I wanted to rename all of the files in a directory that ended in .txt so that they ended in .xml . Clearly some sort of script was needed, but what language to use? I chose elisp, because I was working in emacs and the interactive help made it easy to find the functions I wanted. Thinking about it later I wondered how I'd do this in other languages so here also are python and perl versions with the same functionality. None are heavily tested but all work if run in the directory containing the files to rename. My final impression? The elisp one was the quickest to write and I think is the easiest to manage for this job.

In elisp:

(defun rename-some-files (dir)
  "Rename some files."
  (interactive "DDirectory name ")
  (mapc (lambda (f) (rename-file (concat f ".txt") 
				 (concat f ".xml")))
	(mapcar 'file-name-sans-extension
		(directory-files dir nil "\.txt$")))

In python:

import os
import re

pat = re.compile('\.txt$')
map(lambda f: os.rename(f + '.txt', f + '.xml'),
    [os.path.splitext(f)[0] for f in os.listdir(os.getcwd()) \
     if pat.search(f)])

In perl:

use Cwd;
use File::Basename;

opendir(DIR, cwd());
map(rename($_ . '.txt', $_ . '.xml'),
    map(basename($_, '.txt'),
	grep(/\.txt$/, readdir(DIR))));
closedir(DIR);