flexi_logger

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.

Usage

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

[dependencies]
flexi_logger = "0.19"
log = "0.4"

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

[dependencies]
flexi_logger = { version = "0.19", features = ["async", "specfile", "compress"] }
log = "0.4"

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

[dependencies]
flexi_logger = { version = "0.19", 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.

The minimal rust version is currently "1.46.0".

Versions

See the change log for more details.

Code examples

See the documentation of module code_examples.

Options

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,
• 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,

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).

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 a dependency to crossbeam.

colors

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.

compress

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.

specfile

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.

specfile_without_notification

Pretty much like specfile, except that updates to the file are being ignored. See issue-59 for more details.

syslog

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

textfilter

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

trc

An experimental feature that allows using flexi_logger functionality with tracing.