NAME
        Execute -- Execute a CLI command

    SYNOPSIS
        success = Execute( commandString, input, output )
        D0                 D1             D2     D3

        bool execute(strptr, bptr, bptr)

    FUNCTION
        This function attempts to execute the string commandString as a
        Shell command and arguments. The string can contain any valid input
        that you could type directly in a Shell, including input and output
        redirection using < and >.  Note that Execute() doesn't return until
        the command(s) in commandstring have returned.

        The input file handle will normally be zero, and in this case
        Execute() will perform whatever was requested in the commandString
        and then return. If the input file handle is nonzero then after the
        (possibly empty) commandString is performed subsequent input is read
        from the specified input file handle until end of that file is
        reached.

        In most cases the output file handle must be provided, and is used
        by the Shell commands as their output stream unless output
        redirection was specified. If the output file handle is set to zero
        then the current window, normally specified as *, is used. Note
        that programs running under the Workbench do not normally have a
        current window.

        Execute() may also be used to create a new interactive Shell process
        just like those created with the NewShell command. In order to do
        this you would call Execute() with an empty commandString, and pass
        a file handle relating to a new window as the input file handle.
        The output file handle would be set to zero. The Shell will read
        commands from the new window, and will use the same window for
        output. This new Shell window can only be terminated by using the
        EndCLI command.

        Under V37, if an input filehandle is passed, and it's either
        interactive or a NIL: filehandle, the pr_ConsoleTask of the new
        process will be set to that filehandle's process (the same applies
        to systemtaglist()).

        For this command to work the program Run must be present in C: in
        versions before V36 (except that in 1.3.2 and any later 1.3 versions,
        the system first checks the resident list for Run).

    INPUTS
        commandString - pointer to a null-terminated string
        input         - BCPL pointer to a file handle
        output        - BCPL pointer to a file handle

    RESULTS
        success - BOOLEAN indicating whether Execute was successful
                  in finding and starting the specified program.  Note this
                  is NOT the return code of the command(s).
    SEE ALSO
        systemtaglist(), newshell, endcli, run