MKVToolNix

Video walkthrough: YouTube

A toolkit for working with Matroska (.mkv) files — merging, splitting, editing metadata, extracting tracks. No re-encoding, so changes are fast and lossless.

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Install

sudo apt install mkvtoolnix mkvtoolnix-gui

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The four tools

Tool	Purpose
mkvmerge	Combine video, audio, subtitle tracks into one MKV. The workhorse.
mkvextract	Pull individual tracks out of an MKV (audio, subs, attachments).
mkvinfo	Inspect an MKV's structure and metadata.
mkvpropedit	Edit metadata (title, language, default flags) in place — no re-encode.

Plus MKVToolNix GUI (mkvtoolnix-gui) — graphical front-end, easiest for one-off jobs.

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Common tasks

Merge a video with separate audio and subtitle files:

mkvmerge -o output.mkv video.mkv audio.aac subtitles.srt

Extract a subtitle track (track ID from mkvinfo):

mkvextract tracks input.mkv 2:subs.srt

Set a track's default-flag without re-encoding:

mkvpropedit input.mkv --edit track:a1 --set flag-default=1

Inspect file structure:

mkvinfo input.mkv

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When it's the right tool

• Bundling movie + multiple subtitle/audio tracks into one file
• Splitting long recordings without re-encoding
• Fixing track language/title metadata that media players read wrong
• Pulling audio out of a video for separate editing

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Related

• My Setup — recording and editing rig