如何用shell 实现某天减1天的日期

如何用shell 实现某天减1天的日期：

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#!/bin/sh

CurYear=date +%Y
CurMonth=date +%m
CurDay=date +%d

GetYear="$CurYear"
GetMonth="$CurMonth"
GetDay="expr $CurDay - 1"

if [ "$GetDay" -le 0 ] ; then

GetMonth=expr $CurMonth - 1

if [ "$GetMonth" -le 0 ] ; then

GetYear=expr $CurYear - 1
GetMonth=12

fi

case "$GetMonth"
in
1 &brvbar3 &brvbar5 &brvbar7 &brvbar8 &brvbar10 &brvbar12)

GetDay=31;;
4 &brvbar6 &brvbar9 &brvbar11)

GetDay=30;;
2)

if [ expr "$CurYear" % 400 -eq 0 ] ; then

GetDay=29
elif [ expr "$CurYear" % 4 -eq 0 -a expr "$CurYear" % 100 -ne 0 ] ; then

GetDay=29
else

GetDay=28
fi
esac

fi

if [ echo "$GetMonth" ¦ wc -m -ne 3 ] ; then

GetMonth=0$GetMonth
fi

if [ echo "$GetDay" ¦ wc -m -ne 3 ] ; then

GetDay=0$GetDay
fi

echo
echo " === Get Last Date ==="
echo
echo "\t\t""$GetYear""$GetMonth""$GetDay"
echo