How to log to stderr?

Koszal

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

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2024-05-30 19:00:07 UTC

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D4no0

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

Post #1

Koszal

Not sure what you mean by running as a daemon - if I enclose the calls to the logger in a module and then spawn a function that does the loging then yes - it still goes to stdout.

Post #2

dwark

The Logger docs have a section on configuration
which mentions:

“See :logger_std_h for all relevant configuration, including overload protection.”

That page includes bits on how to set the type of device the logger writes to:

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

Maybe that helps?

Post #3

Koszal

Thanks for the links. I did try a few things but could not get it to work. It looks more involved so I need to put some time aside and investigate how Erlang logging works.

I understand that simple scripts is not where BEAM shines but I thought that would be a fun way to play with the language (and also to pitch it to my colleagues)

Koszal

Post #4

D4no0

You want a fun way to play, try livebook, the scripts (especially in that format) are not useful and don’t show any of the capabilities of erlang/OTP.

Post #5

pdgonzalez872

@Koszal could you get away with IO instead? Since you only wanted it for scripting anyways →

eflogger [master] $ cat stderr.exs
IO.puts("stdout")
IO.puts(:stderr,"stderr")
eflogger [master] $ elixir stderr.exs > stdout.log 2> stderr.log
eflogger [master] $ cat stdout.log
stdout
eflogger [master] $ cat stderr.log
stderr
eflogger [master] $

Post #6

dwark

Gleaned from this post, the following seemed to work for me:

# in config/config.exs
import Config

config :logger, :console,
  format: "$date $time [$level] $message\n",
  device: :standard_error

Post #7

dwark

btw, your original sample works as intended for me it seems like:

dnscheck on  main [!] via  
→  elixir -e 'require Logger; Logger.configure_backend(:console, device: :standard_error); Logger.error("why stdout?")' 2>scr/delme

dnscheck on  main [!] via  
→  cat scr/delme

18:55:55.984 [error] why stdout?

Perhaps due to elixer/erlang versions being used?
I’m on:

→  iex
Erlang/OTP 25 [erts-13.1.2] [source] [64-bit] [smp:4:4] [ds:4:4:10] [async-threads:1] [jit:ns]

Interactive Elixir (1.14.2) - press Ctrl+C to exit (type h() ENTER for help)

Post #8