Steam – Can you play Half-Life 2 without Steam

I don't believe there is. Terraria is solely run from Steam, and the only possible way you could run it outside of Steam was if it was Pirated (which is illegal.) You can still play it if you're not connected to the internet, however. You simply run Steam in offline mode.

According to this article:

By default the Half Life 2 engine looks for autoexec.cfg at startup, and if it finds it, it will automatically load and execute all the valid commands in it along with those in config.cfg while starting up Half Life 2.

Additionally, you can create any other .cfg files you need, name them as you wish (as long as they end in .cfg), such as mytweaks.cfg, place them in the same directory as config.cfg, and then run the commands they contain from the Half Life 2 console (or from autoexec.cfg) by using the "exec" command. For example typing the command: exec mytweaks.cfg in the console will execute the command variables contained in the mytweaks.cfg file. They will not run automatically at startup however - only autoexec.cfg and config.cfg can do that.