If that program runs into trouble, it dies, but does not affect the affect the kernel or the other programs running on the computer. You communicate with a UNIX computer through a comand program known as a shell . The shell interprets comands that you type on the keyboard. There are actually many different shells available for UNIX computers, and on some systems you can choose the shell in which you wish to work. UNIX commands are short and cryptic like vi or rm .
Every command has a host of modifiers which are generally single letters preceded by a hyphen.
Capital letters have different functions than small letters, often completely unrelated. A comand also generally requires an argument, meaning some file on which it will act. You type Control characters by holding down the key while also pressing the speicified character.