There are several ways to "output to null" in FFmpeg (like a null sink). This can be helpful for benchmarking purposes, for instance testing encoding speeds (without having to worry about the cost of disk writes) etc.

There is the null muxer, use it like this:

         ffmpeg -i INPUT -f null out.null # file out.null will not be created, no encoding will occur

or just output to /dev/null (or the reserved filename NUL in windows) for your file to just disappear, if you want to perform encoding but not create a file

     ffmpeg -i INPUT -f mpeg4 -f nut -y /dev/null # no file is created, encoding occurs and packaging occurs

See also the "-benchmark" parameter.