title: NCO: Extract a variable from NetCDF file using NCO ncks
body: <p>I am trying to extract a variable from a multi-variable netcdf file, by entering the command: </p>
  
  <pre><code>ncks -v ta temp1.nc out.nc
  </code></pre>
  
  <p>However then I look at the out.nc header, all the variables are still there.</p>
  
  <p>The headers of both temp1.nc and out.nc are below:</p>
  
  <pre><code>--- temp1.nc header ---
  dimensions:
          time = UNLIMITED ; // (124 currently)
          lon = 256 ;
          bnds = 2 ;
          lat = 128 ;
          lev = 40 ;
  variables:
          double time(time) ;
                  time:standard_name = "time" ;
                  time:long_name = "time" ;
                  time:units = "days since 1850-1-1" ;
                  time:calendar = "365_day" ;
                  time:axis = "T" ;
          double lon(lon) ;
                  lon:standard_name = "longitude" ;
                  lon:long_name = "longitude" ;
                  lon:units = "degrees_east" ;
                  lon:axis = "X" ;
                  lon:bounds = "lon_bnds" ;
          double lon_bnds(lon, bnds) ;
          double lat(lat) ;
                  lat:standard_name = "latitude" ;
                  lat:long_name = "latitude" ;
                  lat:units = "degrees_north" ;
                  lat:axis = "Y" ;
                  lat:bounds = "lat_bnds" ;
          double lat_bnds(lat, bnds) ;
          double p0 ;
                  p0:long_name = "reference pressure" ;
                  p0:units = "Pa" ;
          double lev(lev) ;
                  lev:standard_name = "atmosphere_hybrid_sigma_pressure_coordinat
                  lev:axis = "Z" ;
                  lev:positive = "down" ;
                  lev:long_name = "hybrid sigma pressure coordinate" ;
                  lev:units = "1" ;
                  lev:formula_terms = "a: a b: b p0: p0 ps: ps" ;
                  lev:bounds = "lev_bnds" ;
                  lev:formula = "p = a*p0 + b*ps" ;
          double lev_bnds(lev, bnds) ;
                  lev_bnds:standard_name = "atmosphere_hybrid_sigma_pressure_coor
                  lev_bnds:units = "1" ;
                  lev_bnds:formula_terms = "a: a_bnds b: b_bnds p0: p0 ps: ps" ;
          double a(lev) ;
                  a:long_name = "vertical coordinate formula term: ap(k)" ;
                  a:units = "Pa" ;
          double b(lev) ;
                  b:long_name = "vertical coordinate formula term: b(k)" ;
                  b:units = "1" ;
          double a_bnds(lev, bnds) ;
                  a_bnds:long_name = "vertical coordinate formula term: ap(k+1/2)
                  a_bnds:units = "Pa" ;
          double b_bnds(lev, bnds) ;
                  b_bnds:long_name = "vertical coordinate formula term: b(k+1/2)"
                  b_bnds:units = "1" ;
          float ps(time, lat, lon) ;
                  ps:standard_name = "surface_air_pressure" ;
                  ps:long_name = "Surface Air Pressure" ;
                  ps:units = "Pa" ;
                  ps:comment = "surface pressure, not mean sea level pressure" ;
                  ps:original_units = "hPa" ;
                  ps:history = "2011-10-17T12:45:37Z altered by CMOR";
                  ps:cell_measures = "area: areacella" ;
          float ta(time, lev, lat, lon) ;
                  ta:standard_name = "air_temperature" ;
                  ta:long_name = "Air Temperature" ;
                  ta:units = "K" ;
                  ta:_FillValue = 1.e+20f ;
                  ta:missing_value = 1.e+20f ;
                  ta:original_name = "T" ;
                  ta:cell_measures = "area: areacella" ;
                  ta:history = "2011-10-17T12:45:37Z altered by CMOR ;
  // global attributes:
  </code></pre>
  
  <hr>
  
  <p>--- out.nc header ---</p>
  
  <pre><code>dimensions:
          lev = 40 ;
          bnds = 2 ;
          lat = 128 ;
          lon = 256 ;
          time = UNLIMITED ; // (124 currently)
  variables:
          double a(lev) ;
                  a:long_name = "vertical coordinate formula term: ap(k)" ;
                  a:units = "Pa" ;
          double a_bnds(lev, bnds) ;
                  a_bnds:long_name = "vertical coordinate formula term: ap(k+1/2)" ;
                  a_bnds:units = "Pa" ;
          double b(lev) ;
                  b:long_name = "vertical coordinate formula term: b(k)" ;
                  b:units = "1" ;
          double b_bnds(lev, bnds) ;
                  b_bnds:long_name = "vertical coordinate formula term: b(k+1/2)" ;
                  b_bnds:units = "1" ;
          double lat(lat) ;
                  lat:standard_name = "latitude" ;
                  lat:long_name = "latitude" ;
                  lat:units = "degrees_north" ;
                  lat:axis = "Y" ;
                  lat:bounds = "lat_bnds" ;
          double lat_bnds(lat, bnds) ;
          double lev(lev) ;
                  lev:standard_name = "atmosphere_hybrid_sigma_pressure_coordinate" ;
                  lev:axis = "Z" ;
                  lev:positive = "down" ;
                  lev:long_name = "hybrid sigma pressure coordinate" ;
                  lev:units = "1" ;
                  lev:formula_terms = "a: a b: b p0: p0 ps: ps" ;
                  lev:bounds = "lev_bnds" ;
                  lev:formula = "p = a*p0 + b*ps" ;
          double lev_bnds(lev, bnds) ;
                  lev_bnds:standard_name = "atmosphere_hybrid_sigma_pressure_coordinate" ;
                  lev_bnds:units = "1" ;
                  lev_bnds:formula_terms = "a: a_bnds b: b_bnds p0: p0 ps: ps" ;
          double lon(lon) ;
                  lon:standard_name = "longitude" ;
                  lon:long_name = "longitude" ;
                  lon:units = "degrees_east" ;
                  lon:axis = "X" ;
                  lon:bounds = "lon_bnds" ;
          double lon_bnds(lon, bnds) ;
          double p0 ;
                  p0:long_name = "reference pressure" ;
                  p0:units = "Pa" ;
          float ps(time, lat, lon) ;
                  ps:standard_name = "surface_air_pressure" ;
                  ps:long_name = "Surface Air Pressure" ;
                  ps:units = "Pa" ;
                  ps:comment = "surface pressure, not mean sea level pressure" ;
                  ps:original_units = "hPa" ;
                  ps:history = "2011-10-17T12:45:37Z altered by CMOR" ;
                  ps:cell_measures = "area: areacella" ;
          float ta(time, lev, lat, lon) ;
                  ta:standard_name = "air_temperature" ;
                  ta:long_name = "Air Temperature" ;
                  ta:units = "K" ;
                  ta:_FillValue = 1.e+20f ;
                  ta:missing_value = 1.e+20f ;
                  ta:original_name = "T" ;
                  ta:cell_measures = "area: areacella" ;
                  ta:history = "2011-10-17T12:45:37Z altered by CMOR" ;
          double time(time) ;
                  time:standard_name = "time" ;
                  time:long_name = "time" ;
                  time:units = "days since 1850-1-1" ;
                  time:calendar = "365_day" ;
                  time:axis = "T" ;
  
  // global attributes:
  </code></pre>
  
  <hr>
  
  <p>I expected to have only ta on out.nc.  I don't get any error message from ncks.</p>
  
  <p>What am I doing wrong here?  I appreciate any help.</p>
  
  <p>Thank you.</p>