This discussion assumes that Ferret is already installed on your system. Installation documentation is available separately from node abyss.pmel.noaa.gov. The Ferret program requires access to several files and directories. Unless your system manager has modified the typical setup, this will occur automatically when you execute the above command.
Instructions are inside the file.
The file is self-explanatory. STEP 3 Execute the command below interactively or add it to your .login file. This optional file holds a list of Ferret commands that will be executed immediately each time Ferret is started, permitting Ferret to be tailored to individual needs and styles. Your system manager may have set this variable to include other data areas as well. This is the list of directories searched to locate NetCDF files.