That said, if you're just looking for a simple one-liner you can paste into the terminal, my go-to is this one:
cat *.jpg *.JPG *.jpeg | ffmpeg -r .5 -i - -vf "scale=w=1280:h=720:force_original_aspect_ratio=2,crop=1280:720" -y slideshow.mp4
Is there a sane way to extend this out to check if a directory exists, and if not then download and unzip a file to create that directory?
The problem I always have with make and bash is that, for example, I only ever want two possible states-- the successfully unzipped directory or no state at all. And the more steps I add the more little state boogers my build system acquires (e.g., partial download, dangling zip file, etc.)