created_at2015-07-12 15:32:35.33522
updated_at2024-06-01 09:51:14.594884
descriptionAn easy-to-configure and flexible logger that writes logs to stderr or stdout and/or to files. It allows custom logline formats, and it allows changing the log specification at runtime. It also allows defining additional log streams, e.g. for alert or security messages.





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.

Latest version Documentation License Build unsafe forbidden


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):

flexi_logger = "0.28"
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:


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")?

There are many more configuration options to e.g.

  • decide whether you want to write your logs to stdout or to a file,
  • configure the path and the filenames of the log files,
  • use file rotation,
  • specify the line format for the log lines,
  • apply a stateful filter before log lines are really written,
  • define additional log streams, e.g for alert or security messages,
  • support changing the log specification on the fly, while the program is running.


Minimal rust version

The minimal supported rust version is currently "1.70.0".

Crate Features

Make use of the non-default features by specifying them in your Cargo.toml, e.g.

flexi_logger = { version = "0.28", features = ["async", "specfile", "compress"] }
log = "0.4"

or, to get the smallest footprint (and no colors), switch off even the default features:

flexi_logger = { version = "0.28", default_features = false }
log = "0.4"


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:

  • it activates the optional dependency to nu_ansi_term and
  • provides additional colored pendants to the existing uncolored format functions
  • it uses colored_default_format() for the output to stderr, and the non-colored default_format() for the output to files
  • it activates the optional dependency to is-terminal to being able to switch off coloring if the output is not sent to a terminal but e.g. piped to another program.

Colors, 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 and choosing feature is-terminal explicitly (see usage) you can remove the nu_ansi_term-based coloring but keep the capability to switch off your own coloring.


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.


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.


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.


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.


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.


Pretty much like specfile, except that updates to the file are being ignored. See here for more details.


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.


An experimental feature that allows using flexi_logger functionality with tracing.


See the change log for more details.

Commit count: 394

cargo fmt