Crate fern [] [src]

Efficient, configurable logging in Rust.

Depending on fern

Ensure you require both fern and log in your project's Cargo.toml:

log = "0.4"
fern = "0.5"

Then declare both in or

extern crate log;
extern crate fern;

Example setup

With fern, all logger configuration is done via builder-like methods on instances of the Dispatch structure.

Here's an example logger which formats messages, and sends everything Debug and above to both stdout and an output.log file:

extern crate fern;
extern crate log;

extern crate chrono;

fn setup_logger() -> Result<(), fern::InitError> {
        .format(|out, message, record| {
                "{}[{}][{}] {}",

Let's unwrap this:


Create an empty configuration.

.format(|...| ...)

Add a formatter to the logger, modifying all messages sent through.


Get the current time in the local timezone using the chrono library. See the time-and-date docs.


Use chrono's lazy format specifier to turn the time into a readable string.


Call the fern::FormattingCallback to submit the formatted message.

This roundabout way is slightly odd, but it allows for very fast logging. No string allocation required!

format_args!() has the same format as println!() (and every other std::fmt-based macro).


Set the minimum level needed to output to Debug.


Add a child to the logger. All messages which pass the filters will be sent to stdout.

Dispatch::chain accepts Stdout, Stderr, File and other Dispatch instances.


Add a second child sending messages to the file "output.log".

See fern::log_file() for more info on file output.


Consume the configuration and instantiate it as the current runtime global logger.

This will fail if and only if .apply() or equivalent form another crate has already been used this runtime.

Since the binary crate is the only one ever setting up logging, the apply result can be reasonably unwrapped: it's a bug if any crate is calling this method more than once.

The final output will look like:

[2017-01-20][12:55:04][crate-name][INFO] Hello, world!
[2017-01-20][12:56:21][crate-name][WARN] Ahhh!
[2017-01-20][12:58:00][crate-name][DEBUG] Something less important happened.


Once the logger has been set, it will pick up all logging calls from your crate and all libraries you depend on.

    // ...

trace!("Trace message");
debug!("Debug message");
info!("Info message");
warn!("Warning message");
error!("Error message");


The Dispatch documentation has example usages of each method, and the full example program might be useful for using fern in a larger application context.

See the colors module for examples using ANSI terminal coloring.

See the syslog module for examples outputting to the unix syslog, or the syslog full example program for a more realistic sample.

See the meta module for information on getting logging-within-logging working correctly.



Support for ANSI terminal colors via the colored crate.


Fern supports logging most things by default, except for one kind of struct: structs which make log calls to the global logger from within their Display or Debug implementations.


Example usage of fern with the syslog crate.



The base dispatch logger.


Callback struct for use within a formatter closure


Configuration for a logger output.


Logger which will panic whenever anything is logged. The panic will be exactly the message of the log.



Convenience error combining possible errors which could occur while initializing logging.



Convenience method for opening a log file with common options.

Type Definitions


A type alias for a log filter. Returning true means the record should succeed - false means it should fail.


A type alias for a log formatter.