1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
|
# env_logger
[](https://crates.io/crates/env_logger)
[](https://docs.rs/env_logger)
[](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.
|
