Crate env_logger
source ·Expand description
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: 12You 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: 12And 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: 12If 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: 12This 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
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:
example::log::target=levelThe 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 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:
exampleexample::testexample::test::module::submoduleexamples::and_more_examples
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:
errorwarninfodebugtrace
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:
helloturns on all logging for the ‘hello’ moduletraceturns on all logging for the application, regardless of its nameTRACEturns on all logging for the application, regardless of its name (same as previous)infoturns on all info loggingINFOturns on all info logging (same as previous)hello=debugturns on debug logging for ‘hello’hello=DEBUGturns on debug logging for ‘hello’ (same as previous)hello,std::optionturns on hello, and std’s option loggingerror,hello=warnturn on global error logging and also warn for helloerror,hello=offturn on global error logging, but turn off logging for hellooffturns off all logging for the applicationOFFturns 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/footurns on all logging for the ‘hello’ module where the log message includes ‘foo’.info/f.oturns on all info logging where the log message includes ‘foo’, ‘f1o’, ‘fao’, etc.hello=debug/foo*footurns on debug logging for ‘hello’ where the log message includes ‘foofoo’ or ‘fofoo’ or ‘fooooooofoo’, etc.error,hello=warn/[0-9]scopesturn 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:
#[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.alwayswill 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.neverwill 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:
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:
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();Similar to the universe of log level names, the
offpseudo log level feature is also provided by the underlyinglogcrate. ↩
Re-exports
pub use super::TimestampPrecision;Modules
Structs
Builder acts as builder for initializing a Logger.Enums
stdout, stderr or a custom pipe.