#!/bin/bash
mdoys=( 31 59 90 120 151 181 212 243 273 304 334 365 )
months=( 'January' 'February' 'March' 'April' 'May' 'June' 'July' 'August' 'September' 'October' 'November' 'December' )
this_month=0
files=""
for ((doy=1;doy<365;doy++)) do

  r.sun -s elev=elevation.dem slop=slope asp=aspect
        beam_read=beam_${doy} diff_rad=diff_${doy} day=${doy}

  if [[ ${doy} -lt ${mdoys[${this_month}]} ]] ;  then
   files=${files},beam_${doy}
  fi
  if [[ ${doy} -eq ${mdoys[${this_month}]} ]] ; then
   files=${files},beam_${doy}
   echo ${files} > rasters_${months[$this_month]}
   this_month=$((this_month+1))
   # Use r.series to process the files in rasters_January, for example
  fi
done