If you have the files a.txt, b.txt, and c.txt in the directory in which this is run, this program will output a_2.txt, b_2.txt, and c_2.txt with foo appended to each (replace the foo line with your processing commands).
for f in *.txt;
do f2=${f%.*}_2.txt;
cp $f $f2;
echo "Processing $f2 file...";
echo "foo" >> $f2; # Your processing command here
done