If you run mgv without a file name or with a file name of "-"
then one of two things will happen:
$ mgv
will just start an instance of mgv without a file;
if you run mgv like this:
$ mgv somefile.ps
then mgv will open somefile.ps as the initial file;
if you run mgv like this:
$ mgv < somefile.ps
$ mgv - < somefile.ps
$ cat somefile.ps | mgv
$ cat somefile.ps | mgv -
then mgv will end up reading somefile.ps from the standard input.
If you have asked mgv to copy files from the standard input using the
copyStdin resource, then mgv will
copy the standard input to a temporary file and work from the temporary
file. Note that if a file is read from standard input and not copied,
you will only be able to move to the next page: you won't be able to
jump to an arbitrary page and you won't be able to move backwards.