This is a simple UNIX command line interpreter
Displays a prompt "#cisfun$ " and waits for user input.
Runs all commands of type "executable program" (ls and /bin/ls).
Runs the following build_in commands: exit, env, setenv and unsetenv.
Handles commands with arguments.
Handles the PATH global variable.
Handles The EOF (End Of File) condition.
Handles the Ctrl + C signal -> It doesn't exit the shell