根据日期获取星期的shell脚本

自己对shell脚本不熟悉，在chinaunix上cp了一段根据日期获取星期的脚本。结果测试没问题，项目上线了，出错了。最后测试那上面的那个脚本部分时段管用。为了防止害人无数，我把从网上找到的正确的贴出来。

#!/bin/sh

# SAVE THIS CODE AS dayofweek.sh
#
# Uses Zellers Congruence calculation to use a date and give
# the day of the week that date was.
#
# This function expects 1 Arguments,
# YYYYMMDD
# example:20090520
# then
# Returns a value between 0 and 6 to represent the day of the
# week where 0=Sun,1=Mon,...6=Sat
#
# e.g dayofweek.sh 20090520
#
# This formula is Year 2000 compliant.
# It is not compliant using dates previous to Oct 1752

#
YMD=$1
YEAR=`echo $YMD|cut -c0-4`
MONTH=`echo $YMD|cut -c5-6`
DAY=`echo $YMD|cut -c7-9`

# Adjust Month such that March becomes 1 month of
# year and Jan/Feb become 11/12 of previous year
# =============================================

if [ $MONTH -ge 3 ]
then
  MONTH=`expr $MONTH - 2`
else
  MONTH=`expr $MONTH + 10`
fi


if [ $MONTH -eq 11 ] || [ $MONTH -eq 12 ] ; then
  YEAR=`expr $YEAR - 1`
fi
# ==============================================

# Split YEAR into YEAR and CENTURY
# ================================
CENTURY=`expr $YEAR / 100`
YEAR=`expr $YEAR % 100`
# ================================

# Black Magic Time
# ================
#Z=(( 26*$MONTH - 2 ) / 10) + $DAY + $YEAR + ( $YEAR/4 ) + ( $CENTURY/4 ) - (2 * $CENTURY) + 77) % 7
Z=`expr \( $MONTH \* 26 - 2 \) / 10`
Z=`expr $Z + $DAY + $YEAR`
Z=`expr $Z + $YEAR / 4`
Z=`expr $Z + $CENTURY / 4`
Z=`expr $Z - $CENTURY - $CENTURY + 77`
Z=`expr $Z % 7`
if [ $Z -lt 0 ] ; then
  Z=`expr $Z + 7`
fi
# ================

echo $Z
# Sun 0
# Mon 1
# Tue 2
# Wed 3
# Thu 4
# Fri 5
# Sat 6
# ========================