vendor/env_logger/README.md

   1 # env_logger
   2
   3 [![Maintenance](https://img.shields.io/badge/maintenance-actively%20maintained-brightgreen.svg)](https://github.com/env-logger-rs/env_logger)
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   7 ==========
   8
   9 Implements a logger that can be configured via environment variables.
  10
  11 ## Usage
  12
  13 ### In libraries
  14
  15 `env_logger` makes sense when used in executables (binary projects). Libraries should use the [`log`](https://doc.rust-lang.org/log) crate instead.
  16
  17 ### In executables
  18
  19 It must be added along with `log` to the project dependencies:
  20
  21 ```toml
  22 [dependencies]
  23 log = "0.4.0"
  24 env_logger = "0.8.3"
  25 ```
  26
  27 `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.
  28
  29 ```rust
  30 #[macro_use]
  31 extern crate log;
  32
  33 fn main() {
  34     env_logger::init();
  35
  36     info!("starting up");
  37
  38     // ...
  39 }
  40 ```
  41
  42 Then when running the executable, specify a value for the **`RUST_LOG`**
  43 environment variable that corresponds with the log messages you want to show.
  44
  45 ```bash
  46 $ RUST_LOG=info ./main
  47 [2018-11-03T06:09:06Z INFO  default] starting up
  48 ```
  49
  50 The letter case is not significant for the logging level names; e.g., `debug`,
  51 `DEBUG`, and `dEbuG` all represent the same logging level. Therefore, the
  52 previous example could also have been written this way, specifying the log
  53 level as `INFO` rather than as `info`:
  54
  55 ```bash
  56 $ RUST_LOG=INFO ./main
  57 [2018-11-03T06:09:06Z INFO  default] starting up
  58 ```
  59
  60 So which form should you use? For consistency, our convention is to use lower
  61 case names. Where our docs do use other forms, they do so in the context of
  62 specific examples, so you won't be surprised if you see similar usage in the
  63 wild.
  64
  65 The log levels that may be specified correspond to the [`log::Level`][level-enum]
  66 enum from the `log` crate. They are:
  67
  68    * `error`
  69    * `warn`
  70    * `info`
  71    * `debug`
  72    * `trace`
  73
  74 [level-enum]:  https://docs.rs/log/latest/log/enum.Level.html  "log::Level (docs.rs)"
  75
  76 There is also a pseudo logging level, `off`, which may be specified to disable
  77 all logging for a given module or for the entire application. As with the
  78 logging levels, the letter case is not significant.
  79
  80 `env_logger` can be configured in other ways besides an environment variable. See [the examples](https://github.com/env-logger-rs/env_logger/tree/master/examples) for more approaches.
  81
  82 ### In tests
  83
  84 Tests can use the `env_logger` crate to see log messages generated during that test:
  85
  86 ```toml
  87 [dependencies]
  88 log = "0.4.0"
  89
  90 [dev-dependencies]
  91 env_logger = "0.8.3"
  92 ```
  93
  94 ```rust
  95 #[macro_use]
  96 extern crate log;
  97
  98 fn add_one(num: i32) -> i32 {
  99     info!("add_one called with {}", num);
 100     num + 1
 101 }
 102
 103 #[cfg(test)]
 104 mod tests {
 105     use super::*;
 106
 107     fn init() {
 108         let _ = env_logger::builder().is_test(true).try_init();
 109     }
 110
 111     #[test]
 112     fn it_adds_one() {
 113         init();
 114
 115         info!("can log from the test too");
 116         assert_eq!(3, add_one(2));
 117     }
 118
 119     #[test]
 120     fn it_handles_negative_numbers() {
 121         init();
 122
 123         info!("logging from another test");
 124         assert_eq!(-7, add_one(-8));
 125     }
 126 }
 127 ```
 128
 129 Assuming the module under test is called `my_lib`, running the tests with the
 130 `RUST_LOG` filtering to info messages from this module looks like:
 131
 132 ```bash
 133 $ RUST_LOG=my_lib=info cargo test
 134      Running target/debug/my_lib-...
 135
 136 running 2 tests
 137 [INFO my_lib::tests] logging from another test
 138 [INFO my_lib] add_one called with -8
 139 test tests::it_handles_negative_numbers ... ok
 140 [INFO my_lib::tests] can log from the test too
 141 [INFO my_lib] add_one called with 2
 142 test tests::it_adds_one ... ok
 143
 144 test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured
 145 ```
 146
 147 Note that `env_logger::try_init()` needs to be called in each test in which you
 148 want to enable logging. Additionally, the default behavior of tests to
 149 run in parallel means that logging output may be interleaved with test output.
 150 Either run tests in a single thread by specifying `RUST_TEST_THREADS=1` or by
 151 running one test by specifying its name as an argument to the test binaries as
 152 directed by the `cargo test` help docs:
 153
 154 ```bash
 155 $ RUST_LOG=my_lib=info cargo test it_adds_one
 156      Running target/debug/my_lib-...
 157
 158 running 1 test
 159 [INFO my_lib::tests] can log from the test too
 160 [INFO my_lib] add_one called with 2
 161 test tests::it_adds_one ... ok
 162
 163 test result: ok. 1 passed; 0 failed; 0 ignored; 0 measured
 164 ```
 165
 166 ## Configuring log target
 167
 168 By default, `env_logger` logs to stderr. If you want to log to stdout instead,
 169 you can use the `Builder` to change the log target:
 170
 171 ```rust
 172 use std::env;
 173 use env_logger::{Builder, Target};
 174
 175 let mut builder = Builder::from_default_env();
 176 builder.target(Target::Stdout);
 177
 178 builder.init();
 179 ```
 180
 181 ## Stability of the default format
 182
 183 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`.
 184
 185 If you want to capture or interpret the output of `env_logger` programmatically then you should use a custom format.