Disk usage monitoring and record the disk used in last 24 hour

HI I am Trying to edit the below code to send email every day with difference of disk utilized in for last 24 hours but instead getting same usage everyday. can you please help me to point out where my calculation is going wrong. Thank you.

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#!/bin/bash


TODAY="at $(date '+%H:%M on %d-%b-%y')"
OutputFilename=$(date +"%b_%d_%Y".html)
LastReport=/tmp/YesReport.txt
NowReport=/tmp/TodReport.txt
CurDate=`date +%D\ %T\ %Z`

CriticalPercentage=80
WarningPercentage=70
NormalPercentage=68

IPADDRESS=`hostname`

if [ -f /tmp/${OutputFilename} ]; then
 rm -f /tmp/${OutputFilename}
fi

if [ -f ${TodReport} ]; then
     mv ${TodReport} ${YesReport}
         cp /dev/null ${TodReport}
fi


df -Ph | grep -vE "^Filesystem|tmpfs|cdrom" | awk 'NR>0 && NF==6' | awk '{print $3}' > ${TodReport}
if [ -f ${YesReport} ]; then
 DiffValue=(`awk '{ getline getdifference < "/tmp/YesReport.txt"; $1 -= getdifference; print }' /tmp/TodReport.txt`)
fi

(
 echo '<HTML><HEAD><TITLE>Disk Usage Statistics</TITLE></HEAD>'
 echo '<BODY>'
 echo '<H3><font color="#5555E7" face="verdana" size="3">Disk Usage Report for server - '$(uname -n) DEV ${IPADDRESS}'</font></H3>'
 echo '<P style="color:#5555E7" face="verdana" size="3">Report Generated '${TODAY}'</P>'
 echo '<TABLE BORDER=1 CELLSPACING=0 CELLPADDING=2 bordercolor=#DEDEE2>'
echo '<P style="color:#5555E7" face="verdana" size="3">Report Generated '${TODAY}'</P>'
 echo '<TABLE BORDER=1 CELLSPACING=0 CELLPADDING=2 bordercolor=#DEDEE2>'
 echo '<style>
 body {
     font-family: verdana;
         font: 16px verdana, sans-serif;
}
tbody th { text-align:right; background: #F8F8FA; color:red}
</style>'
# echo '<TR BGCOLOR=\"#BBFFFF\"> <TH><font color="#5555E7"><font face="verdana"><b>Filesystem</b></font></TH> <TH>Total</TH> <TH>Disk Used</TH>
#<TH>Available</TH> <TH>Percentage Info</TH> <TH>Mounted On</TH> <TH>Critical Alert</TH>
#<TH>Report Date</TH> <TH>12 Hrs Difference </TH><//TR>'
echo '<TR BGCOLOR=\"#4863A0\"> <TH><font color="#5555E7" face="verdana" size="2"><b>Filesystem</b></font></TH> <TH><font color="#5555E7" face="verdana" size="2"><b>Total</b></font></TH> <TH><font color="#5555E7" face="verdana" size="2"><b>Disk Used</b></font></TH><TH><font color="#5555E7" size="2" face="verdana">
<b>Available</b></font></TH> <TH><font color="#5555E7" size="2" face="verdana"><b>Percentage Info</b></font></TH> <TH><font color="#5555E7" size="2" face="verdana">
<b>Mounted On</b></font></TH> <TH><font color="#5555E7" size="2" face="verdana"><b>Critical Alert</b></font></TH> <TH><font color="#5555E7" size="2" face="verdana">
<b>Report Date</b></font></TH> <TH><font color="#5555E7" size="2" face="verdana"><b>24 Hrs Difference</b></font></TH></TR>'




ArryCount=0
df -Ph | grep -vE "^Filesystem|tmpfs|cdrom" | awk 'NR>0 && NF==6'|sort|while read FileSystem Size DiskUsed DiskFree DiskPercentUsed MountPoint
 do
 PERCENT=${DiskPercentUsed%%%}


 TDiffValue=(`awk '{ getline getdifference < "/tmp/YesReport.txt"; $1 -= getdifference; print }' /tmp/TodReport.txt`)


 if [[ ${PERCENT} -ge ${CriticalPercentage} ]];
 then
 COLOR=red
 CRITICALALERT="Yes, Notify"
 elif [ ${PERCENT} -ge ${WarningPercentage} ] && [ ${PERCENT} -le 90 ];
 then
 COLOR=orange
 CRITICALALERT=No
 else
 COLOR=green
 CRITICALALERT=NA
 fi
CRITICALALERT=NA
 fi
 echo '<TR><TD>'$FileSystem'</TD><TD ALIGN=RIGHT>'$Size'</TD>'
 echo '<TD ALIGN=RIGHT>'$DiskUsed'</TD><TD ALIGN=RIGHT>'$DiskFree'</TD>'
 echo '<TD><TABLE BORDER=0 CELLSPACING=3 CELLPADDING=0>'
 echo '<TR><TD WIDTH='$((2 * $PERCENT))' BGCOLOR="'$COLOR'"></TD>'
 echo '<TD WIDTH='$((2 * (100 - $PERCENT)))' BGCOLOR="gray"></TD>'
 echo '<TD><FONT FONT-WEIGHT="bold" SIZE=-1
COLOR="'$COLOR'">'$DiskPercentUsed'</FONT></TD>'
 echo '<TR></TABLE><TD>'$MountPoint'</TD>'
 echo '<TD><FONT font-weight="bold">'$CRITICALALERT'</TD></FONT>'
 echo '<TD><FONT font-weight="bold">'`date`'</TD></FONT>'
 echo '<TD><FONT font-weight="bold">'${TDiffValue[ArryCount]} \(in bytes\)'</TD></FONT></TR>'
 echo $DiskUsed >> `hostname`.usage.txt
 ArryCount=$ArryCount+1
 done
 echo '</TABLE>'
 echo '</P><BR>'
 echo '<TABLE BORDER=1 CELLSPACING=3 CELLPADDING=0>'
 echo '<TR><TH FONT font-weight="bold">Legend Information</TH></TR>'
 echo '<TR><TD FONT color="white" BGCOLOR="RED">Critical Alert</TD></TR>'
 echo '<TR><TD FONT color="white" BGCOLOR="ORANGE">Warning Alert</TD></TR>'
 echo '<TR><TD FONT color="white" BGCOLOR="GREEN">No Action Alert</TD></TR>'
 echo '</TABLE>'
 echo '<BODY></HTML>'
 echo '<P><FONT font-weight="bold">Report Generated by MGT Big Data Admin Team</P>'
) | tee `hostname`_${0##*/}.html