NCL can be run in either a script mode or interactive mode. Either
mode can be used to explore the contents of a grib file:

INTERACTIVE MODE:

% ncl <return>
  ncl > g = addfile ("/cgd/cas/murphys/Data/ERS40/ers40_u01276.grb", "r") 
  ncl > print (g)                ; print overview of file's contents
  ncl > z = g->Z_GDS0_ISBL_123   ; read a selected variable to memory 
  ncl > printVarSummary(z)       ; print overview of the variable
  ncl > print (z)                ; print values
  ncl > quit



SCRIPT MODE:

begin
;************************************************
; create pointer to file and read in data
;************************************************
  g = addfile("/cgd/cas/murphys/Data/ERS40/ers40_u01276.grb","r")
  print(g)
  [other statements]
end

Output of print(g):

filename:       ers40_u01276                                                 
    path:   ers40_u01276.grb                                                  
       file global attributes:                                               
       dimensions:                                                           
          initial_time0 = 16                                                 
          lv_ISBL1 = 23                                                      
          g0_lat_2 = 73                                                      
          g0_lon_3 = 144                                                     
      variables:                                                             
      float PV_GDS0_ISBL_123 ( initial_time0, lv_ISBL1, g0_lat_2, g0_lon_3 )
         center :       European Center for Medium-Range Weather Forecasts - 
	 Reading                                                             
         long_name :    Potential vorticity                                  
         units :        K m**2 kg**-1 s**-1                                 
         _FillValue :   -999                                                 
         level_indicator :      100 
         grid_number :  255
         parameter_number :     60
         forecast_time :        0

      float Z_GDS0_ISBL_123 ( initial_time0, lv_ISBL1, g0_lat_2, g0_lon_3 )
         center :       European Center for Medium-Range Weather Forecasts - Reading
         long_name :    Geopotential
         units :        m**2 s**-2
         _FillValue :   -999
         level_indicator :      100
         grid_number :  255
         parameter_number :     129
         forecast_time :        0

     [SNIP other variables]