Node.js Piping-how to write?

I need to figure out how I can give exec permissions to write.

var exec = require('child_process').exec;
exec('ffmpeg -i file.ogv -f mp4 file.mp4', function(err, stdout, stderr){
});
// ffmpeg c开发者_如何学Goonverts one video file to another.

How can I do this?

I used node-fluent to take care of this, it works better/grants permissions.