Rename ?

rename '2.txt' '3.txt' REP2.txt && ls

rename '1.txt' '.txt'  REP1.txt

lami20j@debian:~$ touch REP1.txt
lami20j@debian:~$ ls -l REP1.txt
-rw-r--r-- 1 lami20j lami20j 0 2008-09-27 16:44 REP1.txt
lami20j@debian:~$ rename 's/1//' REP1.txt
lami20j@debian:~$ ls -l REP1.txt
ls: REP1.txt: Aucun fichier ou répertoire de ce type
lami20j@debian:~$ ls -l REP.txt
-rw-r--r-- 1 lami20j lami20j 0 2008-09-27 16:44 REP.txt
lami20j@debian:~$

mv REP1.txt REP.txt

~/Desktop/compt> ls
REP1.txt  REP2.txt  REP.txt  URL1.txt  URL2.txt  URL.txt

rename 's/2/3/' REP2.txt && ls
REP1.txt  REP2.txt  REP.txt  URL1.txt  URL2.txt  URL.txt

lami20j@debian:~/eZula$ touch REP1.txt  REP2.txt  REP.txt  URL1.txt  URL2.txt  URL.txt
lami20j@debian:~/eZula$ ls -l
total 0
-rw-r--r-- 1 lami20j lami20j 0 2008-09-27 16:56 REP1.txt
-rw-r--r-- 1 lami20j lami20j 0 2008-09-27 16:56 REP2.txt
-rw-r--r-- 1 lami20j lami20j 0 2008-09-27 16:56 REP.txt
-rw-r--r-- 1 lami20j lami20j 0 2008-09-27 16:56 URL1.txt
-rw-r--r-- 1 lami20j lami20j 0 2008-09-27 16:56 URL2.txt
-rw-r--r-- 1 lami20j lami20j 0 2008-09-27 16:56 URL.txt
lami20j@debian:~/eZula$ rename 's/2/3/' REP2.txt && ls
REP1.txt  REP3.txt  REP.txt  URL1.txt  URL2.txt  URL.txt

RENAME(1)                                                                       Perl Programmers Reference Guide                                                                      RENAME(1)

NAME
       rename - renames multiple files

SYNOPSIS
       rename [ -v ] [ -n ] [ -f ] perlexpr [ files ]

DESCRIPTION
       "rename" renames the filenames supplied according to the rule specified as the first argument.  The perlexpr argument is a Perl expression which is expected to modify the $_ string in
       Perl for at least some of the filenames specified.  If a given filename is not modified by the expression, it will not be renamed.  If no filenames are given on the command line, file-
       names will be read via standard input.

       For example, to rename all files matching "*.bak" to strip the extension, you might say

               rename 's/\.bak$//' *.bak

       To translate uppercase names to lower, you'd use

               rename 'y/A-Z/a-z/' *

OPTIONS
       -v, --verbose
               Verbose: print names of files successfully renamed.

       -n, --no-act
               No Action: show what files would have been renamed.

       -f, --force
               Force: overwrite existing files.

ENVIRONMENT
       No environment variables are used.

AUTHOR
       Larry Wall

SEE ALSO
       mv(1), perl(1)

DIAGNOSTICS
       If you give an invalid Perl expression you'll get a syntax error.

BUGS
       The original "rename" did not check for the existence of target filenames, so had to be used with care.  I hope I've fixed that (Robin Barker).

perl v5.8.8                                                                                2008-04-25                                                                                 RENAME(1)
 Manual page rename(1) line 1/51 (END)          

~/Desktop/compt> ls
Compteur.txt
REP1.txt
REP2.txt

compt=$(cat Compteur.txt)
for i in `seq $compt` ; do j=$(( $compt - 1 ));rename "$compt" "$j" REP$compt.txt ; done

for i in $(seq $(cat Compteur.txt));do rename "$i.txt" "$(($i-1)).txt" REP*;done

[eZula]$ ls
foo.sh*  REP1.txt  REP2.txt  REP3.txt  REP4.txt

[eZula]$ cat foo.sh
#! /bin/bash

for i in REP*
do
number=$(grep -o '[ [:digit:]]' <<<$i)
mv ${i} ${i/$number/$(($number -1))}
done

[eZula]$ ./foo.sh

[eZula]$ ls
foo.sh*  REP0.txt  REP1.txt  REP2.txt  REP3.txt

[eZula]$