Gets the beginning of a mp3 file and looks for chapters until it finds them

get-mp3-chapters.sh

#!/usr/bin/env bash
# fetch the smallest possible head of an MP3 to list chapters (HH:MM:SS)
# deps: curl, ffprobe (ffmpeg), jq, column, awk
set -euo pipefail

usage() {
  cat <<EOF
Usage: $0 <mp3_url> [--json] [--max-mb N] [--quiet]
  --json        Output chapters as JSON (seconds), not a table
  --max-mb N    Stop after N megabytes (default: 32)
  --quiet       Suppress progress messages
EOF
}

if [ $# -lt 1 ]; then usage; exit 1; fi

URL="$1"; shift || true
JSON_OUT="no"
MAX_MB=32
QUIET="no"
while [ $# -gt 0 ]; do
  case "$1" in
    --json) JSON_OUT="yes" ;;
    --max-mb) shift; MAX_MB="${1:-32}" ;;
    --quiet) QUIET="yes" ;;
    -h|--help) usage; exit 0 ;;
    *) echo "Unknown arg: $1" >&2; usage; exit 2 ;;
  esac
  shift || true
done

TMP="$(mktemp -t mp3head_chaps.XXXXXX).mp3"
HDR="$(mktemp -t mp3head_hdr.XXXXXX)"
trap 'rm -f "$TMP" "$HDR"' EXIT

# 1 MB chunk size
MB=$((1024*1024))
chunk_mb=1
start_byte=0
end_byte=$((chunk_mb*MB - 1))

say() { [ "$QUIET" = "yes" ] || echo "$@"; }

fetch_range() {
  local s="$1" e="$2"
  : > "$HDR"
  # Append bytes s..e to TMP
  curl -fsSL --range "${s}-${e}" -D "$HDR" "$URL" -o - >> "$TMP" || return $?

  # Detect if server honored range (206) or sent full file (200)
  local code
  code="$(awk 'BEGIN{c=""} /^HTTP\/[0-9.]+ /{c=$2} END{print c}' "$HDR")"
  if [ "$code" = "200" ]; then
    say "(server ignored range request; likely sent full file)"
    # No point continuing — we already have the entire response
    return 200
  fi
  if grep -qi '^HTTP/.* 416' "$HDR"; then
    return 416  # past EOF
  fi
  return 0
}

have_chapters() {
  ffprobe -v error -print_format json -show_chapters "$TMP" \
    | jq -e '.chapters | length > 0' >/dev/null
}

print_chapters_table() {
  ffprobe -v error -print_format json -show_chapters "$TMP" \
    | jq -r '
        def two(n): ("0\((n|floor))" | (.[-2:] // .));
        def hms(t):
          (t|tonumber|floor) as $s
          | ($s/3600|floor) as $h
          | (($s%3600)/60|floor) as $m
          | ($s%60|floor) as $sec
          | (two($h) + ":" + two($m) + ":" + two($sec));
        .chapters[]
        | [hms(.start_time), hms(.end_time), (.tags.title // .tags.TIT2 // "")]
        | @tsv
      ' \
    | awk 'BEGIN{print "start_time\tend_time\ttitle"}1' \
    | column -t -s $'\t'
}

print_chapters_json() {
  ffprobe -v error -print_format json -show_chapters "$TMP" \
    | jq '.chapters
          | map({start: (.start_time|tonumber),
                  end:   (.end_time|tonumber),
                  title: (.tags.title // .tags.TIT2 // "")})'
}

# Incrementally fetch until chapters appear or we hit the cap
while : ; do
  say "Fetching bytes ${start_byte}..${end_byte} (chunk: ${chunk_mb} MB; total: $((end_byte+1)) bytes)…"
  if ! fetch_range "$start_byte" "$end_byte"; then
    rc=$?
    if [ $rc -eq 200 ]; then
      # Full content delivered despite range; proceed to parse once.
      :
    elif [ $rc -eq 416 ]; then
      say "Reached end of file without finding chapters."
      break
    else
      echo "Download failed (curl exit $rc)." >&2
      exit $rc
    fi
  fi

  if have_chapters; then
    say "—— Chapters ——"
    if [ "$JSON_OUT" = "yes" ]; then
      print_chapters_json
    else
      print_chapters_table
    fi
    exit 0
  fi

  # If server ignored range (got full file), no point continuing
  if grep -q '^HTTP/.* 200' "$HDR"; then
    say "No chapter markers found."
    exit 3
  fi

  # Next chunk
  chunk_mb=$((chunk_mb + 1))
  if [ $chunk_mb -gt $MAX_MB ]; then
    echo "Gave up after ${MAX_MB} MB without finding chapters." >&2
    exit 4
  fi
  start_byte=$((end_byte + 1))
  end_byte=$((end_byte + MB))
done

# If we exit the loop without chapters:
say "No chapter markers found."
exit 3