Wanna merge MP4?

concat demuxer

Use this method when you want to avoid a re-encode and your format does not support file-level concatenation (most files used by general users do not support file-level concatenation).

$ cat mylist.txt file '/path/to/file1' file '/path/to/file2' file '/path/to/file3' $ ffmpeg -f concat -safe 0 -i mylist.txt -c copy output.mp4

source:
https://stackoverflow.com/questions/7333232/how-to-concatenate-two-mp4-files-using-ffmpeg/11175851#11175851