Running games through Steam's Proton is great. But what if there is a secondary exe or configuration application bundled with the game? How can you launch it if Steam itself only launches the game?
Simply run this command in a terminal:
cd /path/to/steam/steamapps/compatdata/20920/pfx STEAM_COMPAT_DATA_PATH="/path/to/steam/steamapps/compatdata/20920" WINEPREFIX=$PWD \ "$HOME/.steam/root/steamapps/common/Proton 5.0/proton" run ./drive_c/path/to/custom_application.exe
20920 is the game id.
cd (change directory) to the proton prefix in question. If you don't have any custom steam libraries it likely will all be within your home directory. To find it easier you may run
find . | grep compatdata from
~/ and when you find it,
cd to it.
Real world example:
cd /media/michael/SSDGAMES/steam/steamapps/compatdata/20920/pfx STEAM_COMPAT_DATA_PATH="/media/michael/SSDGAMES/steam/steamapps/compatdata/20920" WINEPREFIX=$PWD \ "/home/michael/.steam/root/steamapps/common/Proton 5.0/proton" run \ ./drive_c/flawless/FlawlessWidescreen.exe
Someone in the comments below created a bash script that makes this easier: https://gist.github.com/michaelbutler/f364276f4030c5f449252f2c4d960bd2?permalink_comment_id=3921629#gistcomment-3921629
So... I wrote a little bash script to help me with this problem and it works pretty good. wstart
It's command line but creates dynamic menus to alleviate some of the tedium of typing. Personally, I've been using it for years to run programs within Steam Protons and game prefixes. That said, you can't launch Proton, via Steam, and then another program within that running context. Steam locks the prefix.
Steam does host some non-Steam DRM protected games. I recently used wstart with "Mafia: The City of Lost Heaven" and the "MafiaCon" loader. That works fine (I had to disable winegstreamer to prevent crash on video) using the Steam Proton and prefix without actually launching it from Steam.