exec command

    (A) Function
        Forking a child process, and making a shell start up. 

    (B) Format

        exec  [ <command> ]

    (C) Parameter
       <command>
         This command must consist of one token character string for 
         the execution of a shell. 
         If <command> is specified, a shell executes it.
         If <command> is not specified, a shell starts up and waits for a command. 
         SHELL environment variables determine what shell starts up. 

    (D) Notes
        The stdin, stdout and stderr of child processes are set to the stdin,
        stdout and stderr of SECONDS, respectively, and cannot be changed with 
        the listen/speak/claim commands. 
        Because <command> is executed in a child process, you cannot change 
        any working directory or environment variable in the SECONDS processes.
        To change these directories and variables, use the dir/setenv commands. 

    (E) Related items
        dir command
        ldir command
        setenv command