open filename
Opens filename, which should be the name of a GrADS data descriptor file, also called a control file.
.ctl. Provided you adhere to this standard, there is no need to type the extension .ctl when issuing the open command.
open jandata.1966
open jandata.1966.ctl
Both of these commands will have the same effect -- opening the GrADS data descriptor file "jandata.1966.ctl".