Shell Programming and Scripting

Hi Gurus
I need your kind help sorting the below query
I have two text files

Code:

 
File1.txt
ID   Name       Address 
101  Srinath      BBB
102  Sidharth     CCC

File2.txt
ID   Name      Address
102  Siddharth   DDD
103  Suman       EEE

Now the requirement is if the second file has new data which is not exist in file1 then it will flag as I (insert)
if in file2.txt any data exist in file1.txt but changed in file2.txt it will have flag value as U (Update)
if in file2.txt any data not exist from file1.txt it will update as D(Delete)

So the o/p needed in a new filw is

Code:

 
File3.txt
ID   Name     Address   Flag
101  Srinath   BBB       D
102  Sidharth  DDD       U
103  Suman     EEE       I

Thanks in advance