Did you try some searches ?
BASH rename Greek characters upper, lower case
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Unless there is a reason it has to be "bash", 'perl' might be a better choice:
rename Greek characters upper, lower case using perl
Do you have the command 'rename' ?
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, filenames will be read via standard input.
If you don't, you will need to install it, to use 'rename'. There are details on that
in the results of the first searches,...
Also in this thread/topic,
http://forums.debian.net/viewtopic.php?f=30&t=129864
The most recent post mentions: ' pyrenamer '
pyrenamer
mass file renamer written in PyGTK
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You can rename files using patterns, search and replace,
substitutions, insert or delete text, or even rename
files manually.
You can also rename images using their EXIF tags
and music using their internal tags
I don't know if it works very well, have never tried it myself,
Perl or bash is usually sufficient.