I have 15 video files which I want to merge with ffmpeg. I want to save time by merge all of the video files in a single command line, not cut it one by one and merge it. Thanks.
Use the concat demuxer.
First, create a text file with the filenames.
file '1.mp4' file '2.mp4' file '3.mp4' ... file '13.mp4' file '14.mp4' file '15.mp4'
Then, run the concat command.
ffmpeg -f concat -i textfile -c copy -fflags +genpts merged.mp4
For this to work, all videos should have same properties such as codec, resolution, framerate, sample rate, etc.
If they are not, you can encode the concat.
ffmpeg -f concat -i textfile -fflags +genpts merged.mp4
Also see FFmpeg Wiki: Concatenate.