In the following script when I get SIZE1 and SIZE2, they are of type string. I can not do any mathematical operations on them. I wanted to find out if I can convert string in to Number. Thanks in Advance.

FILE1='/export/registry/incoming/reg.dmp'
FILE2='/export/registry/reg.dmp'

SIZE1=`ls -l $FILE1 | awk '{print $5}'`
SIZE2=`ls -l $FILE2 | awk '{print $5}'`

if [ $SIZE1 > $SIZE2 ]
then
echo $SIZE1 - $SIZE2
else
echo "The 1st file is smaller."
fi

You didn't mention which shell you are using. Form Bourne shell or bash shell, the following comparison operators exist for comparing integer values:

int1 -eq int2 True if integer one is equal to integer two
int1 -ge int2 True if integer one is greater than or equal to integer two
int1 -gt int2 True if integer one is greater than integer two
int1 -le int2 True if integer one is less than or equal to integer two
int1 -lt int2 True if interger one is less then interger two.
int1 -ne int2 True if integer one is not equal to integer two

So, you should be able to do something like:

if [ $SIZE1 -gt $SIZE2 ]
then
...
else
...
fi

And if you actually want the result of $SIZE1 - $SIZE2, you'll have to replace

echo $SIZE1 - $SIZE2

with:

echo `expr $SIZE1 - 500`