Crate env_logger

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.

Example

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

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);
}

Assumes the binary is main:

$ RUST_LOG=error ./main
[2017-11-09T02:12:24Z ERROR main] this is printed by default

$ RUST_LOG=info ./main
[2017-11-09T02:12:24Z ERROR main] this is printed by default
[2017-11-09T02:12:24Z INFO main] the answer was: 12

$ RUST_LOG=debug ./main
[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:

$ RUST_LOG=main=info ./main
[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:

$ RUST_LOG=main ./main
[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:

$ RUST_LOG=my_app ./my-app
[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 for more information about its API.

Enabling logging

By default all logging is disabled except for the error level

The RUST_LOG environment variable controls logging with the syntax:

RUST_LOG=[target][=][level][,...]

Or in other words, its a comma-separated list of directives. Directives can filter by target, by level, or both (using =).

For example,

RUST_LOG=data=debug,hardware=debug

target is typically the path of the module the message in question originated from, though it can be overridden. 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 hello.

Furthermore, the log can be filtered using prefix-search based on the specified log target.

For example, RUST_LOG=example would match the following targets:

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.

level is the maximum log::Level to be shown and includes:

Logging level names are case-insensitive; 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.

Some examples of valid values of RUST_LOG are:

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:

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:

#[cfg(test)]
mod tests {
    use log::info;

    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:

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:

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][crate::fmt::Formatter] and log::Record as arguments:

use std::io::Write;

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

See the fmt 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:

use env_logger::Env;

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

Modules