Move files into folders by extension

extension.sh

#!/bin/bash

recup_dir="${1%/}"
echo "$recup_dir";

[ -d "$recup_dir" ] || {
    echo "Usage: $0 recup_dir";
    echo "Mirror files from recup_dir into recup_dir.by_ext, organized by extension";
    exit 1
};
find "$recup_dir" -type f | while read k; do
    ext="${k##*.}";
    ext_dir="$ext";
    fn=`basename "$k" "$ext"`;
    pre=${fn:0:3};
    [ -d "$ext_dir" ] || mkdir -p "$ext_dir";
    [ -d "$ext_dir/$pre" ] || mkdir -p "$ext_dir/$pre";
    echo "${k%/*}"
    mv "$k" "$ext_dir/$pre/";
done