of use. However, it is also possible to create the appmanifest files manually, without the script.

1. Find the AppID of the app you're trying to download. This can be easily done by going on SteamDB and searching for it.
2. Go to ~/.steam/steam/SteamApps or wherever your main SteamApps folder is.
3. Create and open a new file called "appmanifest_APPID.acf", replace APPID with the actual AppID you found in Step 1.
4. Copy and paste the following and replace APPID (the all-caps one) with the one you found in Step 1 and APPNAME with the folder name to download to:

    "AppState"
    {
      "AppID"  "APPID"
      "Universe" "1"
      "installdir" "APPNAME"
      "StateFlags" "1026"
    }

Save and restart Steam.

This method isn't guaranteed to work. I've tested it on several games and they all seemed to download fine except for Civilization V, which just made an empty directory.

Steam won't download apps you don't own.