You can use the following function to get you started. The backupFile argument is the new file name, I believe it will store it in the nsa or nas execution folder.
function ConfigFileBackup(controllerAddr,probeDir,cfgFile,backupFile)
mypds = pds.create()
pds.putString ( mypds, "directory", probeDir )
pds.putString ( mypds, "file", cfgFile )
pds.putInt ( mypds, "buffer_size", 10240000 )
output1,return_status1 = nimbus.request ( controllerAddr, "text_file_get", mypds )
if return_status1 == NIME_OK then
value1 = output1["file_content"]
fname1 = backupFile
file.create ( fname1 )
file.write ( fname1, value1 )
print ("Success")
else
print ("Failed")
pds.delete (mypds)
exit (2)
end
pds.delete (mypds)
end