An operating system command is any command that NeXtMidas needs to pass to the underlying operating system for execution. NeXtMidas automatically treats any command it doesn't recognize as an operating system command.
An operating system command can optionally be installed in the dictionary of a NeXtMidas option tree. Doing so allows NeXtMidas to supply default parameter values at run time. If there is an installed NeXtMidas intrinsic, primitive, or macro with the same name as an uninstalled operating system command, it is necessary to use a '$' prefix on the command line to force NeXtMidas to pass the command to the operating system.