WOM -- ISO8601 and ICU compliant week of a month

wom.sh

#! /bin/sh

#  WOM -- ISO8601 and ICU compliant week of a month
#  Copyright (C) 2012 Matous J. Fialka, <http://mjf.cz/>
#  Released under the terms of The MIT License

if [ $# -ne 1 ] || [ "$1" != +%w -a "$1" != +%u ]
then
	cat 1>&2 <<- EOT
	Usage:  wom +%[wu];

	Where:  "w" for weeks starting with Sunday,
	        "u" for weeks starting with Monday;

	Environment:
	        WOM_TS - Unix timestamp (Epoch time);
	EOT

	exit 1
fi

WOM_TS=${WOM_TS:-`date +%s`}

# Print time in date(1) format according to the actual WOM_TS variable.

now()
{
	date -d 1970-01-01\ 00:00:00\ $WOM_TS\ SEC "$*"
}

# Print the week of the month (from 0 to 5).  The first week of the
# month is the first week that contains a Thursday.  This is based
# on the ICU definition of week of month, and correlates to the ISO8601
# week of year definition.  A day in the week before the week with the
# first Thursday will be week 0.

printf -- %d\\n $(( ( ( `now +%e` + 4 - `now $1` ) + 6 ) / 7 ))