Deleting and adding attributes and variables with NCO
Here are some more common tasks I’ve come across when needing to edit netCDF files. This is usually when they need to be ingested into different models or post-processing scripts that require the netCDF files to be in a certain format.
Deleting a global attribute
You want to delete a single global attribute from a netCDF file.
This can be done using ncatted, e.g.:
ncatted -a global_attr_name,d,, infile.nc outfile.nc
This command takes for arguments separated by commas. Since we are specifying deletion, (d), only the first two arguments are needed, but the remaining commas bust be typed in.
Convert a variable type
A variable is of incorrect type and you need to change it. You can use ncap2 (nc arithmetic processing).
ncap2 -s 'time=float(time)'
Assumes you already have the variable defined. The -s option specifies that we are providing an inline script, within the quote marks.
Add a variable mapped over a certain dimension
You want to a variable that iterates over a given dimension, such as time. The variable should increase montonically (i.e. increase by n each time until the end of the dimension length is reached. I often find I need to do this after having merged netCDF files that were single time slices from a model output or satellite data or otherwise. ncap2 is used.
ncap2 -s 'time[$time]=array(54760,30,$time)' infile.nc outfile.nc
We are assigning the current time variable (assuming we have already added this) an array of values, specified by the (start_point, step, dimension). In this case, we get an array of values starting at 54760, increasing by 30 each point, as long as the time dimension. The -s option simply means we are giving an inline script as the input to the ncap2 program.
Add an attribute one at a time
You want to an attribute to a variable. (I.e. metadata attributes for variables, such as units, etc.). We can use ncatted for this. (netCDF attribute editor).
ncatted -a attribute,variable,a,c,"Atrribute Value" infile.nc
The -a option specifies append mode, and so we only need to supply the input file infile.nc. The value of the attribute is given in the quotation marks. The nco documentation suggested also putting single quotation marks around the comma-separated arguments as well, but I found this produced unexpected results where the double quotes were escaped and inserted into the actual attribute value as well. Could possibly be a unix thing though…
