generate silence with ffmpeg doesn't work

[lenberg]

Hi!
I want to generate a silent audio track with the same duration of a video.
The solution should work for every video file.
I have tried

ffmpeg.exe -i input1.mkv -f lavfi -i anullsrc -vn -c:a flac -shortest silence.flac

ffmpeg.exe -f lavfi -i anullsrc=channel_layout=5.1(side):sample_rate=48000 -i input1.mkv -vn -c:a flac -shortest silence.flac

ffmpeg.exe -f lavfi -i anullsrc=channel_layout=5.1:sample_rate=48000 -i input1.mkv -vn -c:a flac -shortest silence.flac

ffmpeg.exe -f lavfi -i anullsrc=channel_layout=5.1:sample_rate=48000 -i input1.mkv -shortest -vn -c:a flac silence.flac

ffmpeg.exe -f lavfi -i anullsrc=r=48000:cl=1 -i input1.mkv -shortest -vn -c:a flac silence.flac

None worked.
It always creates either a flac with audio in it, or a silent track which duration is longer than the video (I stop the process because it would continue producing silence), it ignores the -shortest flag...

What am I doing wrong?
Thanks.

[jagabo]

Since you are discarding the video -shortest is the length of the generated audio. Output video and audio to an MKV file then extract the audio.

[lenberg]

Thank you for your reply!
Then what should I do so that the video part is as small as possible, so the process is quite quick and resulting file is as small as possible?
something like creating a continuous blank frame...