While command line flags cannot be specified directly when starting MrEd on the Macintosh, startup files from the Finder (sent to MrEd by drag-and-drop on the MrEd Finder icon or because the user double-clicked on a file with the 'MrEd' creator) can be used to pass command line arguments indirectly.
The method for passing command line flags to MrEd on the Macintosh is a strange extension of an old Unix hack. To pass command line arguments, MrEd must be started with a single startup file. The data fork of this file must begin with a pound sign followed by an exclamation point (``#!''). Starting with the next whitespace, the rest of the first line in the file is used as the command line for MrEd.
There are several special parsing considerations for this ``command line'':
The Macintosh version of MrEd recognizes an additional command line flag at startup time: ``-r ''. This flag must appear before all other flags, and it must be followed by a filename. This resource fork of this file will be opened while MrEd is running. The primary use for the ``-r'' flag is to change the ``splash screen '', the dialog that appears while MrEd is loading. This dialog is read as from the 'DLOG' resource 128. Multiple ``-r'' flags can be provided, but they must precede all other flags in the command line.