Shell Programming and Scripting

#!/bin/sh
#       Display today and derive yesterday date YYMMDD. 
#       Works between 02/01/00 - 31/12/20
#
# Today
YY="`date +'%y'`"
MM="`date +'%m'`"
DD="`date +'%d'`"
TODAY="${YY}${MM}${DD}"
#
case "$MM" in
"01"|"02"|"04"|"06"|"08"|"09"|"11") # Last month was a 31 day month
        LASTDAY="31"    # Last day of last month
        ;;
"05"|"07"|"10"|"12")    # Last month was a 30 day month
        LASTDAY="30"    # Last day of last month
        ;;
"03")   # Last month was a 28/29 day month
        # Leap year adjustment
        # (does not work for year 2100 which is not a leap year)
        LEAP=`expr ${YY} % 4`
        if [ ${LEAP} -eq 0 ]
        then
                LASTDAY="29"
        else
                LASTDAY="28"
        fi
        ;;
*)
        echo "Invalid month: ${MM}"
        ;;
esac
#
if [ "${DD}" = "01" ]
then
        DD2="${LASTDAY}"        # Last day of last month
        if [ "${MM}" = "01" ]
        then
                MM2="12"
                YY2=`expr ${YY} - 1`
        else
                MM2=`expr ${MM} - 1`
                YY2="${YY}"
        fi
else
        DD2=`expr ${DD} - 1`
        MM2="${MM}"
        YY2="${YY}"
fi
#
YESTERDAY=`printf '%.2d%.2d%.2d' $YY2 $MM2 $DD2`
echo "${TODAY}"
echo "${YESTERDAY}"