Simple script to convert mp3 files to aac with size limit (generated by chatgpt)

convert_mp3_to_aac.sh

#!/bin/bash

LIMIT_MB=199
LIMIT_BYTES=$((LIMIT_MB * 1024 * 1024))
JOBS=8

mkdir -p out

convert_file() {
  local f="$1"

  local size_bytes
  size_bytes=$(stat -f%z "$f")

  if (( size_bytes <= LIMIT_BYTES )); then
    echo "OK (copy): $f"
    cp "$f" "out/$f"
    return
  fi

  local duration
  duration=$(ffprobe -v error -show_entries format=duration \
    -of default=noprint_wrappers=1:nokey=1 "$f")

  local bitrate
  bitrate=$(echo "$LIMIT_BYTES * 8 / $duration / 1000 * 0.95" | bc)

  local out_file="out/${f%.mp3}.m4a"

  while true; do
    echo
    echo "Encode: $f → ${bitrate} kbps"

    ffmpeg -y -i "$f" \
      -map_metadata 0 -vn \
      -c:a aac_at \
      -b:a ${bitrate}k \
      -stats -nostdin \
      "$out_file"

    local out_size
    out_size=$(stat -f%z "$out_file")

    if (( out_size <= LIMIT_BYTES )); then
      echo "OK: $f ($(echo "$out_size/1024/1024" | bc) MB)"
      break
    fi

    echo "Too big, retry..."
    bitrate=$(echo "$bitrate * 0.95" | bc)
  done
}

export -f convert_file
export LIMIT_BYTES

running=0

for f in *.mp3; do
  [ -f "$f" ] || continue

  convert_file "$f" &

  ((running++))
  if (( running >= JOBS )); then
    wait -n
    ((running--))
  fi
done

wait