logging.md

log crate

The log crate is the de-facto logging API in Rust.

One important note is that the log crate only provides the API, not implementations.

log crate takes an environment variable RUST_LOG to configure its log level: std::env::set_var("RUST_LOG", "debug");.

There are 5 log levels:

• error (highest priority);
• warn;
• info;
• debug;
• trace (lowest priority).

To log a message, there are corresponding marcos:

• error!();
• warn!();
• info!();
• debug!();
• trace!().

These marcos behave like println! and supports the syntax of format!.

That is:

• {} calls display() on an object;
• {:?} calls debug() on an object;
• {:#?} pretty-print the debug formatting.

Other crates

There are many available implementations

• Simple minimal loggers:
  • env_logger
• Complex configurable frameworks
  • log4rs
  • fern
• Adaptors for other facilities
  • syslog

env_logger crate

By far, the most commonly used logging library in Rust is the env_logger crate.

env_logger takes an environment variable RUST_LOG to configure its log level: std::env::set_var("RUST_LOG", "debug");.

Initialize logger: env_logger::init();.

use log::debug;
use log::error;
use log::info;
use log::warn;

fn main() {
    env_logger::init();

    debug!("Mary has a little lamb");
    error!("{}", "Its fleece was white as snow");
    info!("{:?}", "And every where that Mary went");
    warn!("{:#?}", "The lamb was sure to go");
}

log4rs crate

Another logging library in Rust is log4rs crate.

use log::error;
use log::info;
use log::warn;
use log::{debug, LevelFilter};
use log4rs::append::console::ConsoleAppender;
use log4rs::config::{Appender, Root};
use log4rs::Config;

fn main() {
    let stdout = ConsoleAppender::builder().build();    
    let config = Config::builder().appender(Appender::builder().build("stdout", Box::new(stdout)))
        .build(Root::builder().appender("stdout").build(LevelFilter::Trace)).unwrap();    
    let _handle = log4rs::init_config(config).unwrap();    

    debug!("Mary has a little lamb");
    error!("{}", "Its fleece was white as snow");
    info!("{:?}", "And every where that Mary went");
    warn!("{:#?}", "The lamb was sure to go");
}