# Running code when a script exits

**URL:** <https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340>\
**Category:** Questions\
**Tags:** mix-run\
**Created:** [June 14, 2020, 6:08am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340 "2020-06-14T06:08:30Z")\
**Posts on this page:** 20\
**Page:** 1

<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 14, 2020, 6:08am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/1 "2020-06-14T06:08:30Z")

</div>

I have a script executed with `mix run` that has an infinite loop that collects some data in an agent. I’d like to print that data when I interrupt the script.

Tried `System.at_exit/1` without success, maybe because the only way I know to interrupt the execution is `C-c C-c`.

Is there a way to accomplish that?

---

<div class="post-metadata">

**Author:** ![NobbZ](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/nobbz/32/27235_2.png) [@NobbZ](https://forum.elixirforum.com/u/NobbZ)\
**Post date:** [June 14, 2020, 8:10am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/2 "2020-06-14T08:10:00Z")

</div>

Ctrl+C, Ctrl+C kills the Virtual Machine. Ctrl+G, Q should exit in a way that the exit handler gets called, alternatively you can try `kill $BEAM_PID` from another terminal.

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<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 14, 2020, 8:53am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/3 "2020-06-14T08:53:47Z")

</div>

I am trying

```elixir
System.at_exit(fn _status ->
  IO.puts("Bye!")
end)

IO.puts("Interrupt me!")
Process.sleep(:infinity)

```

`C-g` + `q` does not seem to interrupt the script here, and `kill` does, but does not trigger `System.at_exit/1`.

---

<div class="post-metadata">

**Author:** ![rvirding](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/rvirding/32/1409_2.png) [@rvirding](https://forum.elixirforum.com/u/rvirding)\
**Post date:** [June 14, 2020, 3:39pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/4 "2020-06-14T15:39:29Z")

</div>

That is weird. What output do you get when you do Ctrl-G? It should be something like

```
User switch command
 --> 

```

Try doing ?

Btw you did press Enter after the q? It is not stated anywhere but otherwise nothing happens.

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<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 14, 2020, 4:58pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/5 "2020-06-14T16:58:09Z")

</div>

When I type Ctrl-G nothing happens apparently, I have recorded a GIF.

![CleanShot 2020-06-14 at 18.54.59](https://forum.elixirforum.com/uploads/default/original/3X/c/0/c01572b280615bd9117c5098b134e5eafcd22671.gif)

Is that strange? This is macOS and `elixir -v` returns

```plaintext
Erlang/OTP 23 [erts-11.0.2] [source] [64-bit] [smp:16:16] [ds:16:16:10] [async-threads:1] [hipe]

Elixir 1.10.3 (compiled with Erlang/OTP 23)

```

They two are installed with `asdf`.

---

<div class="post-metadata">

**Author:** ![axelson](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/axelson/32/26351_2.png) [@axelson](https://forum.elixirforum.com/u/axelson)\
**Post date:** [June 14, 2020, 6:10pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/6 "2020-06-14T18:10:26Z")

</div>

Just throwing out a random idea, but is it different if you execute the script with `mix run`?

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<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 14, 2020, 7:23pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/7 "2020-06-14T19:23:56Z")

</div>

With `mix run` happens the same, indeed I was trying `elixir` to remove Mix from the equation.

I have revised shortcuts in iTerm2 in case `C-g` is intercepted, but don’t find anything. Terminal.app makes no difference either.

`C-g q` shows “User switch command” in IEx and erl, which to me suggests the character is actually not intercepted by my terminals or my OS. However, `System.at_exit/1` does not seem to be invoked:

```plaintext
iex(1)> System.at_exit(fn _ -> IO.puts("Bye!") end)
:ok
iex(2)>
User switch command
 --> q
~/tmp/x %

```

Also, I have replaced the sleep call with a dummy `receive` block, just in case sleeping meant whatever code needs to be triggered by `C-g` is just within the slept process. No luck either.

Does

```elixir
System.at_exit(fn _status ->
  IO.puts("Bye!")
end)

IO.puts("Interrupt me!")
Process.sleep(:infinity)

```

print “Bye!” in your machines using `C-g q`?

---

<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 14, 2020, 7:24pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/8 "2020-06-14T19:24:44Z")

</div>

Ah, I also tried in a Docker container that runs Linux, same.

---

<div class="post-metadata">

**Author:** ![ChrisYammine](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/chrisyammine/32/19254_2.png) [@ChrisYammine](https://forum.elixirforum.com/u/ChrisYammine)\
**Post date:** [June 14, 2020, 9:27pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/9 "2020-06-14T21:27:03Z")

</div>

This may be of interest to you: [https://stackoverflow.com/questions/42324276/catching-exit-signals-in-elixir-escript](https://stackoverflow.com/questions/42324276/catching-exit-signals-in-elixir-escript)

AFAIK erts will trap the TERM signal and call init:stop, but when you’re C-c ing that’s an INT signal. You should be able to achieve your desired behavior with a bash wrapper like in the linked stack overflow question.

---

<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 14, 2020, 10:38pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/10 "2020-06-14T22:38:23Z")

</div>

However, several people in this thread say `C-g q` should work. Note that `C-g` does not send a signal, you get the control character 7. If that is possible, a shell script wrapper would be overkill.

I just need to interrupt a script in an ordered way. I have an `at_exit` callback, but could be taking down a supervision tree orderly or whatever.

---

<div class="post-metadata">

**Author:** ![rvirding](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/rvirding/32/1409_2.png) [@rvirding](https://forum.elixirforum.com/u/rvirding)\
**Post date:** [June 14, 2020, 11:58pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/11 "2020-06-14T23:58:51Z")

</div>

A thing to realise here is that the handling of Ctrl-G and its commands only work when you are running a shell. Without a shell this interface is not generally not started. You can see this if you look in the `elixir` shell script and see that when an iex shell is not start then there is a `-noshell` argument to the resultant `erl` command. Doing `elixir foo.exs` does not start a shell and, hence, no handling of Ctrl-G and q.

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<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 15, 2020, 10:10am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/12 "2020-06-15T10:10:31Z")

</div>

### Temporary hack

Thanks @rvirding! That makes sense. Since Ctrl-G does not trigger a signal, I guess someone needs to be monitoring standard input to do something about it.

By now, I am moving on with a dirty solution, which is to monitor a file:

```elixir
defmodule Halter do
  @halter "halt"

  def run(on_halt) do
    if File.exists?(@halter) do
      File.rm(@halter)
      on_halt.()
      System.stop()
    else
      Process.sleep(1000)
      run(on_halt)
    end
  end
end

Halter.run(fn -> IO.puts("done") end)

```

You leave the script running, and `touch halt` whenever you want it to stop. Enough for my purposes right now.

That solution prints these noisy traces, so it is definitely not clean:

```plaintext
** (exit) exited in: GenServer.call(Mix.ProjectStack, {:get_stack, #Function<10.12591541/1 in Mix.ProjectStack.peek/0>}, :infinity)
    ** (EXIT) no process: the process is not alive or there's no process currently associated with the given name, possibly because its application isn't started
    (elixir 1.10.3) lib/gen_server.ex:1013: GenServer.call/3

```

`System.at_exit/1` is not invoked (that is why `run/1` above receives a callback), I do not know why.

### Is this a missing feature?

Not being able to do this easily surprises to me. It is a fundamental aspect of Erlang/OTP that you start applications and, symmetrically, shut them down orderly.

So, given that `mix run` starts the application by default, you’d expect that there would be interface to shut it down too, no? @josevalim is there a technical reason this is not in place? Or is it something that could be contributed?

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<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 15, 2020, 10:22am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/13 "2020-06-15T10:22:27Z")

</div>

BTW, if you do not call `System.stop/1` and let `run/1` and the script finish by itself, then `System.at_exit/1` gets invoked, but the application is apparently not stopped. At least, I do not see `terminate/2` callbacks being invoked.

---

<div class="post-metadata">

**Author:** ![josevalim](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/josevalim/32/1787_2.png) [@josevalim](https://forum.elixirforum.com/u/josevalim)\
**Post date:** [June 16, 2020, 7:44am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/14 "2020-06-16T07:44:35Z")

</div>

There are two questions here:

1. How to shut the system down cleanly

2. How to handle certain signals (interrupt)

To answer 1, `System.stop/1` is the answer. If `terminate/2` callbacks are not invoked, make sure those processes are trapping exits.

To answer 2, since Erlang/OTP 20, there is OS signal handling: [http://erlang.org/doc/man/kernel\_app.html#os-signal-event-handler](http://erlang.org/doc/man/kernel_app.html#os-signal-event-handler)

It is a bit of a long answer. In a nutshell, you need to implement a gen\_event handler and react to the callbacks you are interested. There is a more detailed guide here: [https://medium.com/@ellispritchard/graceful-shutdown-on-kubernetes-with-signals-erlang-otp-20-a22325e8ae98](https://medium.com/@ellispritchard/graceful-shutdown-on-kubernetes-with-signals-erlang-otp-20-a22325e8ae98)

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<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 16, 2020, 9:12am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/15 "2020-06-16T09:12:50Z")

</div>

Thanks @josevalim!

However, `System.stop/1` has three problems:

1. It does not invoke `System.at_exit/1`, that is why my “halter” solution above needs to get passed a function.
2. The script ends printing the noisy “exit” traces I quoted above. Maybe that is cosmetic, but a user does not know if “no process: the process is not alive or…” is a normal message or something to worry about, and in any case the output is not clean as you’d expect from running a program that finishes normally.
3. The programmer needs to do this by hand. I am touching a file to finish the script! Alternatively, I could write a signal handler, but all this seems boilerplate for something `mix run` should have builtin in my view.

The question is: `mix run` starts the application by default, why does not provide a builtin way to cleanly stop the application too?

---

<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 16, 2020, 9:24am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/16 "2020-06-16T09:24:24Z")

</div>

@josevalim ah, re `terminate/2`, what I said was that if you remove `System.stop/1` and let the script finish by itself, then `System.at_exit/1` is called, but `terminate/2` callbacks are not invoked (meaning that is an observable property that suggests to me the application is not being shutdown, there’s just the VM halting.)

---

<div class="post-metadata">

**Author:** ![josevalim](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/josevalim/32/1787_2.png) [@josevalim](https://forum.elixirforum.com/u/josevalim)\
**Post date:** [June 16, 2020, 5:52pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/17 "2020-06-16T17:52:49Z")

</div>

Could put the logic you want to execute on exit in a terminate/2 callback of a process in your app’s sup tree?

We could execute at\_exit for System.stop as well… but we do have an issue in that they would be executed way too late in the termination process, when almost everything would be shut down, so it is likely not useful. That’s why System.at\_exit is about the script exiting normally.

---

<div class="post-metadata">

**Author:** ![josevalim](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/josevalim/32/1787_2.png) [@josevalim](https://forum.elixirforum.com/u/josevalim)\
**Post date:** [June 16, 2020, 5:57pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/18 "2020-06-16T17:57:54Z")

</div>

Another trick you could do is `System.at_exit(fn int -> your_code(); System.stop(int) end)`. 🙂

---

<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 19, 2020, 9:25pm UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/19 "2020-06-19T21:25:29Z")

</div>

I have investigated a bit the solution with the signal handler.

Erlang executes `:init.stop()` on `SIGTERM` [by default](https://github.com/erlang/otp/blob/76bacff3e0f0d4ef1631323db28fb35c0b84f534/lib/kernel/src/erl_signal_handler.erl#L48). That means that a simple `kill PID` shuts the thing down orderly.

However, you need to figure out the PID of the process, and open a different terminal to execute the command. Besides, custom code passed to `System.at_exit/1` is not invoked by the event handler.

`Ctrl-C` sends `SIGINT`. The only shortcut that I’ve seen sends a different signal is `Ctrl-\`, which sends `SIGQUIT`. So, I am using this event handler:

```elixir
defmodule QuitScript do
  @behaviour :gen_event

  def init(_), do: {:ok, nil}
  def handle_event(:sigquit, _state), do: :remove_handler
  def handle_event(signal, state), do: :erl_signal_handler.handle_event(signal, state)
  def handle_call(_, state), do: {:ok, :ok, state}

  def terminate(_reason, _state) do
    IO.puts("My exit code")
    System.stop()
  end
end

:gen_event.swap_handler(:erl_signal_server, {:erl_signal_handler, []}, {QuitScript, []})

```

Notice that the `terminate/2` callback allows you to run custom code on exit.

If you throw that to a script and execute it for example like this

```plaintext
mix run --no-halt foo.exs

```

`Ctrl-C` still behaves as always, and additionally `Ctrl-\` shuts the thing down orderly (you only see `^\` echoed in the terminal, not too bad).

---

<div class="post-metadata">

**Author:** ![fxn](https://forum.elixirforum.com/user_avatar/forum.elixirforum.com/fxn/32/20274_2.png) [@fxn](https://forum.elixirforum.com/u/fxn)\
**Post date:** [June 20, 2020, 7:59am UTC](https://forum.elixirforum.com/t/running-code-when-a-script-exits/32340/20 "2020-06-20T07:59:40Z")

</div>

Alternatively, we could move `System.stop()` to the callback:

```elixir
defmodule QuitScript do
  @behaviour :gen_event

  def init(_), do: {:ok, nil}
  def handle_event(:sigquit, _state), do: System.stop()
  def handle_event(signal, state), do: :erl_signal_handler.handle_event(signal, state)
  def handle_call(_, state), do: {:ok, :ok, state}

  def terminate(_reason, _state) do
    IO.puts("My exit code")
  end
end

:gen_event.swap_handler(:erl_signal_server, {:erl_signal_handler, []}, {QuitScript, []})

```

Looks more natural to me this way, though in the previous example the order in which the custom code runs is more obvious.