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pub struct Builder { /* private fields */ }
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Builder acts as builder for initializing a Logger.

It can be used to customize the log format, change the environment variable used to provide the logging directives and also set the default log level filter.

Examples

use env_logger::Builder;
use log::LevelFilter;

let mut builder = Builder::from_default_env();

builder
    .format(|buf, record| writeln!(buf, "{} - {}", record.level(), record.args()))
    .filter(None, LevelFilter::Info)
    .init();

error!("error message");
info!("info message");

Implementations

Initializes the log builder with defaults.

NOTE: This method won’t read from any environment variables. Use the filter and write_style methods to configure the builder or use from_env or from_default_env instead.

Examples

Create a new builder and configure filters and style:

use log::LevelFilter;
use env_logger::{Builder, WriteStyle};

let mut builder = Builder::new();

builder
    .filter(None, LevelFilter::Info)
    .write_style(WriteStyle::Always)
    .init();

Initializes the log builder from the environment.

The variables used to read configuration from can be tweaked before passing in.

Examples

Initialise a logger reading the log filter from an environment variable called MY_LOG:

use env_logger::Builder;

let mut builder = Builder::from_env("MY_LOG");
builder.init();

Initialise a logger using the MY_LOG variable for filtering and MY_LOG_STYLE for whether or not to write styles:

use env_logger::{Builder, Env};

let env = Env::new().filter("MY_LOG").write_style("MY_LOG_STYLE");

let mut builder = Builder::from_env(env);
builder.init();

Applies the configuration from the environment.

This function allows a builder to be configured with default parameters, to be then overridden by the environment.

Examples

Initialise a logger with filter level Off, then override the log filter from an environment variable called MY_LOG:

use log::LevelFilter;
use env_logger::Builder;

let mut builder = Builder::new();

builder.filter_level(LevelFilter::Off);
builder.parse_env("MY_LOG");
builder.init();

Initialise a logger with filter level Off, then use the MY_LOG variable to override filtering and MY_LOG_STYLE to override whether or not to write styles:

use log::LevelFilter;
use env_logger::{Builder, Env};

let env = Env::new().filter("MY_LOG").write_style("MY_LOG_STYLE");

let mut builder = Builder::new();
builder.filter_level(LevelFilter::Off);
builder.parse_env(env);
builder.init();

Initializes the log builder from the environment using default variable names.

This method is a convenient way to call from_env(Env::default()) without having to use the Env type explicitly. The builder will use the default environment variables.

Examples

Initialise a logger using the default environment variables:

use env_logger::Builder;

let mut builder = Builder::from_default_env();
builder.init();

Applies the configuration from the environment using default variable names.

This method is a convenient way to call parse_env(Env::default()) without having to use the Env type explicitly. The builder will use the default environment variables.

Examples

Initialise a logger with filter level Off, then configure it using the default environment variables:

use log::LevelFilter;
use env_logger::Builder;

let mut builder = Builder::new();
builder.filter_level(LevelFilter::Off);
builder.parse_default_env();
builder.init();

Sets the format function for formatting the log output.

This function is called on each record logged and should format the log record and output it to the given Formatter.

The format function is expected to output the string directly to the Formatter so that implementations can use the std::fmt macros to format and output without intermediate heap allocations. The default env_logger formatter takes advantage of this.

Examples

Use a custom format to write only the log message:

use std::io::Write;
use env_logger::Builder;

let mut builder = Builder::new();

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

Use the default format.

This method will clear any custom format set on the builder.

Whether or not to write the level in the default format.

Whether or not to write the module path in the default format.

Whether or not to write the target in the default format.

Configures the amount of spaces to use to indent multiline log records. A value of None disables any kind of indentation.

Configures if timestamp should be included and in what precision.

Configures the timestamp to use second precision.

Configures the timestamp to use millisecond precision.

Configures the timestamp to use microsecond precision.

Configures the timestamp to use nanosecond precision.

Configures the end of line suffix.

Adds a directive to the filter for a specific module.

Examples

Only include messages for info and above for logs in path::to::module:

use env_logger::Builder;
use log::LevelFilter;

let mut builder = Builder::new();

builder.filter_module("path::to::module", LevelFilter::Info);

Adds a directive to the filter for all modules.

Examples

Only include messages for info and above for logs in path::to::module:

use env_logger::Builder;
use log::LevelFilter;

let mut builder = Builder::new();

builder.filter_level(LevelFilter::Info);

Adds filters to the logger.

The given module (if any) will log at most the specified level provided. If no module is provided then the filter will apply to all log messages.

Examples

Only include messages for info and above for logs in path::to::module:

use env_logger::Builder;
use log::LevelFilter;

let mut builder = Builder::new();

builder.filter(Some("path::to::module"), LevelFilter::Info);

Parses the directives string in the same form as the RUST_LOG environment variable.

See the module documentation for more details.

Sets the target for the log output.

Env logger can log to either stdout, stderr or a custom pipe. The default is stderr.

The custom pipe can be used to send the log messages to a custom sink (for example a file). Do note that direct writes to a file can become a bottleneck due to IO operation times.

Examples

Write log message to stdout:

use env_logger::{Builder, Target};

let mut builder = Builder::new();

builder.target(Target::Stdout);

Sets whether or not styles will be written.

This can be useful in environments that don’t support control characters for setting colors.

Examples

Never attempt to write styles:

use env_logger::{Builder, WriteStyle};

let mut builder = Builder::new();

builder.write_style(WriteStyle::Never);

Parses whether or not to write styles in the same form as the RUST_LOG_STYLE environment variable.

See the module documentation for more details.

Sets whether or not the logger will be used in unit tests.

If is_test is true then the logger will allow the testing framework to capture log records rather than printing them to the terminal directly.

Initializes the global logger with the built env logger.

This should be called early in the execution of a Rust program. Any log events that occur before initialization will be ignored.

Errors

This function will fail if it is called more than once, or if another library has already initialized a global logger.

Initializes the global logger with the built env logger.

This should be called early in the execution of a Rust program. Any log events that occur before initialization will be ignored.

Panics

This function will panic if it is called more than once, or if another library has already initialized a global logger.

Build an env logger.

The returned logger implements the Log trait and can be installed manually or nested within another logger.

Trait Implementations

Formats the value using the given formatter. Read more

Returns the “default value” for a type. Read more

Auto Trait Implementations

Blanket Implementations

Gets the TypeId of self. Read more

Immutably borrows from an owned value. Read more

Mutably borrows from an owned value. Read more

Returns the argument unchanged.

Calls U::from(self).

That is, this conversion is whatever the implementation of From<T> for U chooses to do.

The type returned in the event of a conversion error.

Performs the conversion.

The type returned in the event of a conversion error.

Performs the conversion.