[FFmpeg-user] Can anyone explain why need append "</dev/null &" at the end of command for ffmpeg working in background?

[littlebat]

Hi,

I try to convert video using ffmpeg in background, but it failed, only
get 44 bytes result file. The command as below:

<code>
ffmpeg -y -i /home/mdx/pipetest/t1.mkv \
-vcodec h263 -b 200k -r 15 -s 176x144 -aspect 11:9 \
-acodec libopencore_amrnb \
-ac 1 -ar 8000 -ab 10.2k -f 3gp 1.3gp & 
</code>

But, when I replace "&" with "</dev/null &", it works. 

Can anyone explain why need append "</dev/null &" at the end of command
for ffmpeg working in background?

thanks.

littlebat