wcal

#! /bin/sh
# wcal (Bourne shell script) -- Shows a calendar, with week numbers, for a given month
#
# Usage: wcal [ month year | delta ]
# A delta is a single arg with "+" or "-" prefix to adjust relative to currently month
#
# TO-DO:
#   + Replace today-highlight that's omitted because stdout is a pipe

case $# in
  0) datespec=$(date +%m)/1 ;;
  1) # Handle a delta
     current_month=$(date +%m)
     case $1 in
       +*) relative_month=$(expr $current_month + ${1#+})
           month=$(expr $relative_month % 12)
           # Have to make month 0-based when working out the year delta
           year=$(expr $(date +%Y) + $(expr $(expr $relative_month - 1) / 12))
           ;;
       -*) relative_month=$(expr $current_month - ${1#-})
           if [ $relative_month -ge 1 ] ; then
             month=$relative_month
             year=$(date +%Y)
           else
             # Account for negative values or 0
             month=$(expr 12 + $(expr $relative_month % 12))
             # Have to increase negative year delta by 1, i.e. subtract 1
             year=$(expr $(date +%Y) + $(expr $(expr $relative_month / 12) - 1))
           fi
           ;;
       *) echo "Usage: wcal [ month year | delta ]" >&2
          exit 1
          ;;
     esac
     datespec=$month/1
     ;;
  2) month=$1
     year=$2
     datespec=$month/1
     ;;
  *) echo "Usage: wcal [ month year | delta ]" >&2
     exit 1
     ;;
esac

# Calculate the week number of the first (partial) week of the month and adjust
# it (to be corrected later) if it's actually the last week of the previous year.
if [ -n "$year" ] ; then
  year_prefix=$year/
fi
num=$(date -d "$year_prefix$datespec" +%V)
if [ $num -gt 50 ] ; then
  pnum=$num
  num=0
fi

ncal -bM $month $year |
  sed -e '# Insert nl header & body delimiters before the relevant lines' \
      -e '1i\
\\:\\:\\:' -e '3i\
\\:\\:' \
      -e '# Strip out blank lines inserted by ncal' \
      -e '/^ \+$/ d' |
  nl --number-width=3 --number-format=ln --number-separator='  ' \
     --starting-line-number=$num |
  sed -e '# Modify header to include "wk" column' \
      -e '3 s/^../wk/' \
      -e '# Strip out blank lines inserted by nl' \
      -e '1d' -e '4d' \
      -e '# Correct the first week number if necessary' \
      -e "s/^0 /$pnum/"