# Mask subregion using coorainate in the R1.dat file:
# Input file: Data/pr_exp28_daily.nc
# Masked file: ofile_R1.nc

cdo maskregion,R1.dat Data/pr_exp28_daily.nc ofile_R1.nc

#Or:
count=2
cat lonlat_r2r20.dat | while read -r lon1 lon2 lat1 lat2; do
         cdo sellonlatbox,${lon1},${lon2},${lat1},${lat2} ifile ofile_R${count}.nc
         count=$((count+1))
done