sudo apt-get install steam

if steam fails to start, open console and start steam.

If the output is something like this:

Running Steam on arch  64-bit
STEAM_RUNTIME is enabled automatically
Installing breakpad exception handler for appid(steam)/version(0_client)
libGL error: unable to load driver: r600_dri.so
libGL error: driver pointer missing
libGL error: failed to load driver: r600
libGL error: unable to load driver: swrast_dri.so
libGL error: failed to load driver: swrast

Hit CTRL+C and to get more debugging output run:

LIBGL_DEBUG=verbose steam

Steam bundles several libraries in case they're not already installed and adds them to the steam directory in the home directory. These libraries may be outdated and incompatible with the drivers.

Deleting the libraries from the home directory will resolve the issue.

Done on mint for ati radeon hd6850:

rm /home/wuff/.local/share/Steam/ubuntu12_32/steam-runtime/i386/lib/i386-linux-gnu/libgcc_s.so.1
rm /home/wuff/.local/share/Steam/ubuntu12_32/steam-runtime/i386/usr/lib/i386-linux-gnu/libstdc++.so.6

running steam then starts updating itself (and re-downloads the libraries), then fails with the libstdc++.so.6 error again.

rm /home/wuff/.local/share/Steam/ubuntu12_32/steam-runtime/i386/usr/lib/i386-linux-gnu/libstdc++.so.6

running steam now starts properly.

Game does not start, create an appid file to be able to launch it directly:

vi ~/.steam/steam/steamapps/common/Unepic/steam_appid.txt

and enter

233980

Now, edit the start script and adjust the binaries that are called. There seems to be a mismatch between the filenames and the entries in the script.

vi ~/.steam/steam/steamapps/common/Unepic/unepic.sh

and replace unepic32steam and unepic64steam with unepic32s and unepic64s respectively.

Now edit the launcher and replace the default

steam steam://rungameid/233980

with (replace username with actual username in the command. Referring to ~/.steam does not work.
<code>
/home/username/.steam/steam/steamapps/common/Unepic/unepic.sh
<code>

Possible alternative is renaming unepic32s and unepic64s into unepic32steam and unepic64steam respectively.