The value of the logical name DBG$INPUT determines the debugger
    input device. By default, this is SYS$INPUT.

    The value of the logical name DBG$OUTPUT determines the debugger
    output device. By default, this is SYS$OUTPUT.

    If you plan to debug a program that takes its input from a
    file and your debugger input from the terminal, establish the
    following definitions before starting the debugger:

    $ DEFINE SYS$INPUT program-input-file
    $ DEFINE/PROCESS DBG$INPUT 'F$LOGICAL("SYS$COMMAND")

    That is, define DBG$INPUT to point to the translation of
    SYS$COMMAND.

    If you define DBG$INPUT to point to SYS$COMMAND, the debugger
    will try to get its input from the file.