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#determinate screen resolution
xdpyinfo | grep 'dimensions:'|awk '{print $2}'
#how to grab you display where 0.0, same as the DISPLAY environment variable
ffmpeg -f x11grab -i :0.0 /tmp/out.mpg
#here we grub display, -r will be used to set the output file to 25 fps.
ffmpeg -f x11grab -s `xdpyinfo | grep 'dimensions:'|awk '{print $2}'` -r 25 -i :0.0 -sameq videoOutput.mpg