David's Blog

Using the `log` Crate in Rust

By David Li on Saturday, 2 June 2023 13:00:00 GMT

Using the log Crate in Rust

Rust is a powerful systems programming language that prioritizes safety, performance, and concurrent programming. Logging is a vital aspect of any application, as it provides valuable information during development, debugging, and production. In Rust, one of the most popular logging libraries is the log crate.

In this article, we’ll explore how to use the log crate in Rust to create a flexible and extensible logging system.


To follow this tutorial, you need to have Rust installed. If you don’t have it already, you can follow the official installation guide here.

Getting Started

To begin, create a new Rust project:

$ cargo new log_example
$ cd log_example

Now, open the Cargo.toml file and add the following dependencies:

log = "0.4"

env_logger = "0.9"

Here, we’re adding the log crate and the env_logger crate. The log crate provides logging macros, while the env_logger crate provides a simple logging implementation that outputs logs to the terminal based on environment variables.

Setting Up the Logger

Next, we’ll set up the logger. In the src/main.rs file, add the following lines:

extern crate log;
extern crate env_logger;

use log::{info, trace, warn, error};
use env_logger::Env;

fn main() {

    info!("Logger initialized!");

Here, we import the log and env_logger crates, along with the logging macros from the log crate. We then create a new env_logger::Builder instance, using the from_env function to configure the logging filter based on environment variables. The default_filter_or function sets a default log level if no environment variable is provided.

Now, run the application:

$ cargo run

You should see the following output:

INFO [log_example] Logger initialized!

Logging Messages

With our logger initialized, we can now log messages at different levels. Let’s examine the various log levels:

  • trace!: Very detailed logs, mostly used for development and debugging.
  • debug!: Detailed logs, used to provide context during debugging.
  • info!: General informational logs, used to inform users about the application’s status.
  • warn!: Warning logs, used to indicate potential issues or problems.
  • error!: Error logs, used to indicate critical failures or problems in the application.

Add the following lines to src/main.rs:

fn main() {
    // ...

    trace!("This is a trace log!");
    debug!("This is a debug log!");
    info!("This is an info log!");
    warn!("This is a warning log!");
    error!("This is an error log!");

By default, our logger is set to the info level. If you run the application again, you’ll see logs for info, warn, and error:

$ cargo run
INFO [log_example] Logger initialized!
INFO [log_example] This is an info log!
WARN [log_example] This is a warning log!
ERROR [log_example] This is an error log!

Changing Log Levels

To change the log level, set the RUST_LOG environment variable. For example, if you want to display logs for debug and higher levels, run:

$ RUST_LOG=debug cargo run

You’ll now see logs for debug, info, warn, and error.


In this article, we explored how to use the log crate in Rust to create a flexible and extensible logging system. By leveraging the env_logger crate, we can easily control the log level at runtime using environment variables. Now, you can integrate logging into your Rust applications and enhance your development, debugging, and production monitoring experiences.

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