A simple logger configured via environment variables which writesto stdout or stderr, for use with the logging facade exposed by thelog crate.

Example

#[macro_use]externcratelog;externcrateenv_logger;uselog::Level;fnmain() {env_logger::init();debug!("this is a debug {}","message");error!("this is printed by default");iflog_enabled!(Level::Info) {letx=3*4;// expensive computationinfo!("the answer was: {}",x);    }}

Assumes the binary ismain:

$ 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 replacethem 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 hyphensin their name, althoughcargo continues to accept them.

See the documentation for thelog crate for moreinformation about its API.

Enabling logging

Log levels are controlled on a per-module basis, and by default all loggingis disabled except forerror!. Logging is controlled via theRUST_LOGenvironment variable. The value of this environment variable is acomma-separated list of logging directives. A logging directive is of theform:

path::to::module=level

The path to the module is rooted in the name of the crate it was compiledfor, so if your program is contained in a filehello.rs, for example, toturn on logging for this file you would use a value ofRUST_LOG=hello.Furthermore, this path is a prefix-search, so all modules nested in thespecified module will also have logging enabled.

The actuallevel is optional to specify. If omitted, all logging willbe enabled. If specified, it must be one of the stringsdebug,error,info,warn, ortrace.

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

Some examples of valid values ofRUST_LOG are:

• hello turns on all logging for the 'hello' module
• info turns on all info logging
• hello=debug turns on debug logging for 'hello'
• hello,std::option turns on hello, and std's option logging
• error,hello=warn turn on global error logging and also warn for hello

Filtering results

ARUST_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 onlylogged if it matches. Note that the matching is done after formatting thelog string but before adding any logging meta-data. There is a single filterfor all modules.

Some examples:

• hello/foo turns on all logging for the 'hello' module where the logmessage 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 logmessage includes 'foofoo' or 'fofoo' or 'fooooooofoo', etc.
• error,hello=warn/[0-9]scopes turn on global error logging and alsowarn for hello. In both cases the log message must include a single digitnumber followed by 'scopes'.

Capturing logs in tests

Records logged duringcargo test will not be captured by the test harness by default.TheBuilder::is_test method can be used in unit tests to ensure logs will be captured:

#[cfg(test)]modtests {fninit() {let_=env_logger::builder().is_test(true).try_init();    }#[test]fnit_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 supportand may have performance implications.

Disabling colors

Colors and other styles can be configured with theRUST_LOG_STYLEenvironment 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 theBuilder.The following example excludes the timestamp from the log output:

env_logger::builder()    .default_format_timestamp(false)    .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 during0.x.

If you want to capture or interpret the output ofenv_logger programmatically then you should use a custom format.

Using a custom format

Custom formats can be provided as closures to theBuilder.These closures take aFormatter andlog::Record as arguments:

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

See thefmt 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 theEnv type.The following example defaults to logwarn and above if theRUST_LOG environment variableisn't set:

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