Finally, $? may be set to non-0 value if the external
       program /cdrom/install fails.  Upper bits of the
       particular value may reflect specific error conditions
       encountered by this program (this is program-dependent),
       lower-bits reflect mode of failure (segfault, completion,
       etc.).  Note that in contrast to $@, $!, and $^E, which
       are set only if error condition is detected, the variable
       $? is set on each wait or pipe close, overwriting the old
       value.