[Awesome Rust] env_logger : A simple logger that can be configured via environment variables

Posted on Edited on In , , Views: Word count in article: 7.6k Reading time ≈ 7 mins.

env_logger

env_logger is a simple logger that can be configured via environment variables, for use with the logging facade exposed by the log crate.

Despite having env in its name, env_logger can also be configured by other means besides environment variables. See the examples in the source repository for more approaches.

By default, env_logger writes logs to stderr, but can be configured to instead write them to stdout.

Dependencies

By default, env_logger can be depended on with:

1
2
3
4
5
# Cargo.toml

[dependencies]
log = "0.4.0"
env_logger = "0.7.1"

Example

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
// src/main.rs

use log::{debug, error, log_enabled, info, Level};

fn main() {
    env_logger::init();

    debug!("this is a debug {}", "message");
    error!("this is printed by default");

    if log_enabled!(Level::Info) {
        let x = 3 * 4; // expensive computation
        info!("the answer was: {}", x);
    }
}

Run it:

1
2
$ RUST_LOG=error cargo run
[2017-11-09T02:12:24Z ERROR main] this is printed by default
1
2
3
$ RUST_LOG=info cargo run
[2017-11-09T02:12:24Z ERROR main] this is printed by default
[2017-11-09T02:12:24Z INFO main] the answer was: 12
1
2
3
4
$ RUST_LOG=debug cargo run
[2017-11-09T02:12:24Z DEBUG main] this is a debug message
[2017-11-09T02:12:24Z ERROR main] this is printed by default
[2017-11-09T02:12:24Z INFO main] the answer was: 12

You can also set the log level on a per module basis:

1
2
3
$ RUST_LOG=main=info cargo run
[2017-11-09T02:12:24Z ERROR main] this is printed by default
[2017-11-09T02:12:24Z INFO main] the answer was: 12

And enable all logging:

1
2
3
4
$ RUST_LOG=main cargo run
[2017-11-09T02:12:24Z DEBUG main] this is a debug message
[2017-11-09T02:12:24Z ERROR main] this is printed by default
[2017-11-09T02:12:24Z INFO main] the answer was: 12

If the binary name contains hyphens, you will need to replace them with underscores:

1
2
3
4
$ RUST_LOG=my_app cargo run
[2017-11-09T02:12:24Z DEBUG my_app] this is a debug message
[2017-11-09T02:12:24Z ERROR my_app] this is printed by default
[2017-11-09T02:12:24Z INFO my_app] the answer was: 12

This is because Rust modules and crates cannot contain hyphens in their name, although cargo continues to accept them.

See the documentation for the log crate - https://docs.rs/log/ for more information about its API.

Enabling logging

Log levels are controlled on a per-module basis, and by default all logging is disabled except for the error level.

Logging is controlled via the RUST_LOG environment variable. The value of this environment variable is a comma-separated list of logging directives. A logging directive is of the form:

1
example::log::target=level

The log target is typically equal to the path of the module the message in question originated from, though it can be overriden.

The path is rooted in the name of the crate it was compiled for, so if your program is in a file called, for example, hello.rs, the path would simply be be hello.

Furthermore, the the log can be filtered using prefix-search based on the specified log target. A value of, for example, RUST_LOG=example, would match all of the messages with targets:

  • example

  • example::test

  • example::test::module::submodule

  • examples::and_more_examples

1
2
3
4
5

When providing the crate name or a module path, explicitly specifying the log level is optional. If omitted, all logging for the item will be enabled.

The names of the log levels that may be specified correspond to the variations of the log::Level enum from the log crate. They are:

  • error

  • warn

  • info

  • debug

  • trace

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 significant1.

The letter case is not significant for the logging level names; e.g., debug, DEBUG, and dEbuG all represent the same logging level. For consistency, our convention is to use the 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.

As the log level for a module is optional, the module to enable logging for is also optional. If only a level is provided, then the global log level for all modules is set to this value.

Some examples of valid values of RUST_LOG are:

  • hello turns on all logging for the ‘hello’ module

  • trace turns on all logging for the application, regardless of its name

  • TRACE turns on all logging for the application, regardless of its name (same as previous)

  • info turns on all info logging

  • INFO turns on all info logging (same as previous)

  • hello=debug turns on debug logging for ‘hello’

  • hello=DEBUG turns on debug logging for ‘hello’ (same as previous)

  • hello,std::option turns on hello, and std’s option logging

  • error,hello=warn turn on global error logging and also warn for hello

  • error,hello=off turn on global error logging, but turn off logging for hello

  • off turns off all logging for the application

  • OFF turns off all logging for the application (same as previous)

Filtering results

A RUST_LOG directive may include a regex filter. The syntax is to append / followed by a regex. Each message is checked against the regex, and is only logged if it matches. Note that the matching is done after formatting the log string but before adding any logging meta-data. There is a single filter for all modules.

Some examples:

  • hello/foo turns on all logging for the ‘hello’ module where the log message includes ‘foo’.

  • info/f.o turns on all info logging where the log message includes ‘foo’, ‘f1o’, ‘fao’, etc.

  • hello=debug/foo*foo turns on debug logging for ‘hello’ where the log message includes ‘foofoo’ or ‘fofoo’ or ‘fooooooofoo’, etc.

  • error,hello=warn/[0-9]scopes turn on global error logging and also warn for hello. In both cases the log message must include a single digit number followed by ‘scopes’.

Capturing logs in tests

Records logged during cargo test will not be captured by the test harness by default. The Builder::is_test method can be used in unit tests to ensure logs will be captured:

1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
// src/test.rs

#[cfg(test)]
mod tests {
    fn init() {
        let _ = env_logger::builder().is_test(true).try_init();
    }

    #[test]
    fn it_works() {
        init();

        info!("This record will be captured by `cargo test`");

        assert_eq!(2, 1 + 1);
    }
}

Enabling test capturing comes at the expense of color and other style support and may have performance implications.

Disabling colors

Colors and other styles can be configured with the RUST_LOG_STYLE environment variable. It accepts the following values:

  • auto (default) will attempt to print style characters, but don’t force the issue. If the console isn’t available on Windows, or if TERM=dumb, for example, then don’t print colors.

  • always will always print style characters even if they aren’t supported by the terminal. This includes emitting ANSI colors on Windows if the console API is unavailable.

  • never will never print style characters.

Tweaking the default format

Parts of the default format can be excluded from the log output using the Builder. The following example excludes the timestamp from the log output:

1
2
3
env_logger::builder()
    .format_timestamp(None)
    .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.

Using a custom format

Custom formats can be provided as closures to the Builder. These closures take a Formatter and log::Record as arguments:

1
2
3
4
5
6
7
use std::io::Write;

env_logger::builder()
    .format(|buf, record| {
        writeln!(buf, "{}: {}", record.level(), record.args())
    })
    .init();

See the fmt - https://docs.rs/env_logger/0.9.0/env_logger/fmt/index.html module for more details about custom formats.

Specifying defaults for environment variables

env_logger can read configuration from environment variables. If these variables aren’t present, the default value to use can be tweaked with the Env type. The following example defaults to log warn and above if the RUST_LOG environment variable isn’t set:

1
2
3
use env_logger::Env;

env_logger::Builder::from_env(Env::default().default_filter_or("warn")).init();

Similar to the universe of log level names, the off pseudo log level feature is also provided by the underlying log crate. ↩

References

[1] env-logger-rs/env_logger: A logging implementation for log which is configured via an environment variable. - https://github.com/env-logger-rs/env_logger

[3] env_logger - crates.io: Rust Package Registry - https://crates.io/crates/env_logger

[3] env_logger - Rust - https://docs.rs/env_logger/0.9.0/env_logger/

[4] log - Rust - https://docs.rs/log/