webm

copy video codec

ffmpeg -i input.mkv -c:v copy -c:a libopus output.webm

two pass

ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 0 -crf 30 -pass 1 -an -f null /dev/null && \
ffmpeg -i input.mp4 -c:v libvpx-vp9 -b:v 0 -crf 30 -pass 2 -c:a libopus output.webm

single pass

Note: vp9 is designed for a 2-pass encoding so using a single pass will give sub-optimal compression, but it can be done as so in a pinch:

ffmpeg -i <oldfile> -c:a libopus \
  -c:v libvpx-vp9 -crf <quality> \
  -b:v 0 -pix_fmt yuv420p -movflags faststart <newfile>

lossless

ffmpeg -i input.mp4 -c:v libvpx-vp9 -lossless 1 output.webm

trim

To trim a video, first seek to the correct position in the file with -ss then using the -t option to set a duration. Alternatively, and often much easier to use is the -to option which is to indicate a position in time to stop reading.

The only other key detail is to use the options to simply copy the existing video and audio codecs without any conversion.

ffmpeg -ss 00:01:00 -to 00:02:00 -i input.mp4 -c copy output.mp4

accurate trimming

The above example is fine if you're just quickly extracting a clip from a movie or something, but if you need perfect accuracy you need to actually re-encode the video. You can of course re-encode it to a lossless format and you don't have to re-encode the audio, but something like this would trim off the first 193 frames accurately:

ffmpeg -ss 193 -i vid.mp4 -c:v libx264 -crf 20 -c:a copy -t 4 accurateoutput.mp4

screen record

xorg

ffmpeg -f x11grab -framerate 30 -i :0.0 out.mpg

combine

$ cat mylist.txt
file '/path/to/file1'
file '/path/to/file2'
file '/path/to/file3'

$ ffmpeg -f concat -i mylist.txt -c copy output.mp4