i.vi writing null data to output raster on first attempt

Question

I have the following lines of code in a bash script I wrote using osgeo's GRASS command line tools:

i.vi viname=ndvi output=$ndvi_fn nir=$nir_fn red=$red_fn --overwrite
while r.info $ndvi_fn -r | grep NULL;do
  echo "Failed to write NDVI file, trying again"
  sleep 2
  i.vi viname=ndvi output=$ndvi_fn nir=$nir_fn red=$red_fn --overwrite
done

This will loop indefinitely. If I stop my script, and rerun it, usually the NDVI band is actually written to the file. Sometimes the third try or more (not always on the second attempt). I can not understand for the life of me why restarting the script makes a difference from sleeping and running it again.

The script will only work if I comment out the while loop, and I rerun the script. I have also tried using g.remove type=raster name=$ndvi_fn -f in the while loop to remove and recreate the file every iteration. no luck.

Has anyone else seen erratic behavior with i.vi? I checked the docs.. but it seems like I'm using it correctly.

Answer 1

Please be sure if the computational region been set to the input map(s) prior to the i.vi command. In your case, just use the variable to pass on the raster map name:

g.region raster=$nir_fn [-p]