How to create and run a custom task in Phoenix? Like "rake something" in Ruby

You need to create a module that is prefixed by Mix.Tasks., is your taskname with caps and implements Mix.Task-behaviour.

The process in general is explained there as well.

How to manually, from the command line run my task on a server?

If your mix task module is named Mix.Tasks.App.Foo you can run it as mix app.foo.

@NobbZ already provided you great answers, but please take a look on this article. Basically it’s very easy.

Also you have to remember that whenever you would like to use any application, which is project dependency, in the mix task, you have to use ensure_started/2, example of using Repo is also here.

Have you tried to run a task on a server this way?

This is how you run a mix task. If it doesn’t work, please tell us more about your environment.

If by server you mean a computer that runs a release, build for production without mix Bering bundled then of course you can’t run mix tasks. For such scenarios you need to create escripts that connect to your nose and call maintenance functions or do a remsh and call those maintenance functions manually, the latter were the way I’d prefer since it doesn’t require an elixir or Erlang installation besides the one bundled in the release.

If by server you mean something else, please elaborate.

server = production server where my website runs

How exactly should I create a task for a production server ?

So you are running a release there? As I said, write maintenance functions, connect via remote shell and then run the functions.

Mix is usually not available on those systems, since it is a build tool and on the prod you usually do not build.

yes
my question is - how exactly?

It depends of the release tool you’ll use. When using distillery, you have to live without mix at all. Consider this example for replacing Repo migrations.