A flexible and easy-to-use logger that writes logs to stderr and/or to files, and/or to other output streams, and that can be influenced while the program is running.
Add flexi_logger
and log
to the dependencies section in your project's Cargo.toml
(log
is needed because flexi_logger
plugs into the standard Rust logging facade given
by the log crate,
and you use the log
macros to write log lines from your code):
[dependencies]
flexi_logger = "0.29"
log = "0.4"
To provide the log specification via env variable RUST_LOG
and get the log written to stderr,
add to an early place in your main:
flexi_logger::init();
Or, to provide a default log spec programmatically, use
flexi_logger::Logger::try_with_env_or_str("info, my::critical::module=trace")?.start()?;
or, to get the log e.g. written with high performance to a file,
use flexi_logger::{FileSpec, Logger, WriteMode};
let _logger = Logger::try_with_str("info, my::critical::module=trace")?
.log_to_file(FileSpec::default())
.write_mode(WriteMode::BufferAndFlush)
.start()?;
There are many more configuration options to e.g.
See
The minimal supported rust version is currently "1.70.0".
Make use of the non-default features by specifying them in your Cargo.toml
, e.g.
[dependencies]
flexi_logger = { version = "0.29", features = ["async", "specfile", "compress"] }
log = "0.4"
or, to get the smallest footprint (and no colors), switch off even the default features:
[dependencies]
flexi_logger = { version = "0.29", default_features = false }
log = "0.4"
async
Adds an additional write mode that decouples flexi_logger
's I/O from your application threads.
Works with log_to_stdout()
, log_to_stderr()
, and log_to_file()
.
See here for a performance comparison of some write modes.
Adds dependencies to
crossbeam-channel
and crossbeam-queue
.
colors
(default feature)Getting colored output is also possible without this feature, by implementing and using your own coloring format function.
The default feature colors
simplifies this by doing three things:
nu_ansi_term
andcolored_default_format()
for the output to stderr,
and the non-colored default_format()
for the output to filesCo<span style="color:green">lo<span style="color:magenta">rs, or styles in general, are a matter of taste, and no choice will fit every need. So you can override the default formatting and coloring in various ways.
With switching off the default features
(see usage) you can remove the nu_ansi_term
-based coloring
but keep the capability to switch off your own coloring.
compress
Adds two variants to the enum
Logger::Cleanup
, which allow keeping some
or all rotated log files in compressed form (.gz
) rather than as plain text files.
dont_minimize_extra_stacks
Normally, flexi_logger
reduces the stack size of all threads that it might spawn
(flusher, specfile-watcher, async writer, cleanup) to a bare minimum.
For usecases where this is not desirable
(see here for some motivation),
you can activate this feature.
json
Adds an additional format function json_format
that prints the whole log line in json format,
like this:
{"level":"WARN","timestamp":"2024-03-14 10:04:57.299908 +01:00","thread":"XY","module_path":"test_json","file":"src/test_json.rs","line":32,"text":"More foo than bar."}
Adds dependencies to serde
, serde_derive
, serde_json
.
kv
If you use the kv
feature of the log
crate to enrich the log-macro calls with key-value pairs,
then you should also use the kv
feature of flexi_logger
so that these key-value pairs are also written by the
provided format functions.
specfile
Adds a method Logger::start_with_specfile(specfile)
.
If started with this method, flexi_logger
uses the log specification
that was given to the factory method (one of Logger::with...()
) as initial spec
and then tries to read the log specification from the named file.
If the file does not exist, it is created and filled with the initial spec.
By editing the log specification in the file while the program is running, you can change the logging behavior in real-time.
The implementation of this feature uses some additional crates that you might not want to depend on with your program if you don't use this functionality. For that reason the feature is not active by default.
specfile_without_notification
Pretty much like specfile
, except that updates to the file are being ignored.
See here for more details.
syslog_writer
Adds SyslogWriter
, a LogWriter
implementation that sends log entries to the syslog.
textfilter
(default feature)Adds the ability to filter logs by text, but also adds a dependency on the regex crate.
trc
An experimental feature that allows using flexi_logger
functionality with tracing
.
See the change log for more details.