I have previously written about how to trim video files with FFmpeg. It is also easy to crop a video file. Here is a short how-to guide for myself and others.
Cropping is not the same as trimming
This may be basic, but I often see the concepts of cropping and trimming used interchangeably. So, to clarify, trimming a video file means making it shorter by removing frames in the beginning and/or end. That is not the same as cropping a video file, which only selects a particular part of the video for export.
If you want to get it done, here is the one-liner:
ffmpeg -i input.avi ffmpeg -i output2.mp4 -vf crop=1920:1080:0:200 output.avi
There is more information about this command in many places (for example, here). the most critical parts of the above command are:
-vf is the command for “video filter”. It can also be spelt out as
crop=1920:1080:0:200 means to make a 1920x1080 video starting from the leftmost side of the source image (=0 pixels horizontally) and 200 pixels down from the top.
Of course, this command can be combined with other FFmpeg combinations.