Concatenate and convert differently sized videos into one

ffmpeg_concat_and_scale.sh

#!/usr/bin/env bash

join_files() {
  local input_n=()
  local scale_to_n=("$SEP")
  local concat_n=()
  for i in $(seq $#); do
    input_n+=("-i ${!i}")
    ((n = i - 1))

    concat_n+=("[v${n}][${n}:a]")

    local scale_to=""
    scale_to+="[${n}:v]scale=$SCALE:force_original_aspect_ratio=decrease:eval=frame,"
    scale_to+="pad=$SCALE:-1:-1:color=black[v${n}];$SEP"
    scale_to_n+=("$scale_to")
  done
  local filter_complex=""
  filter_complex+="${scale_to_n[*]}"
  filter_complex+="${concat_n[*]}"
  filter_complex+=" concat=n=$#:v=1:a=1 [v] [a]"
  # shellcheck disable=SC2068
  command $FFMPEG ${input_n[@]} -n -filter_complex "$filter_complex" \
    -map "[v]" -map "[a]" -s hd480 -vcodec libx264 "$OUT_FILE"
}

FFMPEG="ffmpeg"
ARGS=()
SEP=""
while [[ $# -gt 0 ]]; do
  case "$1" in
  --scale)
    SCALE="$2"
    shift
    shift
    ;;
  --out)
    OUT_FILE="$2"
    shift
    shift
    ;;
  --debug)
    FFMPEG="echo -e \nffmpeg"
    SEP="\n"
    shift
    ;;
  *)
    ARGS+=("$1")
    shift
    ;;
  esac
done
OUT_FILE=${OUT_FILE:?"--out <file> is undefined"}
SCALE=${SCALE:?"--scale <width:height> is undefined"}
# shellcheck disable=SC2068
join_files ${ARGS[@]}