# `ExUnit.CaptureLog`
[🔗](https://github.com/elixir-lang/elixir/blob/v1.20.0-rc.4/lib/ex_unit/lib/ex_unit/capture_log.ex#L5)

Functionality to capture logs for testing.

## Examples

    defmodule AssertionTest do
      use ExUnit.Case

      import ExUnit.CaptureLog
      require Logger

      test "example" do
        {result, log} =
          with_log(fn ->
            Logger.error("log msg")
            2 + 2
          end)

        assert result == 4
        assert log =~ "log msg"
      end

      test "check multiple captures concurrently" do
        fun = fn ->
          for msg <- ["hello", "hi"] do
            assert capture_log(fn -> Logger.error(msg) end) =~ msg
          end

          Logger.debug("testing")
        end

        assert capture_log(fun) =~ "hello"
        assert capture_log(fun) =~ "testing"
      end
    end

# `capture_log_opts`

```elixir
@type capture_log_opts() :: [
  level: Logger.level() | nil,
  formatter: {module(), term()} | nil
]
```

# `capture_log`

```elixir
@spec capture_log(capture_log_opts(), (-&gt; any())) :: String.t()
```

Captures Logger messages generated when evaluating `fun`.

Returns the binary which is the captured output. The captured log
messages will be formatted using `Logger.default_formatter/1` by
default. Any option, besides `:level` or `formatter`, will be
forwarded as an override to the default formatter.

This function mutes the default logger handler and captures any log
messages sent to Logger from the calling processes. It is possible
to ensure explicit log messages from other processes are captured
by waiting for their exit or monitor signal.

Note that when the `async` is set to `true` on `use ExUnit.Case`,
messages from other tests might be captured. This is OK as long
you consider such cases in your assertions, typically by using
the `=~/2` operator to perform partial matches.

It is possible to configure the level to capture with `:level`,
which will set the capturing level for the duration of the
capture, for instance, if the log level is set to `:error`, then
any message with the lower level will be ignored.
The default level is `nil`, which will capture all messages.
Note this setting does not override the overall `Logger.level/0` value.
Therefore, if `Logger.level/0` is set to a higher level than the one
configured in this function, no message will be captured.
The behaviour is undetermined if async tests change Logger level.

It is possible to use an alternative log formatter with `:formatter`,
which must be provided as `{module, config}`. If `:formatter` is not
provided, the formatter configuration from `Logger.default_formatter/1`
will be used.

To get the result of the evaluation along with the captured log,
use `with_log/2`.

# `with_log`
*since 1.13.0* 

```elixir
@spec with_log(capture_log_opts(), (-&gt; result)) :: {result, log :: String.t()}
when result: any()
```

Invokes the given `fun` and returns the result and captured log.

It accepts the same arguments and options as `capture_log/2`.

## Examples

    {result, log} =
      with_log(fn ->
        Logger.error("log msg")
        2 + 2
      end)

    assert result == 4
    assert log =~ "log msg"

---

*Consult [api-reference.md](api-reference.md) for complete listing*