Make sure you have the latest version of the client before you follow the instructions.
Although specific instructions are given for Old School RuneScape, a similar procedure can be used with a different directory/folder name with a matching .prm or info.plist file to enable access to different servers. Refer to prm Files for specific settings to be included. Although the file format is slightly different with the info.plist file, most of the same settings are available.
Warning! If you change runescape.prm file to oldschool.prm you'll get a Missing Paramenter error! Don't change the file name just the code inside it.
To return the client back to the default live game, simply follow this guide again, and replace the line <string>http://oldschool.runescape.com/jav_config.ws </string> with <string>http://www.runescape.com/k=3/l=$(Language:0)/jav_config.ws</string>.
NOTE: The Runescape Unix Client is regularly updated, and will probably have support for the old school servers out-of-the-box very soon. Check in your application menu to see if this is the case. If not, continue on.
Note: The & at the end of the command is important- it makes it so the process is launched in the background, allowing you to close the terminal window while playing runescape.