A Whisper

Lets assume that you have a file of files number sequences. That file named (list.txt) contain only numbers.

$ more list.txt
1
2
3
4
5
8
10
11
12
13
14


Here the sequence 6, 7 and 9 is missed. How to find the missing sequence? I have tried this script and it works fine:

> complete.txt
sort -nu list.txt > sorted.txt
s=`head -1 sorted.txt`
e=`tail -1 sorted.txt`
while [ $s -le $e ]
do
echo $s >> complete.txt
((s = $s + 1 ))
done
echo "Sequence missed:"
diff sorted.txt complete.txt | grep '^>'