Question

How do you register a datum grid shift file in the Unix version of FME

Answer

You simply need to edit the "Nad27ToNad83.gdc" file (or whichever *.gdc text file is applicable for your datum shift). They live in the Reproject directory under the <fme install folder>.

These text files include a path to where your grid file is living. For example:
Remember: The order in which the grids are listed are the order in which FME will use them.

If this is working properly, you should see the information logged in the log file on running a translation.

FYI: Using the Workbench Tools > Options > Coordinate System GUI is just a nice way of doing the same editing to this file!