Koszal

How to log to stderr?

I am trying to write log messages to stderr (rather than to stdout). However, the obvious thing (device: :standard_error) seems to have no effect. In the following one-liner the log output is still written to stdout.

$ elixir -e 'require Logger; Logger.configure_backend(:console, device: :standard_error); Logger.error("why stdout?")' 2>/dev/null 

20:41:11.210 [error] why stdout?

Am I missing something obvious?

I wanted to create a script that writes something to stdout and then that something is piped into another program. Hence the logger must write to stderr (rather than stdout).

Koszal

21 comments

#logger #stderr

0 1414 21

2024-05-30 19:00:07 UTC

First Post!

D4no0

Is this also true when running the application as a daemon?

Post #1

Most Liked

dwark

There have been indeed some changes to Elixir’s Logger since v1.15.0:

[Logger] The :console configuration has been deprecated in favor of :default_formatter
[Logger] The :backends configuration has been deprecated in favor of Logger.add_handlers/1

Logger now uses :logger’s default handler and refers to the logger_std_h documentation, which states:

type = io:standard_io/0 | io:standard_error/0 | file | {device, io:device/0 } - Specifies the log destination.

The value is set when the handler is added, and it cannot be changed in runtime.

Defaults to standard_io, unless parameter file is given, in which case it defaults to file.

So once the require Logger is excuted in the one-liner, the default handler will have been added and its device cannot be changed during runtime. And looking at the actual
configuration of the default handler:

elixir -e 'require Logger; :logger.get_handler_config(:default) |> IO.inspect(); Logger.error("why stdout?")' 2>scr/delme
{:ok,
 %{
   id: :default,
   module: :logger_std_h,
   config: %{
     type: :standard_io,
     burst_limit_enable: true,
     drop_mode_qlen: 200,
     flush_qlen: 1000,
     sync_mode_qlen: 10,
     overload_kill_restart_after: 5000,
     burst_limit_max_count: 500,
     burst_limit_window_time: 1000,
     overload_kill_enable: false,
     overload_kill_mem_size: 3000000,
     overload_kill_qlen: 20000,
     filesync_repeat_interval: :no_repeat
   },
   level: :all,
   filters: [remote_gl: {&:logger_filters.remote_gl/2, :stop}],
   filter_default: :log,
   formatter: {Logger.Formatter,
    %Logger.Formatter{
      template: ["\n", :time, " ", :metadata, "[", :level, "] ", :message, "\n"],
      truncate: 8096,
      metadata: [],
      colors: %{
        error: :red,
        enabled: true,
        info: :normal,
        warning: :yellow,
        debug: :cyan,
        notice: :normal,
        alert: :red,
        critical: :red,
        emergency: :red
      },
      utc_log?: false
    }}
 }}

08:53:01.187 [error] why stdout?

it is indeed using stdio.

Post #11

hauleth

It is possible. You just need to add additional Logger backend that will filter messages to level :error or higher and output to stderr.

Post #13

NobbZ

I took the freedom to edit some code fences in.

As this:

some text [ ]

renders into

some text

Where the is a clickable checkbox.

Post #17

Last Post!

Koszal

My goal was not to write to STDERR - rather to have the standard logger behave sensibly. (And I understand my definition of sensible may not match typical BEAM use cases).

Maybe I’ll provide more context:

I am writing a tiny app that will talk to a legacy XML API (a get-to-know Elixir/LiveView project).
I started with the XML API interface. Request payloads are rendered using EEx templates. I liked how I could pattern match to select the appropriate template and then render it injecting required values. Once I had that ready I whipped up an .exs script that reads key value pairs (from System.argv). I could fire it off and render requests to STDOUT. The next step was piping to | curl -d@- -XPOST http://... The server complained about illegal input. After a bit of poking I realized log messages went to STDOUT rather than STDERR so I was feeding not only XML but logger output to the remote host.

And hence my question about having logger write to STDOUT.

Post #21