Re-assemble HLS segments

hls-concat

1) If applicable, list available formats for this asset.
youtube-dl -F <url>

... then get URL to m3u8 for selected format
youtube-dl -f <format name> -g <url>

2) Download all the segments in the m3u8. Now you have a bag of transport stream segments.
The file names are probably increasing but not necessarily sequentially.
You can do the download with youtube-dl (which automatically re-assembles the results),
but it doesn't support parallel operation. You might find that parallelizing this download
is desirable, depending on where it's coming from.

3) Do a numeric sort (uzing zsh) of the downloaded file names, placing the result into a batch file for ffmpeg.
Edit the glob expression as needed to match only the clips you just downloaded.
cd <path of downloads>
rm concat.txt
echo highlight*ts(n) | tr ' ' '\n' | while read f ; do echo "file '$PWD/$f'" >> concat.txt ; done

Did you catch that? We're using the sort behavior from zsh "(n)", so switch to zsh temporarily if you don't already use it.

4) Pick an appropriate file extension for the media at hand.
Use ffprobe <segment path> to see what's inside:
ffprobe -loglevel 0 -show_format <file>

5) Use ffmpeg to concat a new file from the segments.
ffmpeg -f concat -safe 0 -i concat.txt -c copy <output file>

... we're using 'unsafe' mode here because ffmpeg considers absolute paths
in the concat.txt file to be *less* safe than relative paths, which is... backwards :)