FFmpeg is very powerful open source multimedia tool. The source code is hosted at https://github.com/FFmpeg/FFmpeg, and the pre-compiled binary version can be found at https://ffmpeg.zeranoe.com/builds/. However, sometimes, it not user friendly, and you have to remember many parameters.
Here are some samples, which I regular used:
Converting video and audio into different format
$ ffmpeg -i input.mp4 output.avi
Compressing video
$ ffmpeg -i input.mp4 -s 640x360 output.mp4
Changing bitrate
$ ffmpeg.exe -i input.avi -b 3000k -maxrate 4000k -bufsize 1835k output.avi
Concating video
$ ffmpeg.exe -f concat -i filelist.txt -c copy concat.avi
filelist.txt
file '1.AVI' file '2.AVI' file '3.AVI'
Remove audio from video file
$ ffmpeg -i input.mp4 -c copy -an output.mp4
Extract audio from video
$ ffmpeg -i input.avi -vn -acodec copy output.aac
Combining audio into video
$ ffmpeg -i video.mp4 -i audio.m4a -c:v copy -c:a copy output.mp4
Cutting the videos based on start and end time
$ ffmpeg -i input.mp4 -ss 00:00:45 -codec copy -t 40 output.mp4
Adding audio into video file on start and time
$ ffmpeg -i video.mp4 -ss 00:04:00 -i audio.mp3 -c copy -t 40 output.mkv
Adding Poster Image to an Audio File
$ ffmpeg -loop 1 -y -i image.jpg -i audio.mp3 -acodec copy -vcode c libx264 -shortest ouput.mp4
Download m3u8 url and save as MP4 file
$ ffmpeg -i "http://url/file.m3u8" ouput.mp4