What are the differences between:

	1. system("ls")
	2. exec("ls")
	3. `ls`

1. system("ls") forks and then execs a shell to execute the string "ls".  
   The return code from the system() call is the exit status of the shell.

2. exec("ls") loads the "ls" program onto the calling process.

3. `ls` (backticks) is similar to system("ls"), except that the return from
   the `ls` is the standard output of the command itself, not the return
   status.
