#file #logger


An easy-to-configure and flexible logger that writes logs to stderr 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.

77 releases

new 0.16.1 Sep 20, 2020
0.15.12 Aug 28, 2020
0.15.10 Jul 22, 2020
0.15.2 Mar 25, 2020
0.3.1 Jul 31, 2015

#8 in Debugging

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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 Travis CI


Add flexi_logger to the dependencies section in your project's Cargo.toml, with

flexi_logger = "0.16"
log = "0.4"

or, if you want to use some of the optional features, with something like

flexi_logger = { version = "0.16", features = ["specfile", "compress"] }
log = "0.4"

or, to get the smallest footprint (and no colors), with

flexi_logger = { version = "0.16", default_features = false }
log = "0.4"

Note: 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.

Example 1: log to stderr

To read the log specification from the environment variable RUST_LOG and write the logs to stderr (i.e., behave like env_logger), do this early in your program:


After that, you just use the log-macros from the log crate.

To log differently, you may

  • choose an alternative with... method,
  • and/or add some configuration options,
  • and/or choose an alternative start... method.

Example 2: log to files in a folder

In the folllowing example we

  • provide the loglevel-specification programmatically, as String, while still allowing it to be overridden by the environment variable RUST_LOG,
  • and we configure flexi_logger to write into a log file in folder log_files,
  • and write the log entries with time and location info (opt_format)
use flexi_logger::{Logger, opt_format};
// ...
Logger::with_env_or_str("myprog=debug, mylib=warn")

Example 3: reconfigure the log-spec programmatically

Obtain the ReconfigurationHandle (using .start()):

let mut log_handle = flexi_logger::Logger::with_str("info")
    // ... logger configuration ...

and modify the effective log specification from within your code:

// ...
log_handle.parse_and_push_temp_spec("info, critical_mod = trace");
// ... critical calls ...
// ... continue with the log spec you had before.

Example 4: reconfigure the log-spec dynamically by editing a spec-file

If you start flexi_logger with a specfile, e.g.

    // ... logger configuration ...

then you can change the logspec dynamically, while your program is running, by editing the specfile.

See the API documentation of Logger::start_with_specfile() for more details.


There are configuration options to e.g.

  • decide whether you want to write your logs to stderr 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,
  • define additional log streams, e.g for alert or security messages,
  • support changing the log specification on the fly, while the program is running,

See the API documentation for a complete reference.

Crate Features

Make use of any of these features by specifying them in your Cargo.toml (see above in the usage section).


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 yansi 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 atty 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 --no-default-features --features="atty" you can remove the yansi-based coloring but keep the capability to switch off your own coloring.


The compress feature adds two options to the Logger::Cleanup enum, which allow keeping some or all rotated log files in compressed form (.gz) rather than as plain text files.

The feature was previously called ziplogs. The old name still works, but is deprecated and should be replaced.


The specfile feature 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 issue-59 for more details.


Removes the ability to filter logs by text, but also removes the dependency on the regex crate.


This is still an experimental feature, likely working, but not well tested. Feedback of all kinds is highly appreciated.


See the change log.


~51K SLoC