Script to quickly and automatically remux videos on Synology NAS

I recently set up an old HP ProLiant Microserver N54L as a NAS running Xpenology (Synology DSM 6) and it’s a great little device capable of a lot. It’s also fun to write scripts and configure things to add automation and extra capabilities!

Here’s a script I wrote that will look for mkv and avi files and quickly copy-remux them as mp4. I do this because my playback setup across Apple devices seems to have an easier time with that type of container.

As it’s only the container that is being changed: the video content remain the same so there is no change in quality, the remux process takes only seconds, and the file size remains effectively the same. I also set the faststart flag on the mp4 to help it start playing more quickly.


  1. Find mkv and avi files
  2. Remux the files with ffmpeg to mp4
  3. If successful move the old file to Recycle Bin (so it can be recovered if needed)
  4. Log results to a file and fire off some notifications


I have set this script to run daily using Task Scheduler > Scheduled Task > User-defined script using the full path to the script, which should be located in a Scripts sub-directory inside your video directory.

After the first successful run I decided to comment out all notifications except the failure one, as that’s all I really need to act on. But feel free to set it up as you wish. You could even change it to remux from mp4 to mkv.



I’ve only recently begun to set the faststart movflag on my mp4 files, which meant my older videos did not have it set. So here’s an additional script,, to set the faststart movflag on all mp4 files.

Note: if you have a lot of videos this script will take a long time to complete. You can estimate about 30 seconds per video, for a total of (all your videos divided by two) minutes.

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Comments: @gingerbeardman