#!/bin/bash

set -euo pipefail

in_file="$1"
out_file="$2"
files="$(find "$1" -type f -name '*Z.mp4' | sort)"
concat="concat.txt"
echo '' > concat.txt
for file in $files; do
    echo "file '$file'" >> concat.txt
done
ffmpeg -f concat -safe 0 -i concat.txt -c:v copy -c:a aac "$2.mp4"