avprobe <file>

ffmpeg -i input.video -threads 4 -b:v 1M -crf 10 output.webm

mencoder -idx input.video -ovc copy -oac copy -o output.video

convert to mkv

mkvmerge -o output.mkv <inputfile>

convert to mkv and downgrade quality

avconv -i <inputfile> -map 0 -c:v libx264 -crf 20 -c:a copy <outfile>.mkv

concatenate all *.mp4 files to output.mkv

mkvmerge -o output.mkv $(echo *.mp4 | sed "s| | +|g")