login
compile loop.c which does an infinite loop for future testing
execute the loop executable: start one foreground job and one background job
switch job between background and foreground
view history list and execute history command
logout and quit
when there are suspended (stopped) jobs, user can't logout.
the quit command kills all existing jobs and then quits.
simple redirection
multiple redirection operators
use fd number in redirection operation
redirection can appear anywhere in a command
multiple pipes
both built-in commands and executables can be in pipes
pipes and redirection operators combined
