Shell Programming and Scripting

Hi Folks

I have a file with the following sample input
919416586748,1200,31200
919416283619,3521,33521
919416811900,2440,64940
919416576012,13670,43670

Now i want to subtract the second value (where , is field separator) from the third value and display the output as follows

919416,30000
919416,30000
919416,62500
919416,30000

i wrote a simple program

Code:

for var in `cat 1`
do
a1=`echo $var | cut -d"," -f1|cut  -c1-6`
a2=`echo $var | cut -d"," -f2`
a3=`echo $var | cut -d"," -f3`
a4=`expr $a3 - $a2`
echo "$a1,$a4" >> 2
done

This works fine for files which are of small size. But my input file has over 1 million entries and hence is taking a damn long time to process.
Can anybody recommend a much faster method for the same?

Thanks in advance
Regards

How about:
awk -v FS=, -v OFS=, '{print substr($1,1,6),$3-$2}' < datafile

awk 'BEGIN{FS=OFS=","}{print substr($1,1,6),$3-$2}' datafile

Thanks a lot folks

Both the queries worked. I really need to brush up on the awk command. This is the second time where a one line awk command has bailed me out where i had spent hours using some inefficient scripts.

Regards