[FFmpeg-user] error handling for map_channel

[Elliott Balsley]

I've been using ffmpeg to batch process lots of files by putting it in a find -execdir command.  The source clips I'm encoding have varying numbers of audio tracks, and I always want to encode only the first two.  Below is my command, which is working beautifully...

find $source_dir -maxdepth 1 -iname "*.mov" -execdir ffmpeg -i "{}" -pix_fmt yuv420p -vf scale=hd720 -acodec libfdk_aac -b:a 128k -map_channel 0.1.0 -map_channel 0.1.1 -vcodec libx264 -preset veryfast -crf 18 -level 31 H.264/"{}" \;

But when it comes to a clip that has fewer than two audio tracks, it fails with this error:
mapchan: stream #0.1 is not an audio stream.

In this case, I want the encode to have the same number of channels as the source (either 0 or 1).  Is there some way to make ffmpeg handle this?