Portal 2 (Linux)

I had this problem on 64 bit Ubuntu 14.04 and fixed it by doing the following

  Delete SteamApps/common/Portal 2/bin/libstdc++.so.6
  Run the Steam client
  cd "SteamApps/common/Portal 2"
  LD_LIBRARY_PATH=bin ./portal2_linux -game portal2 -windowed

After running it once this way I could start it from the normal library window

I am sorry that I do not remember the attribution of this answer, but you may be able to google it up if you wish.

Quelle: http://superuser.com/questions/943565/portal-2-wont-launch