exec will never exit

That's not quite true. The manual page says:

       exec PROGRAM LIST
               The "exec" function executes a system command and
               never returns-- use "system" instead of "exec" if
               you want it to return.  It fails and returns false
               only if the command does not exist and it is exe-
               cuted directly instead of via your system's com-
               mand shell (see below).
[download]

It can fail, but that's an exception.

Abigail