Media Mkvtoolnix

Media Mkvtoolnix

Context: $ARGUMENTS

MKVToolNix authors Matroska natively. Four tools: mkvmerge (mux/split), mkvextract (demux), mkvpropedit (in-place metadata edit), mkvinfo (inspect). Never re-encodes — stream-copy only. For re-encoding use ffmpeg; for MKV → MP4 use ffmpeg (MKVToolNix cannot write MP4).

Quick start

• Merge tracks into one MKV: → Step 3 (merge recipe)
• Split by time / size: → Step 3 (split recipe)
• Extract a track (audio/subs/attachment): → Step 3 (extract recipe)
• Edit language / default flag in place: → Step 3 (propedit recipe)
• Inspect / verify: → Step 4

When to use

• You need lossless MKV editing (splitting, merging, metadata).
• You want to change track language, name, default/forced flag without re-muxing.
• You need to extract raw tracks (h264/ac3/srt) for re-authoring.
• You need to add/replace fonts or cover art attachments.
• Skip if: re-encoding required, or output container must be MP4/MOV/WebM-not-Matroska.

Step 1 — Install

# macOS
brew install mkvtoolnix

# Debian/Ubuntu
sudo apt install mkvtoolnix

# Verify
mkvmerge --version && mkvextract --version && mkvpropedit --version && mkvinfo --version

Step 2 — Pick the right tool

Get track IDs first: mkvmerge --identify in.mkv (or -J in.mkv for JSON).

Step 3 — Run

Merge (mkvmerge)

# Combine raw essences into an MKV
mkvmerge -o out.mkv video.h264 audio.ac3 subs.srt

# Merge with language + title metadata
mkvmerge -o out.mkv \
  --language 0:eng --track-name 0:"Main Video" video.h264 \
  --language 0:jpn audio.ac3 \
  --language 0:eng subs.srt \
  --title "My Movie"

# Append (concatenate) MKVs: use `+` between files
mkvmerge -o out.mkv a.mkv + b.mkv + c.mkv

# Re-mux / defragment / strip oddities
mkvmerge -o clean.mkv messy.mkv

# Convert MP4 -> MKV
mkvmerge -o out.mkv in.mp4

Split (mkvmerge --split)

# Split at absolute timestamps
mkvmerge -o part.mkv --split timestamps:00:10:00,00:20:00 in.mkv

# Split by size
mkvmerge -o part.mkv --split size:500M in.mkv

# Keep only selected ranges (parts: absolute)
mkvmerge -o out.mkv --split parts:00:00:00-00:10:00,+00:15:00-00:25:00 in.mkv

# Split every N frames
mkvmerge -o part.mkv --split frames:2500 in.mkv

# Split at chapter boundaries
mkvmerge -o part.mkv --split chapters:all in.mkv

Output name template: --split writes part-001.mkv, part-002.mkv, ...

Extract (mkvextract)

# Extract raw tracks (track_id:output_file)
mkvextract tracks in.mkv 0:video.h264 1:audio.ac3 2:subs.srt

# Extract chapters (XML by default, --simple = OGM)
mkvextract chapters in.mkv > chapters.xml
mkvextract chapters --simple in.mkv > chapters.txt

# Extract global tags
mkvextract tags in.mkv > tags.xml

# Extract attachments (attachment_id:output_file — IDs from mkvmerge -J)
mkvextract attachments in.mkv 1:font.ttf 2:cover.jpg

# Extract cues, timestamps, CUE sheet (advanced)
mkvextract timestamps_v2 in.mkv 0:video_ts.txt

Edit metadata in place (mkvpropedit)

# Set track language + name
mkvpropedit in.mkv --edit track:v1 --set "language=eng" --set "name=Main Video"

# Set default flag on audio track 2 (a2 = second audio track)
mkvpropedit in.mkv --edit track:a2 --set "flag-default=1"

# Unset default flag on audio track 1
mkvpropedit in.mkv --edit track:a1 --set "flag-default=0"

# Set forced flag on a subtitle track
mkvpropedit in.mkv --edit track:s1 --set "flag-forced=1"

# Replace chapters from XML
mkvpropedit in.mkv --chapters chapters.xml

# Delete all chapters
mkvpropedit in.mkv --chapters ""

# Delete all tags
mkvpropedit in.mkv --tags all:""

# Add / replace / delete attachments
mkvpropedit in.mkv --add-attachment font.ttf --attachment-mime-type application/x-truetype-font
mkvpropedit in.mkv --replace-attachment 1:new.ttf
mkvpropedit in.mkv --delete-attachment 1

# Set container title
mkvpropedit in.mkv --edit info --set "title=My Movie"

Track selector syntax: track:v1 (first video), track:a2 (second audio), track:s1 (first subtitle), track:@N (track with MKV track number N), track:=UID (track UID).

Step 4 — Verify with mkvinfo

# Human-readable
mkvinfo in.mkv

# With size info + UTF-8
mkvinfo --output-charset UTF-8 -p in.mkv

# Programmatic: use mkvmerge JSON
mkvmerge -J in.mkv | jq '.tracks[] | {id, type: .type, codec: .codec, lang: .properties.language, default: .properties.default_track, forced: .properties.forced_track}'

Gotchas

• MKVToolNix is not ffmpeg. It reads/writes Matroska natively, preserves metadata exactly, and is often faster for MKV-in/MKV-out work. But it cannot re-encode and cannot write non-Matroska containers.
• Never re-encodes. Every mkvmerge operation is stream-copy. If source codecs can't live in MKV, mkvmerge errors — don't expect implicit conversion.
• Splits are key-frame-aligned. The actual cut may be 1–2 frames off the requested timestamp. For frame-exact cuts you must re-encode via ffmpeg.
• --split value format is strict:
  • size:500M (K/M/G suffixes)
  • timestamps:HH:MM:SS.nnn,HH:MM:SS.nnn (absolute)
  • parts:START-END[,+START-END...] (+ glues into one file, no + = new file)
  • frames:N / chapters:all / chapters:N,M
• mkvpropedit edits in place without re-muxing — extremely fast, no temp file.
• Default / forced flags are MKV conventions:
  • flag-default=1 → "play this one if multiple in same type."
  • flag-forced=1 → "player MUST render this" (forced subs, burn-in translation).
  • Use 0 to unset. Only one track per type should be default.
• Language tags: MKVToolNix accepts BCP-47 (en, ja, pt-BR) and ISO 639-2 three-letter (eng, jpn, por). New versions prefer BCP-47. Use three-letter for max compatibility with older players.
• + in mkvmerge appends files (concatenates). Codecs/resolutions/timebases must match or the append fails — normalize with ffmpeg first if mismatched.
• Track IDs come from mkvmerge --identify in.mkv (or -J for JSON). They are 0-indexed and distinct from the v1/a1/s1 selector used by --edit.
• Attachments are preserved across mkvmerge -o out in.mkv. Strip them with --no-attachments. Common MIME types: application/x-truetype-font, application/x-opentype-font, image/jpeg, image/png.
• MP4 → MKV works (mkvmerge -o out.mkv in.mp4) but strips some MP4-specific atoms. MKV → MP4 is NOT supported — use ffmpeg.
• Chapter XML schema is specific. Generate via mkvextract chapters in.mkv > template.xml and edit, rather than writing from scratch.
• --title "Name" sets the container-level title (seen in players). --chapter-language eng sets chapter language metadata.
• WebM is a Matroska subset. mkvmerge handles WebM; to produce WebM use --webm or output to .webm.
• Segment linking (--link / --segment-linking) creates chained MKVs referring to each other by UID. Advanced; most players don't follow the links.

Examples

Example 1: anime fansub — merge video, two audio, two subtitle tracks + fonts

mkvmerge -o episode.mkv \
  --language 0:jpn --track-name 0:"Video" video.h264 \
  --language 0:jpn --track-name 0:"Japanese" --default-track 0:yes audio_jpn.flac \
  --language 0:eng --track-name 0:"English Dub" --default-track 0:no audio_eng.ac3 \
  --language 0:eng --track-name 0:"English Subs" --default-track 0:yes subs_eng.ass \
  --language 0:eng --track-name 0:"Signs & Songs" --forced-track 0:yes --default-track 0:no subs_signs.ass \
  --attachment-mime-type application/x-truetype-font --attach-file font1.ttf \
  --attachment-mime-type application/x-truetype-font --attach-file font2.otf \
  --title "Show Name - Ep 01"

Example 2: split a season archive into 4 GB chunks

mkvmerge -o season01-%03d.mkv --split size:4000M season01-full.mkv

Example 3: correct a wrong language tag without re-muxing

mkvmerge -J wrong.mkv | jq '.tracks[] | {id, type: .type, lang: .properties.language}'
# Say audio track (a1) is labeled "und" but should be "jpn":
mkvpropedit wrong.mkv --edit track:a1 --set "language=jpn"

Example 4: replace attached fonts in an ASS-subtitled MKV

# Inspect existing attachments
mkvmerge -J in.mkv | jq '.attachments[] | {id, name: .file_name, mime: .content_type}'

# Replace attachment ID 1, delete ID 2, add a third
mkvpropedit in.mkv \
  --replace-attachment 1:new-regular.ttf \
  --delete-attachment 2 \
  --add-attachment new-italic.ttf --attachment-mime-type application/x-truetype-font

Example 5: keep only chapters 2 and 5 of a movie

mkvmerge -o keep.mkv --split parts:00:12:30-00:24:10,+00:58:00-01:12:00 movie.mkv

Example 6: strip all chapters and tags from a release

mkvpropedit release.mkv --chapters "" --tags all:""

Available scripts

• scripts/mkv.py — argparse wrapper around mkvmerge, mkvextract, mkvpropedit, mkvinfo. Subcommands: identify, merge, split-time, split-size, extract-tracks, extract-chapters, extract-attachments, edit, default-flag, add-attachment, replace-chapters. Flags: --dry-run, --verbose. Stdlib only.

# Identify tracks (JSON)
python3 ${CLAUDE_SKILL_DIR}/scripts/mkv.py identify --input in.mkv

# Merge
python3 ${CLAUDE_SKILL_DIR}/scripts/mkv.py merge \
  --output out.mkv --inputs v.h264 a.ac3 s.srt \
  --title "My Movie" --languages eng,jpn,eng

# Split by time
python3 ${CLAUDE_SKILL_DIR}/scripts/mkv.py split-time \
  --input in.mkv --output-pattern part.mkv --times 00:10:00,00:20:00

# Flip default flag
python3 ${CLAUDE_SKILL_DIR}/scripts/mkv.py default-flag \
  --input in.mkv --track a2 --value 1

Reference docs

• Read references/mkvtoolnix.md for full split syntax, track selector grammar, flag reference (default/forced/hearing-impaired/commentary), language code tables, chapter XML schema, MIME types, and recipe book (fansub workflow, season archival, font embedding, language correction).

Troubleshooting

Error: The file could not be opened for reading: Wrong Matroska version

Cause: File isn't Matroska, or is corrupt. Solution: ffprobe file.mkv to confirm. If MP4 mis-named, rename. If corrupt, try mkvmerge -o recovered.mkv --engage keep_bitstream_av1 damaged.mkv.

Error: The track parameters do not match

Cause: Appending (+) two files whose codecs/resolution/timebase differ. Solution: Normalize with ffmpeg first: ffmpeg -i a.mkv -c copy a-norm.mkv won't help — you must re-encode to matching params: ffmpeg -i a.mkv -c:v libx264 -c:a aac a-norm.mp4.

mkvpropedit says Nothing to do but I specified --set

Cause: Track selector didn't match. Selectors are track:v1, track:a1, track:s1 (type+index), not 0-indexed track IDs. Solution: Verify with mkvmerge -J in.mkv and use the right selector form.

Splits land a frame or two off the requested timestamp

Cause: Splits are key-frame aligned; mkvmerge never re-encodes. Solution: For frame-exact cuts, re-encode with ffmpeg seeking, or add a keyframe at the target timestamp via ffmpeg -force_key_frames before splitting.

Fonts in attached MKV don't render in players

Cause: Wrong MIME type or player doesn't auto-load MKV attachments. Solution: Use application/x-truetype-font for .ttf and application/x-opentype-font for .otf. VLC / mpv autoload; some hardware players do not — burn subs in via ffmpeg subtitles= filter instead.

mkvmerge -o out.mp4 in.mkv fails

Cause: MKVToolNix writes only Matroska/WebM. It cannot produce MP4. Solution: ffmpeg -i in.mkv -c copy out.mp4.