This session deals with the different categories of OS configuration files as well as of different programs that we install on our Linux machine. Their importance in correct execution of different programs and services is discussed. Then we will start with the design of UNIX who utility. First we will see what who do? Then we will try to understand how who actually do it? Finally we will start writing different versions of the who utility. I will also discuss the concept off buffering done by our application programs and the OS kernel as well.