Suggestion to the author: A demo link would be great (doesn't need to actually send mails) – but just clicking through the UI would help a lot to understand the scope and features of a product.
mkdir listmonk-demo
sh -c "$(curl -fsSL https://raw.githubusercontent.com/knadh/listmonk/master/install-demo.sh)"
That said, I would personally recommend inspecting the bash script first before running it. The script in this case is not malicious, but it's a good habit to have.
I guess I'm just an old fuddy-duddy, and if you feel it's OK, knock yourself out. But I really think software developers should know better than to rely on this practice simply because it makes their install process into a one-liner. I'm sure they could do better than recommending users execute random scripts from the intarwebs, sight-unseen.
Surely, when downloading a piece of software from a (quasi?) trusted source, no developer is going to read all the lines of source code before running it. And if that's the case, how is downloading and running an .sh from the same source any different?
wget https://raw.githubusercontent.com/knadh/listmonk/master/docker-compose.yml
docker-compose up demo-db demo-app
Now, should you run some random docker image(s)? Maybe not. But it's easier to see what's going on - maybe you spin up a vm and run docker there, isolated?
Docker images are slightly isolated. By default running a random one doesn't have access to your local filesystem or anything listening on your machine, unless you explicitly ask for it ( which a random docker-compose.yaml or bash script around a docker-compose.yaml and up can).
Software from other sources: not so much. Maybe run it in a VM.
Look, I've used such commands myself - once upon a time that was the recommended way to install LetsEncrypt, I think. But I look at the script first.
So I still think these are pretty high requirements to test a product out. Especially since this is a tool being used by non-IT people (E-Mails/Marketing). Might be helpful to have a demo, to help convince people who don't know what docker is, but can still vote/decide/argue for/against certain software solutions.