Last month, I needed to add a periodic job to a simple Rails app to snapshot some data and send it to another system. I discussed the design with a colleague, a Rails expert, who went into the pros and cons of various worker-queue configurations. In the end, I added a hidden API call, and deployed a shell script along the lines of

  while true; do
    curl https://rails.app/do/the/thing
    sleep 3600
  done

The actual version is slightly larger because of error checking etc., but that's the gist. So that's one example of replacing (or rather: avoiding) a distributed system (an asynchronous job queue) with a small shell script.

I don't mean to say that shell scripts always can replace more complex systems, but sometimes they're enough.