exec [ -cl ] [ -a argv0 ] simple command
       Replace the current shell with an external command  rather  than
       forking.   With  -c  clear the environment; with -l prepend - to
       the argv[0] string of the command executed (to simulate a  login
       shell); with -a argv0 set the argv[0] string of the command exe-
       cuted.  See the section `Precommand Modifiers'.
