$?      The status returned by the last pipe close, backtick
             (``)  command or system operator.  Note that this is
             the status word returned by the wait() system  call,
             so  the exit value of the subprocess is actually ($?
             >> 8).  $? & 255 gives which  signal,  if  any,  the
             process  died  from,  and  whether  there was a core
             dump.  MS-DOS users  should  note  that  the  MS-DOS
             return values are fudged internally so that $? gives
             Unix-like results.  (Mnemonic:  similar  to  sh  and
             ksh.)