ffmpeg

trim

ffmpeg -i input.mp4 -ss 00:01:00 -to 00:02:00 -c copy output.mp4

merage

ffmpeg -f concat -i input.txt -c copy output.mp4

input.txt
file '1.flv'
file '2.flv'

flv2mp4

ffmpeg -i input.flv -c copy output.mp4

rename

ls *.mp4 | sed 's/\(.*\).flv\(.*\).*/mv & \1\2/' | sh