The UNIX operating system is made up of three parts; the kernel, the shell and the programs. The kernel of UNIX is the (71) of the operating system: it allocates time and memory to programs and handles the filestore and communications in response to (72) .
The shell acts as an (73) between the user and the kernel. When a user logs in, the (74) program checks the username and password, and then starts another program called the shell. The shell is a command line (75) (CLI). It interprets the commands the user types in and arranges for them to be carded out. The commands are themselves programs: when they terminate, the shell gives the user another prompt (% on our systems).
A．system commands
B．system calls
C．system transfer
D．system deliver
