How to remove logs from `mix test`


I have a considerable test suite for my app with unit tests and integrated tests. The application has a considerable size so it has quite a number of Logger.debug, and Logger.error calls spread around everywhere.


This means that when I run mix test my output gets polluted with hundreds of lines of logs. I need these logs so removing them from production code is not really an option, but I would like to hide them when testing.


To achieve this I changed my test_helper.exs to have the following content:

ExUnit.start(capture_log: true)

However when I run mix test I see a ton of l debug and even info logs.


How can I hide all logs from showing when running mix test ?

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The test config (config/test.exs) generated by mix has this.

# Print only warnings and errors during test
config :logger, level: :warn

I don’t even want those because since I am (also) testing for errors the Logger.error calls will still pollute my output. I am picky :stuck_out_tongue:

config :logger, backends: [] will disable logging. You can find all the options here


for other kinds of outputs, capture_io/1 might help cleanup test output:

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When testing for errors, you can use ExUnit.CaptureLog to capture log messages (which then won’t appear in the output).

In general, I wouldn’t advise muting errors in tests, since they might be indicative of why some tests are failing.


This is … a good point. I have changed my config to the following:

# Print only warnings and errors during test
config :logger, level: :warn

As @jola suggested. This should do it, even though I may see some logs I don’t want to, I think the compromise is fair.

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You can use only in the test that triggers Log output the @moduletag directive:

@moduletag :capture_log
test "Test that trigger log output..." do
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