Here’s a quick tip on how to combine multiple separate videos into a single one, using ffmpeg.
First, create a text file that has the filenames for all your different fragments.
$ cat files.txt
file 'file 1.mkv'
file 'file 2.mkv'
file 'file 3.mkv'
file 'file 4.mkv'
All the files in files.txt will be concatenated to a single output file. Add/remove lines as needed.
Now, use ffmpeg to combine them all.
$ ffmpeg -f concat -safe 0 -i files.txt -c copy output.mkv
[...]
[matroska,webm @ 0x7fc2cb800600] Auto-inserting h264_mp4toannexb bitstream filter/s speed= 450x
frame=103454 fps=11333 q=-1.0 Lsize= 460025kB time=01:08:58.30 bitrate= 910.6kbits/s speed= 453x
video:398150kB audio:59680kB subtitle:0kB other streams:0kB global headers:0kB muxing overhead: 0.479438%
The end result, if this was successful, is an output.mkv file that has all fragments combined.