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+# env_logger
+
+[![crates.io](https://img.shields.io/crates/v/env_logger.svg)](https://crates.io/crates/env_logger)
+[![Documentation](https://docs.rs/env_logger/badge.svg)](https://docs.rs/env_logger)
+[![Documentation](https://img.shields.io/badge/docs-main-blue.svg)](https://env-logger-rs.github.io/env_logger/env_logger/index.html)
+
+Implements a logger that can be configured via environment variables.
+
+## Usage
+
+### In libraries
+
+`env_logger` makes sense when used in executables (binary projects). Libraries should use the [`log`](https://docs.rs/log) crate instead.
+
+### In executables
+
+It must be added along with `log` to the project dependencies:
+
+```toml
+[dependencies]
+log = "0.4.0"
+env_logger = "0.9.0"
+```
+
+`env_logger` must be initialized as early as possible in the project. After it's initialized, you can use the `log` macros to do actual logging.
+
+```rust
+#[macro_use]
+extern crate log;
+
+fn main() {
+    env_logger::init();
+
+    info!("starting up");
+
+    // ...
+}
+```
+
+Then when running the executable, specify a value for the **`RUST_LOG`**
+environment variable that corresponds with the log messages you want to show.
+
+```bash
+$ RUST_LOG=info ./main
+[2018-11-03T06:09:06Z INFO  default] starting up
+```
+
+The letter case is not significant for the logging level names; e.g., `debug`,
+`DEBUG`, and `dEbuG` all represent the same logging level. Therefore, the
+previous example could also have been written this way, specifying the log
+level as `INFO` rather than as `info`:
+
+```bash
+$ RUST_LOG=INFO ./main
+[2018-11-03T06:09:06Z INFO  default] starting up
+```
+
+So which form should you use? For consistency, our convention is to use lower
+case names. Where our docs do use other forms, they do so in the context of
+specific examples, so you won't be surprised if you see similar usage in the
+wild.
+
+The log levels that may be specified correspond to the [`log::Level`][level-enum]
+enum from the `log` crate. They are:
+
+   * `error`
+   * `warn`
+   * `info`
+   * `debug`
+   * `trace`
+
+[level-enum]:  https://docs.rs/log/latest/log/enum.Level.html  "log::Level (docs.rs)"
+
+There is also a pseudo logging level, `off`, which may be specified to disable
+all logging for a given module or for the entire application. As with the
+logging levels, the letter case is not significant.
+
+`env_logger` can be configured in other ways besides an environment variable. See [the examples](https://github.com/env-logger-rs/env_logger/tree/main/examples) for more approaches.
+
+### In tests
+
+Tests can use the `env_logger` crate to see log messages generated during that test:
+
+```toml
+[dependencies]
+log = "0.4.0"
+
+[dev-dependencies]
+env_logger = "0.9.0"
+```
+
+```rust
+#[macro_use]
+extern crate log;
+
+fn add_one(num: i32) -> i32 {
+    info!("add_one called with {}", num);
+    num + 1
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    fn init() {
+        let _ = env_logger::builder().is_test(true).try_init();
+    }
+
+    #[test]
+    fn it_adds_one() {
+        init();
+
+        info!("can log from the test too");
+        assert_eq!(3, add_one(2));
+    }
+
+    #[test]
+    fn it_handles_negative_numbers() {
+        init();
+
+        info!("logging from another test");
+        assert_eq!(-7, add_one(-8));
+    }
+}
+```
+
+Assuming the module under test is called `my_lib`, running the tests with the
+`RUST_LOG` filtering to info messages from this module looks like:
+
+```bash
+$ RUST_LOG=my_lib=info cargo test
+     Running target/debug/my_lib-...
+
+running 2 tests
+[INFO my_lib::tests] logging from another test
+[INFO my_lib] add_one called with -8
+test tests::it_handles_negative_numbers ... ok
+[INFO my_lib::tests] can log from the test too
+[INFO my_lib] add_one called with 2
+test tests::it_adds_one ... ok
+
+test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured
+```
+
+Note that `env_logger::try_init()` needs to be called in each test in which you
+want to enable logging. Additionally, the default behavior of tests to
+run in parallel means that logging output may be interleaved with test output.
+Either run tests in a single thread by specifying `RUST_TEST_THREADS=1` or by
+running one test by specifying its name as an argument to the test binaries as
+directed by the `cargo test` help docs:
+
+```bash
+$ RUST_LOG=my_lib=info cargo test it_adds_one
+     Running target/debug/my_lib-...
+
+running 1 test
+[INFO my_lib::tests] can log from the test too
+[INFO my_lib] add_one called with 2
+test tests::it_adds_one ... ok
+
+test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
+```
+
+## Configuring log target
+
+By default, `env_logger` logs to stderr. If you want to log to stdout instead,
+you can use the `Builder` to change the log target:
+
+```rust
+use std::env;
+use env_logger::{Builder, Target};
+
+let mut builder = Builder::from_default_env();
+builder.target(Target::Stdout);
+
+builder.init();
+```
+
+## Stability of the default format
+
+The default format won't optimise for long-term stability, and explicitly makes no guarantees about the stability of its output across major, minor or patch version bumps during `0.x`.
+
+If you want to capture or interpret the output of `env_logger` programmatically then you should use a custom format.