Video Subtitle Extractor (YouTube + Bilibili)#

Detect platform → download subtitles → clean → save raw → generate summary.

---

Step 1 — Ensure yt-dlp is available#

bash

if ! command -v yt-dlp &>/dev/null; then
  echo "yt-dlp not found, installing..."
  pip install -q yt-dlp || pip3 install -q yt-dlp
fi
yt-dlp -U --quiet 2>/dev/null || true

If installation fails, stop and tell the user to install yt-dlp manually ( pip install yt-dlp or brew install yt-dlp).

---

Step 2 — Detect platform and download subtitles#

Detect whether the URL is Bilibili or YouTube, then use the appropriate strategy.

bash

URL="<user-provided URL>"
TMPDIR=$(mktemp -d)
SUB_FILE=""
SUBTITLE_LANG=""

# Detect platform
if echo "$URL" | grep -qE '(bilibili\.com|b23\.tv)'; then
  PLATFORM="bilibili"
  SITE_NAME="Bilibili"
  SITE_DOMAIN="bilibili.com"
else
  PLATFORM="youtube"
  SITE_NAME="YouTube"
  SITE_DOMAIN="youtube.com"
fi

Bilibili branch#

Bilibili subtitles require login cookies. Always use a cookies file — refresh from Chrome if missing or stale (>30 days):

bash

if [ "$PLATFORM" = "bilibili" ]; then
  BILI_COOKIES="${BILIBILI_COOKIES_FILE:-$HOME/bilibili_cookies.txt}"

  NEED_REFRESH=false
  if [ ! -f "$BILI_COOKIES" ]; then
    NEED_REFRESH=true
  elif [ "$(find "$BILI_COOKIES" -mtime +30 2>/dev/null | wc -l | tr -d ' ')" -gt 0 ]; then
    echo "Bilibili cookies older than 30 days, refreshing..."
    NEED_REFRESH=true
  fi

  if [ "$NEED_REFRESH" = true ]; then
    echo "Reading cookies from Chrome (one-time keychain prompt)..."
    yt-dlp --cookies-from-browser chrome --cookies "$BILI_COOKIES" \
      --skip-download -i "https://www.bilibili.com/" 2>/dev/null
  fi

  COOKIE_ARGS="--cookies $BILI_COOKIES"

  # List available subtitle langs — capture stderr to detect login failure
  LIST_OUTPUT=$(yt-dlp --list-subs $COOKIE_ARGS "$URL" 2>&1)
  if echo "$LIST_OUTPUT" | grep -qi "login\|not logged\|需要登录\|please log"; then
    echo ""
    echo "❌ Bilibili cookies expired or invalid."
    echo "   Fix: delete the cookies file and retry — it will re-read from Chrome."
    echo "   rm \"$BILI_COOKIES\""
    rm -rf "$TMPDIR"
    exit 1
  fi
  AVAIL_LANGS=$(echo "$LIST_OUTPUT" | awk '/^[a-z]/{print $1}' | grep -v "^Language$")

  # Try ai-zh first, then any zh variant, then en
  for lang in ai-zh zh-Hans zh-CN zh en; do
    if echo "$AVAIL_LANGS" | grep -q "^${lang}$"; then
      yt-dlp \
        --write-sub \
        --sub-langs "$lang" \
        --skip-download \
        --retries 3 \
        -o "$TMPDIR/bili_%(id)s" \
        $COOKIE_ARGS \
        "$URL" 2>/dev/null
      SUB_FILE=$(ls "$TMPDIR"/*.${lang}.* 2>/dev/null | head -1)
      if [ -n "$SUB_FILE" ]; then
        SUBTITLE_LANG="$lang"
        break
      fi
    fi
  done
fi

YouTube branch#

bash

if [ "$PLATFORM" = "youtube" ]; then
  for lang in zh-Hans zh-CN zh en; do
    yt-dlp \
      --write-subs \
      --write-auto-subs \
      --sub-langs "$lang" \
      --skip-download \
      --sub-format vtt \
      --retries 3 \
      --sleep-requests 1 \
      -o "$TMPDIR/yt_%(id)s" \
      "$URL" 2>/dev/null
    SUB_FILE=$(ls "$TMPDIR"/*.${lang}.vtt 2>/dev/null | head -1)
    if [ -n "$SUB_FILE" ]; then
      SUBTITLE_LANG="$lang"
      break
    fi
    sleep 1
  done
fi

Fail if no subtitles#

bash

if [ -z "$SUB_FILE" ]; then
  echo "No subtitles found for this video."
  echo "  - No manually uploaded subtitles"
  echo "  - No auto-generated subtitles"
  echo "Cannot proceed without a transcript."
  rm -rf "$TMPDIR"
  exit 1
fi

---

Step 3 — Clean subtitle file → plain text#

Detect format (SRT vs VTT) and clean accordingly:

bash

EXT="${SUB_FILE##*.}"

if [ "$EXT" = "srt" ]; then
  # SRT: remove sequence numbers, timestamps, HTML tags, deduplicate
  grep -v "^[0-9]*$" "$SUB_FILE" \
    | grep -v "^[0-9][0-9]:[0-9][0-9]:[0-9][0-9],[0-9]* --> " \
    | sed 's/<[^>]*>//g' \
    | grep -v "^$" \
    | python3 -c "
import sys, html
seen = set()
for line in sys.stdin:
    line = html.unescape(line).strip()
    if line and line not in seen:
        seen.add(line)
        print(line)
        print()
" > "$TMPDIR/cleaned.txt"
else
  # VTT
  sed 's/<[^>]*>//g' "$SUB_FILE" \
    | grep -v "^WEBVTT" \
    | grep -v "^NOTE" \
    | grep -v "^Kind:" \
    | grep -v "^Language:" \
    | grep -v "^[0-9][0-9]:[0-9][0-9]:[0-9][0-9]" \
    | grep -v "^$" \
    | python3 -c "
import sys, html
seen = set()
for line in sys.stdin:
    line = html.unescape(line).strip()
    if line and line not in seen:
        seen.add(line)
        print(line)
        print()
" > "$TMPDIR/cleaned.txt"
fi

---

Step 4 — Resolve output directory and set filename#

bash

OUTPUT_DIR="${YOUTUBE_SUBTITLES_DIR:-.}"
mkdir -p "$OUTPUT_DIR"

Use the original video title as the filename. Only strip characters illegal on macOS ( / and ASCII :); preserve all other characters including fullwidth punctuation ( ：、 《》、 、). Truncate to 100 chars:

bash

SLUG=$(echo "<title>" | python3 -c "
import sys
title = sys.stdin.read().strip()
title = title.replace('/', '').replace(':', '')
print(title[:100])
")

---

Step 5 — Fetch video metadata#

Use $COOKIE_ARGS (set in Step 2; empty string for YouTube, --cookies FILE for Bilibili):

bash

yt-dlp --dump-json --no-playlist $COOKIE_ARGS "$URL" 2>/dev/null \
  | python3 -c "
import json, sys
d = json.load(sys.stdin)
desc = d.get('description','')
first_para = desc.split('\n\n')[0].replace('\n',' ')[:300]
chapters = d.get('chapters') or []
chapter_lines = '\n'.join(f'  - \"{int(c[\"start_time\"]//60)}:{int(c[\"start_time\"]%60):02d} {c[\"title\"]}\"' for c in chapters)
cats = d.get('categories') or []
print('TITLE:', d.get('title',''))
print('CHANNEL:', d.get('uploader',''))
print('DURATION:', d.get('duration_string',''))
print('DATE:', d.get('upload_date',''))
print('DESCRIPTION:', first_para)
print('CATEGORY:', cats[0] if cats else '')
print('CHAPTERS:')
print(chapter_lines)
"

---

Step 6 — Save raw transcript#

Write $OUTPUT_DIR/$SLUG.md:

bash

NOW=$(date +"%Y-%m-%dT%H:%M")
WORDS=$(wc -w < "$TMPDIR/cleaned.txt" | tr -d ' ')

markdown

---
title: "<title>"
source: "<URL>"
author:
  - "[[<channel>]]"
published: "<YYYYMMDD>"
description: "<DESCRIPTION>"
tags:
  - "<PLATFORM>"
ctime: "<NOW>"
mtime: "<NOW>"
words: "<WORDS>"
site: "<SITE_NAME>"
domain: "<SITE_DOMAIN>"
channel: "<channel>"
duration: "<duration>"
category: "<CATEGORY>"
subtitle_lang: "<SUBTITLE_LANG>"
chapters:
<CHAPTERS or empty>
type: "source"
---

<full cleaned transcript>

---

Step 7 — Check length and prepare summary input#

bash

CHARS=$(wc -c < "$TMPDIR/cleaned.txt" | tr -d ' ')

• ≤ 120000: use full cleaned text as summary input directly.
• > 120000: run map-reduce first (Step 8), then use the combined bullet points as summary input.

---

Step 8 — Map-reduce for long transcripts (> 120k only)#

Split into ~10,000 character chunks:

bash

python3 - <<'EOF'
text = open("$TMPDIR/cleaned.txt").read()
size = 10000
chunks = [text[i:i+size] for i in range(0, len(text), size)]
for i, chunk in enumerate(chunks):
    print(f"=== CHUNK {i+1}/{len(chunks)} ===")
    print(chunk)
EOF

For each chunk, extract structured notes in this format:

text

TOPIC: [inferred topic name for this chunk, e.g. "AI bubble assessment"]
- [key claim or fact — 2 sentences: what was said + supporting detail]
- [key claim or fact — 2 sentences]
...
QUOTES: [1–3 verbatim lines worth preserving]
DATA: [any specific numbers or metrics]

Extract 8–12 entries per chunk. Collect all structured notes, grouped by TOPIC, as the summary input for Step 9. Merge notes under the same topic across chunks before passing to Step 9.

---

Step 9 — Generate summary#

Resolve the summary language:

bash

if [ -n "$YOUTUBE_SUBTITLES_SUMMARY_LANG" ]; then
  SUMMARY_LANG="$YOUTUBE_SUBTITLES_SUMMARY_LANG"
elif [[ "$SUBTITLE_LANG" == zh* ]]; then
  SUMMARY_LANG="zh"
else
  _SYS_LANG="${LANG:-${LANGUAGE:-}}"
  case "$_SYS_LANG" in
    zh*) SUMMARY_LANG="zh" ;;
    *)   SUMMARY_LANG="en" ;;
  esac
fi

Using the summary input from Step 7 or 8, generate the summary body (all sections) and write it to a temp file first:

bash

cat > "$TMPDIR/summary_body.md" <<'SUMMARY_EOF'
<generated summary sections here>
SUMMARY_EOF
SUMMARY_WORDS=$(wc -w < "$TMPDIR/summary_body.md" | tr -d ' ')

Then write the final file $OUTPUT_DIR/$SLUG-summary.md in $SUMMARY_LANG by combining frontmatter + body:

bash

cat > "$OUTPUT_DIR/$SLUG-summary.md" <<EOF
<frontmatter here>
EOF
cat "$TMPDIR/summary_body.md" >> "$OUTPUT_DIR/$SLUG-summary.md"

Frontmatter template:

markdown

---
title: "<title>"
source: "<URL>"
author:
published: "<YYYYMMDD>"
description: "<DESCRIPTION>"
tags:
  - "<PLATFORM>"
ctime: "<NOW>"
mtime: "<NOW>"
words: "<SUMMARY_WORDS>"
site: "<SITE_NAME>"
domain: "<SITE_DOMAIN>"
channel: "<channel>"
duration: "<duration>"
category: "<CATEGORY>"
subtitle_lang: "<SUBTITLE_LANG>"
chapters:
<CHAPTERS or empty>
type: "source"
lang: "<SUMMARY_LANG>"
---

Section headers must match the summary language. Use the headers for the appropriate language:

English ( en): ## Overall Summary / ## Topic Chapters / ## Key Quotes / ## Novel Ideas / ## Counter-intuitive Views / ## Core Tensions / ## Methodology / ## Key Data

Chinese ( zh): ## 总体摘要 / ## 话题章节 / ## 关键引用 / ## 新颖观点 / ## 反直觉观点 / ## 核心张力 / ## 方法论 / ## 关键数据

Voice and attribution#

Write content directly. Never prefix bullets or sentences with attribution phrases like "X says", "X believes", "X points out", "X argues", "according to X", or their equivalents in any language. The speaker is already identified in the frontmatter — repeating their name before every claim is noise. State the idea itself.

Bad: "The speaker believes API businesses have no moat because users have zero loyalty." Good: "API businesses have no moat — users switch instantly to any cheaper or better model."

Depth requirement (applies to ALL bullet points across ALL sections)#

Every bullet point must be 2–3 sentences minimum:

1. Claim: state the specific view or fact
2. Reasoning / evidence: what reasoning, example, or data supports it (quote the transcript if helpful)
3. Nuance or implication: a caveat, consequence, or "so what" the reader needs

One-sentence bullets are not acceptable. If you cannot write 2–3 sentences about a point, the point is too thin to include — omit it rather than padding.

Content rules per section#

• Overall Summary / 总体摘要: 3–5 sentence synthesis of the full content, followed by a " 最值得关注的几个点 / Key Highlights:" bullet list (3–5 items). Each highlight: 2–3 sentences, specific and non-generic, representing the most surprising or high-value insight a busy reader needs.

• Topic Chapters / 话题章节: Identify 4–8 major topics discussed. For each, write a ### [Topic Name] subsection with 3–5 bullets at the 2–3 sentence depth. Topics should reflect the actual conversation flow (e.g. "OpenAI strategy", "AI bubble assessment", "China vs US", "investment philosophy"). Cover all significant topics — do not drop a topic because it seems minor. Even brief or personal moments (a TV show analogy, a hobby mentioned) deserve a short subsection if they carry a meaningful idea.

• Key Quotes / 关键引用: Verbatim quotes from the transcript as blockquotes. Aim for 10–15 quotes; prefer ones that are vivid, specific, or capture a stance in the speaker's own words. After each blockquote, add one sentence of context (who said it, in what context).

• Novel Ideas / 新颖观点: Ideas that are fresh, uncommon, or reframe a familiar concept. Each bullet: state the idea (2–3 sentences including the reasoning behind it). Do not include ideas that are already mainstream.

• Counter-intuitive Views / 反直觉观点: Claims that contradict common belief. Format each bullet as: Common belief: [X] → Actual claim: [Y] — then 1–2 sentences explaining what makes Y non-obvious and what evidence supports it.

• Core Tensions / 核心张力: Opposing forces, unresolved debates, or structural contradictions. Each tension: name both sides (2–3 sentences each), then note whether the speaker resolves it or leaves it open.

• Methodology / 方法论: Frameworks, decision processes, heuristics, or step-by-step approaches. Each entry: describe the framework (what it is, how to apply it, what problem it solves). Sub-bullets are encouraged for multi-step processes.

• Key Data / 关键数据: Specific numbers, statistics, metrics, named comparisons. Include the source context (who said it, about what). Do not fabricate numbers — only include figures explicitly stated in the transcript.

Omit any section where the source has no relevant content. Do not fabricate.

---

Step 10 — Clean up and report#

bash

rm -rf "$TMPDIR"

text

Raw:     $OUTPUT_DIR/$SLUG.md
Summary: $OUTPUT_DIR/$SLUG-summary.md