To explicitly indicate that a command should run in the operating system,
precede the command with a dollar sign ($). This avoids command
ambiguity. For example, typing HELP at the NeXtMidas prompt
invokes the NeXtMidas HELP command, but $HELP
invokes the help facility of the operating system.
To indicate that a command should run in the operating system while
staying inside the NeXtMidas shell, use double dollar signs
($$) before the command name. Otherwise, the NeXtMidas
shell temporarially exits while the operating system runs the command.
(Within a graphical shell such as SHELLGUI, this is
the default execution mode for all operating system commands.)