http://unix.stackexchange.com/q/37121/6040

rename.bash

for album in event-*/album-*/; do
	echo "$album";

	# Execute every directory in its own shell, so we do not need to worry
	# about "cd -" on errors, etc.
	(
		cd "$album";

		# Fill the positional parameter array with all files. "*" is expanded
		# alphabetically by default.
		set -- *;

		# First, determine the number of digits to use for the zero-padded
		# number.
		num_digits=1;
		while [ $# -gt $((10 ** $#)) ]; do
			let num_digits++;
		done;

		# Rename all files.
		i=0;
		for file; do
			let i++;

			new_file="$(printf "%0${num_digits}d.%s" $i "${file##*.}")";
			# Unless they already have the right name.
			[ "$new_file" = "$file" ] && continue;

			# Make sure to ask before overwriting, and be verbose about the operation.
			mv -vi "$file" "$new_file";
		done;
	);
done;