How to use FFmpeg ?

FFmpeg
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A complete, cross-platform solution to record, convert and stream audio and video.
- https://ffmpeg.org/

Download
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You can download it very simply.

 1# Debian / Ubuntu
 2sudo apt-get install ffmpeg
 3 
 4# Arch Linux
 5sudo pacman -S ffmpeg
 6 
 7#REHL / CentOS / Fedora
 8sudo dnf install ffmpeg
 9sudo rpm install ffmpeg
10sudo yum install ffmpeg

Using
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Change FPS
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1ffmpeg -i input.avi -filter:v fps=fps=30 output.avi
It’s means change the FPS of file “input.avi” to 30, and output its filename to “output.avi”.

Convert format
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For default method, you can just using like that:
1ffmpeg -i input.mp4 output.avi

Reduce file size
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Calculate the bitrate you need by dividing your target size (in bits) by the video length (in seconds). For example for a target size of 1 GB (one gigabyte, which is 8 gigabits) and 10 000 seconds of video (2 h 46 min 40 s), use a bitrate of 800 000 bit/s (800 kbit/s):
1ffmpeg -i input.mp4 -b 800k output.mp4
Additional options that might be worth considering is setting the Constant Rate Factor, which lowers the average bit rate, but retains better quality. Vary the CRF between around 18 and 24 — the lower, the higher the bitrate.
1ffmpeg -i input.mp4 -vcodec libx264 -crf 20 output.mp4
- Vicky Chijwani on StackExchange
By the way, you are able to change the video format to H.265, it’s better than H.264.
And also you can add arugument fps-fps=30 to specify the FPS to 30, so it will be looks like that:
1ffmpeg -i input.mp4 -vcodec libx265 -crf 20 fps-fps=30 output.mp4