Running code when a script exits

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.

I am trying

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.

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.

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

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

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.

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

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:

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

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?

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

This may be of interest to you: 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.

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.

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.