lc_ext.sh

#!/bin/bash
# The purpose of this script is to find all (PDF) files in the directory
# specified and turn the file extension to lowercase. If another file already
# has the new name, check its MD5 hash to see if it's an identical file, and if
# so delete it. Also delete files with any name that have identical MD5 hashes
# to other files in the directory.

set -eo pipefail

usage () {
    echo -e "Usage: $(basename "$0") [dir]\n"
    exit
}

if [[ -z $1 || ! -d $1 ]]; then
    usage
fi

declare -A md5s

regex='\.[[:alnum:]]{3}$'

dir=$(readlink -f "$1")

mapfile -t files < <(find "$dir" -type f -iname "*.pdf")

for (( i = 0; i < ${#files[@]}; i++ )); do
    f="${files[${i}]}"
    f_bn=$(basename "$f")

    if [[ ! "$f" =~ $regex ]]; then
        continue
    fi

    f_no_ext="${f%.???}"

    mapfile -d'.' -t name_parts <<<"$f"
    element=$(( ${#name_parts[@]} - 1 ))

    ext="${name_parts[${element}]}"
    lc_ext=$(tr '[[:upper:]]' '[[:lower:]]' <<<"$ext")

    new_f="${f_no_ext}.${lc_ext}"
    new_f_bn=$(basename "$new_f")

    f_md5=$(md5sum -b "$f" | cut -d' ' -f1)

    if [[ -z ${md5s[${f_md5}]} ]]; then
        md5s[${f_md5}]='1'
    else
        echo "rm: ${f}"
        rm "$f"

        continue
    fi

    if [[ "$f" != "$new_f" ]]; then
        if [[ -f "$new_f" ]]; then
            new_f_md5=$(md5sum -b "$new_f" | cut -d' ' -f1)

            if [[ $f_md5 != $new_f_md5 ]]; then
                continue
            fi
        fi

        echo "mv: ${new_f}"
        mv "$f" "$new_f"
    fi
done