Cut video file with ffmpeg

ffmpeg -i input.mp4 -ss 00:00:00 -t 00:06:00 -c copy output.mp4

(ss = start t = length)

and then use 

concatenate

to paste small videos together

batch convert video files with ffmpeg

for i in *.mkv; do ffmpeg -i "$i" -c:v libx265 -c:a aac -b:a 160k "${i%.*}-H265.mkv"; done