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I have some videos (.mp4) that have been tagged with metadata (descriptions, artwork, actors etc) and I now have subtitle files for them that I want to add as a stream.

I can use ffmpeg and set both the video and the .srt file as inputs and get a new output file but the output always seem to lose all of the container level metadata and it sometimes gets annoying always going back and deleting the original files.

Is there a way that I can add the subtitle file as a new stream to the video file without creating a new file? Just by modifying the original video file?

Programs like Subler seem to do this but, out of interest, can it be done from the command line?

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You cannot edit a file in-place with ffmpeg – it always needs to create a new output file. I think that's the default for almost any program, and in many cases the output file needs to be seekable and editable by the program during writing.*

If the real issue is that you lose metadata, try:

ffmpeg -i input.mp4 -i subtitles.srt \
-c copy -c:s mov_text \
-map 0 -map 1 -map_metadata 0 output.mp4

You need to have ffmpeg convert SRT (SubRip) to MP4-compliant subtitles with -c:s mov_text, otherwise ffmpeg refuses to copy the stream over.**

The -map_metadata option should copy over all global metadata from the input MP4 file to the output, i.e. anything you set at the container level. The bitstream-level metadata should be copied automatically without any further options.

* Of course, programs like sponge from moreutils can "soak" up input and overwrite a file in-place, but this only works with muxers that support non-seekable output, and therefore not with MP4.
** There are problems with mov_text encoded subtitles and the QuickTime player, see FFmpeg ticket #1845 and also #2488.

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  • Ah good. I didn't know you could map metadata like that. That will be very handy, although it is annoying that you can't just 'inject' a stream. You say that only works with muxers that support non-seekable output, and therefore not with MP4 so how can Subler do this with mp4s? When I add a subtitle stream to a mp4 using that it takes less than two seconds, so it cant be creating a whole new file and deleting the old...?
    – Sam
    Jul 30 '13 at 20:32
  • I think this is a design choice, and ffmpeg developers never intended to do that. Of course, I don't know what subler does – I haven't looked at its source. It could also be using temporary files, deleting the original, and replacing it with the new one. Since bitstream copies are very fast you wouldn't see the difference. I think MP4Box can also do in-place edits, but I always forget its option syntax… haven't really used it a lot.
    – slhck
    Jul 30 '13 at 20:45
  • Using process substitution and a simple cat, I was able to modify a file in place. Seems fast and to work great. Is this problematic in anyway?
    – rien333
    Oct 30 '18 at 12:28
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    @rien333 Depends on when the output file is being written in relation to the input being read. I can imagine that this depends on the output container as well (for some formats it must be seekable). I would not generally advise to do so, but instead use a temporary file. See also my first footnote in the post above.
    – slhck
    Oct 30 '18 at 12:31

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