How to write a command line software?

For a long time I have been very interested in C language to do a command line inside of the player, such as can play watercress music, to achieve the heart, the next song, switch Channel function. But has not known how the software architecture design, recently read the reference article, feeling a lot of harvest. Under this record. I will summarize the knowledge we learned later.

Reference:

1. http://gansteed.github.io/2014/12/12/abstractions-with-c-01/

2. http://gansteed.github.io/2014/12/18/abstractions-with-c-02/
   

How to write a command line software?