Hi,
I have written a shell script which selects the particular type of file (consider that .xml files) from each sub directories under a parent directory and does some manipulation. Now what happens here at a time it looks for asingle sub directory only. I mean it is executed recursively for each sub directory. Now my intent is to select all the .xml files from all subdirectories and to do some manipulation on the oldest file among all files. How it can be done?

what I did :
I selected all the files from each directory and redirected to them a flat file using command:
ls -lurt $dir/*.xml | awk '{print $6,$7,$8,$9}'>> tmp.txt.
from tmp.txt I can find the file name with full path using the command:
cut -d' ' -f4 tmp.txt
Now I faced difficult to sort the files which are in tmp.txt file.

Using GNU tools you can do:

numfiles=10 #number to process
find $dir -name "*.xml" -printf "%T@\t%p\n" |
sort -k1,1n |
tail -$numfiles |
cut -f2- |
while read file; do
echo processing $file
done

For comparison have a look at:
http://www.pixelbeat.org/scripts/newest