[−][src]Crate env_logger
A simple logger configured via environment variables which writes
to stdout or stderr, for use with the logging facade exposed by the
log
crate.
Example
#[macro_use] extern crate log; extern crate env_logger; use log::Level; fn main() { 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
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=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 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 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' moduleinfo
turns on all info logginghello=debug
turns on debug logging for 'hello'hello,std::option
turns on hello, and std's option loggingerror,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'.
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.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 the Builder
.
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 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::from_env(Env::default().default_filter_or("warn")).init();
Modules
filter | Filtering for log records. |
fmt | Formatting for log records. |
Structs
Builder |
|
Env | Set of environment variables to configure from. |
Logger | The env logger. |
Enums
Target | Log target, either |
WriteStyle | Whether or not to print styles to the target. |
Constants
DEFAULT_FILTER_ENV | The default name for the environment variable to read filters from. |
DEFAULT_WRITE_STYLE_ENV | The default name for the environment variable to read style preferences from. |
Functions
builder | Create a new builder with the default environment variables. |
from_env | Create a builder from the given environment variables. |
init | Initializes the global logger with an env logger. |
init_from_env | Initializes the global logger with an env logger from the given environment variables. |
try_init | Attempts to initialize the global logger with an env logger. |
try_init_from_env | Attempts to initialize the global logger with an env logger from the given environment variables. |