Yes your assumption is correct and this is a real world problem

What iam trying to do is commenting the duplicate Ip's found in the text file ignoring the first occurance as to be valid IP for a valid mac and from the second occurance onwards commenting the duplicate IPs at the starting of the line.

file 1.txt with following entries as shown:

0152364|134444|10.20.30.40|015236433
0233654|122555|10.20.30.50|023365433
**
**
**

file 2.txt with following entries as shown:

0152364|134444|10.20.30.40|015236433
0233654|122555|10.20.30.50|023365433
0789456|332211|10.20.30.40|078945633
1234567|225522|10.20.30.50|123456733
0321654|999999|10.20.30.40|032165433
0456123|777899|10.20.30.40|045612333
***
***
***

i need to have the following output in 2.txt as shown:

0152364|134444|10.20.30.40|015236433
0233654|122555|10.20.30.50|023365433
#0789456|332211|10.20.30.40|078945633
#1234567|225522|10.20.30.50|123456733
#0321654|999999|10.20.30.40|032165433
#0456123|777899|10.20.30.40|045612333
***
***
***

The correct script i wrote is

for i in `cat 1.txt |cut -d"|" -f3`
do
cat 2.txt |sed "s/^\(.*\|$i\|.*)/#\1/g" > tmp.txt
mv tmp.txt 2.txt
done

But the above script will comment all the occurance.

Your help is appreciated.

Thanks
-imas