Today Scripts: notification centre calendar

#!/bin/bash

case $# in
  0) month=$(date +%m) year=$(date +%Y);;
  2) month=$1 year=$2;;
  *) echo 1>&2 "Usage: $0 [MONTH YEAR]"; exit 3;;
esac
month=${month#"${month%%[!0]*}"};
if ((month == 1)); then
  previous_month=12 previous_year=$((year-1))
else
  previous_month=$((month-1)) previous_year=$year
fi
if ((month == 12)); then
  next_month=1 next_year=$((year+1))
else
  next_month=$((month+1)) next_year=$year
fi
paste <(cal $previous_month $previous_year)
paste <(cal $month $year) | grep --before-context 6 --after-context 6 --color -e " $(date +%e)" -e "^$(date +%e)"
paste <(cal $next_month $next_year)