Convert webm to mp4

This worked really well for me: ffmpeg -i screen-capture.webm -c:v copy -c:a aac -b:a 128k output.mp4.

Merge 2+ mp4 files together with ffmpeg

BTW if you also wanted to concat two or more mp4 files you can do the following:

1. Create a text file called inputs.txt.

2. Use file command + each mp4 file path. E.g.

   file '/home/userName/first.mp4'
   file '/home/userName/second.mp4'
   

   Remember that the order here matters. In this example it will append the second.mp4 to first.mp4.

3. cd to where this inputs.txt is located and execute this command:

   ffmpeg -f concat -safe 0 -i inputs.txt -c copy output.mp4