Find difference between 2 files

UNIX for Advanced & Expert Users
1

I have 2 files as follows.
file1.txt

<cell>123</cell>
<cell>345</cell>

file2.txt

<cell>123</cell>
<cell>456</cell>

out out should be

output.txt

<cell>456></cell>

How do we achieve this> The difference betwenn the two files should be wirtten to the output file..

Thanks in advance.
Kantharaj.

2 
comm -13 file1.txt file2.txt
3

Belt and braces:

sort <file1.txt >/tmp/file1.sor
sort <file2.txt >/tmp/file2.sor
comm -13 file1.sor file2.sor