使用ncatted命令结合bash变量修改netCDF文件time变量units属性的报错解决咨询

I ran into an issue where I needed to update the units attribute of the time variable in a netCDF file (FORCING.nc) using ncatted. The original attribute was hours since 2007-12-28 22:00:00, and I wanted to change it to hours since 1996-01-01 22:00:00—where the target date comes from a bash variable $start_date.

When I hardcoded the date, the command worked perfectly:

ncatted -a units,time,o,c,'hours since 2007-12-28 22:00:00' FORCING.nc

But when I tried using the bash variable, I got this error:

ncatted -a units,time,o,c,'hours since '$start_date FORCING.nc

ncatted: ERROR file start_date not found. It does not exist on the local filesystem, nor does it match remote filename patterns (e.g., http://foo or foo.bar.edu:file).
ncatted: HINT file-not-found errors usually arise from filename typos, incorrect paths, missing files, or capricious gods. Please verify spelling and location of requested file. If the file resides on a High Performance Storage System (HPSS) accessible via the 'hsi' command, then add the --hpss option and re-try command.

Why This Happens #

Bash treats spaces as parameter separators. Since $start_date contains a space (like 1996-01-01 22:00:00), the command 'hours since '$start_date gets split into three separate arguments: 'hours since', 1996-01-01, and 22:00:00. ncatted misinterprets the latter parts as filenames, hence the "file not found" error.

Solutions #

You need to ensure the entire attribute value is passed as a single argument to ncatted. Here are two reliable ways to do this:

1. Wrap the entire value in double quotes
   Double quotes let bash expand variables while keeping the entire string intact:

   ncatted -a units,time,o,c,"hours since $start_date" FORCING.nc
   

2. Quote the variable and concatenate
   If you prefer using single quotes for the static part, you can concatenate it with a quoted variable:

   ncatted -a units,time,o,c,'hours since '"$start_date" FORCING.nc
   

   The single quotes preserve hours since as a literal, and the double quotes ensure $start_date is expanded without splitting on spaces.

Verify the Fix #

First, test the parameter expansion with echo to make sure it's correct:

echo "hours since $start_date"

You should see hours since 1996-01-01 22:00:00 as output.

Then, confirm the change worked with Python:

from netCDF4 import Dataset
with Dataset('FORCING.nc', 'r') as nc_file:
    print(nc_file["time"].units)

The output should show your updated units attribute.

内容的提问来源于stack exchange，提问作者Xu Shan