Crate env_logger

A logger configured via an environment variable which writes to standard error.

Example

#[macro_use] extern crate log;
extern crate env_logger;

use log::LogLevel;

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

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

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

Assumes the binary is main:

$ RUST_LOG=error ./main
ERROR:main: this is printed by default

$ RUST_LOG=info ./main
ERROR:main: this is printed by default
INFO:main: the answer was: 12

$ RUST_LOG=debug ./main
DEBUG:main: this is a debug message
ERROR:main: this is printed by default
INFO:main: the answer was: 12

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

$ RUST_LOG=main=info ./main
ERROR:main: this is printed by default
INFO:main: the answer was: 12

And enable all logging:

$ RUST_LOG=main ./main
DEBUG:main: this is a debug message
ERROR:main: this is printed by default
INFO:main: the answer was: 12

See the documentation for the log crate 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 error!. 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:

path::to::module=log_level

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

The actual log_level is optional to specify. If omitted, all logging will be enabled. If specified, it must be one of the strings debug, error, info, warn, or trace.

As the log level for a module is optional, the module to enable logging for is also optional. If only a log_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
• 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

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'.

Structs

LogBuilder	LogBuilder acts as builder for initializing the Logger. It can be used to customize the log format, change the enviromental variable used to provide the logging directives and also set the default log level filter.
Logger	The logger.

Enums

LogTarget

Log target, either stdout or stderr.

Functions

init	Initializes the global logger with an env logger.