How to modify the filename in shell script?

Depending on your distribution, you can use rename:

rename 's/(.*)(\.xml)/$1_R$2/' *.xml

Just basic unix command mv work both on move and rename

mv 1234.xml 1234_R.xml

If you want do it by a large amount, do like this:

[~/bash/rename]$ touch 1234.xml 333.xml cccc.xml
[~/bash/rename]$ ls
1234.xml  333.xml  cccc.xml
[~/bash/rename]$ L=`ls *.xml`
[~/bash/rename]$ for x in $L; do mv $x ${x%%.*}_R.xml; done
[~/bash/rename]$ ls
1234_R.xml  333_R.xml  cccc_R.xml
[~/bash/rename]$

You can use a for loop to iterate over a list of words (e.g. with the list of file names returned by ls) with the (bash) syntax:

for name [ [ in [ word ... ] ] ; ] do list ; done

Then use mv source dest to rename each file.

One nice trick here, is to use basename, which strips directory and suffix from filenames (e.g. basename 333.xml will just return 333).

If you put all this together, the following should work:

for f in `ls *.xml`; do mv $f `basename $f`_R.xml; done