Shell Programming and Scripting

Hi All,

I am trying to manipulate a file using awk in UNIX. My file "test_file" is structured as follows:

field1:field2:field3
field4:field5:field6
field7:field8:field9

My requirement is to calculate another field (which would be let's say sum of 2nd and third column and put as fourth column in this file). so that my final output would be

field1:field2:field3:fieldA
field4:field5:field6:fieldB
field7:field8:field9:fieldC

where fieldA=field2 + field3 and so on...

I have the following piece of code which I guess, is nowhere in the working condition right now:

while read line
do
echo $line | awk -F: 'BEGIN {OFS=":"}
{$4=$2 + $3 ; print $0}' >> new_test_file
done < test_file

I read somewhere that you can dynamically calculate variables in awk, so $4 would be dynamically calculated at the run time. Subsequently when i print $0 should contain the new calculated field too. But I guess that is not the case.

Can anybody look into above and help me out?

Thanks

I guess by mistake i submitted this twice. Plz ignore this one.
Thanks