File monitoring Bash script - linux

Question

I created this script for check whether specific files exist or not in the given location. but when I run this its always showing
Failed - Flag_lms_device_info_20160628.txt do not exist
Failed - Flag_lms_weekly_usage_info_20160628 do not exist
but both files are existing.
PREFIX="/opt/data"
REPORT="/tmp/report.txt"
DATE=$( date -d "${dtd} -1 days" +'%Y%m%d' )
rm -f "$REPORT"
FILENAME="Flag_lms_device_info_${DATE}.txt"
FULLFN="$PREFIX/$FILENAME"
if [ -f "$FULLFN" ]; then
echo "OK - $FILENAME exists" >> $REPORT
else
echo "Failed - $FILENAME do not exist" >> $REPORT
fi
FILENAME="Flag_lms_weekly_usage_info_${DATE}.txt"
FULLFN="$PREFIX/$FILENAME"
if [ -f "$FULLFN" ]; then
echo "OK - $FILENAME exists" >> $REPORT
else
echo "Failed - $FILENAME do not exist" >> $REPORT
fi

Answer 1

First of all, you have strange output in your question: your second line of output lacks a .txt extension. This might be an accident but if it's not it's worth investigating.
Assuming your date command is working correctly (I don't know that particular command), I would reduce your use of variables. In addition, I would use the -e test operator instead of -f because it's more inclusive. (If you haven't put data in the files yet, -f could return an error even if the file exists.) :
REPORT="/tmp/report.txt"
DATE=$( date -d "${dtd} -1 days" +'%Y%m%d' )
echo "" > "$REPORT" # Wipes file instead of completely removing it
filename="/opt/data/Flag_lms_device_info_$DATE.txt"
if [ -e "$filename" ]; then
echo "OK - Flag_lms_device_info_$DATE.txt exists" >> $REPORT
else
echo "Failed - Flag_lms_device_info_$DATE.txt doesn't exist" >> $REPORT
fi
filename="/opt/data/Flag_lms_weekly_usage_info_$DATE.txt"
if [ -e "$filename" ]; then
echo "OK - Flag_lms_weekly_usage_info_$DATE.txt exists" >> $REPORT
else
echo "Failed - Flag_lms_weekly_usage_info_$DATE.txt doesn't exist" >> $REPORT
fi

Answer 2

if [ -f "find "$FULLFN" -type f -name "$FILENAME"" ];then
Here you check for existance of a strange file named find... Use backquotes
if [ -f `find "$FULLFN" -type f -name "$FILENAME"` ];then
or, in bash,
if [ -f $(find "$FULLFN" -type f -name "$FILENAME") ];then
to get the command's output as a string.
Furthermore, your find invocation does not look promising. If you need to find a file named Flag_lms_device and so forth somewhere under /opt/data/, use find "$PREFIX" -type f -name "$FILENAME". If you know for sure that /opt/data is the exact location, then use
if [ -f "$FULLFN" ]
and you don't need to find the file.