I'm looking to batch-convert a directory (or several) of video files from .avi to .mp4 containers, just copying the video and audio streams across. I know the ffmpeg command to do this, and have no trouble doing the files individually - but I'm a noob, hopelessly unfamiliar with ash (as opposed to bash) and its file-handling and looping procedures, which is making it tough to write a shell script. (It has to run in ash 'cos it's running on an underpowered Synology NAS.)

Anyone got any hints? Ta!