Create script for FFMPEG

Job ID: 34498484

Budget: $10 – $30 USD

I need a script for MAC (not Windows) that can do the following batch processing:
Add a 3-second (existing) intro to all files in a folder and copy them to a separate folder.
The videos and the intro are in the mp4 format and the same encoding, so no need for encoding.
The script should be run in a folder called: all-videos
Video files are in a sub-folder called: no_intro
The intro video is in a sub-folder called: intro
Name of the intro video: intro.mp4
Destination folder sub-folder: with_intro (The merged files, with intro, should be copied to this folder)

FFMPEG is installed and I run it in the standard MAC Terminal
Related categories: Mac OS Video Production