A flexible logger for rust programs that can write to stderr or to log files
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A flexible and easy-to-use logger that writes logs to stderr and/or to files.


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

flexi_logger = "^0.10.3"
log = "0.4"

or, if you want to use the specfile feature, with

flexi_logger = { version = "0.10", features = ["specfile"] }
log = "0.4"

(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),

and add this to your crate root:

extern crate flexi_logger;

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:

use flexi_logger::Logger;
// ...
            .unwrap_or_else(|e| panic!("Logger initialization failed with {}", e));

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.

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")
            .unwrap_or_else(|e| panic!("Logger initialization failed with {}", e));


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


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.


See the change log.