FFmpeg CLI to merge a video only mp4 and mp3 audio file

You need to merge video and audio files with ffmpeg when you have separate video and audio tracks that you want to combine into a single multimedia file. This is often necessary when editing videos, as you might need to replace the original audio track with a different one, add a soundtrack, or synchronize audio with existing footage. Merging files allows you to create a complete video file with both visual and sound elements, ready for sharing or playback. 
 

To merge a video-only MP4 file and an MP3 audio file into a single video with FFmpeg, use the following command:

ffmpeg -i video.mp4 -i audio.mp3 -c:v copy -c:a aac -shortest output.mp4

Explanation:

• -i video.mp4: Specifies the video-only input file.
• -i audio.mp3: Specifies the audio input file.
• -c:v copy: Copies the video stream without re-encoding.
• -c:a aac: Encodes the audio in AAC format, which is compatible with MP4.
• -shortest: Ensures the output duration matches the shorter of the two inputs (in case the audio or video is longer than the other).
• output.mp4: The output file with both video and audio.