I can't answer all of the questions Doug asked, but I think I know the
answer to one of them. RENAME never let you rename a file that is in use.
MPE has stayed forward compatible or backward compatible (I'm still not sure
that these are two different things), so that behavior is unlikely to
change. It would break the old hack of trying to rename a file to itself to
see if it is in use or not, and worse. POSIX mv by definition does allow
this, which makes for the neat file shuffling trick. While I kind of prefer
the idea of UDCs calling command files, so the command file can always be
edited, we actually use this on our development box to shuffle XLs (how's
that for scary?) when another developer has one in use. We also use this to
move job streams and command files around, from staging into production (and
from production to the oldest version 'offstage' area), while preserving
create and modify times. RENAME will change the file's modified time.