Master Rust Logging with log4rs: A Step-by-Step Guide

log4rs Basic Concepts

log4rs's functionality consists of appenders and loggers .

Appender : responsible for appending logs to a specified file or console.

Logger : contains multiple appenders; for example, a log can be output to both the console and a file by binding ConsoleAppender and FileAppender in a logger.

log4rs Usage Example

Example Description

We need to record system logs and business logs in logs/sys.log and logs/business.log respectively.

Define appender and logger and initialize

Code location:

src/logger/logger.rs

let sys_file = FileAppender::builder()
    .encoder(Box::new(PatternEncoder::new("{d} - {m}{n}")))
    .build("logs/sys.log")
    .unwrap();
let business_file = FileAppender::builder()
    .encoder(Box::new(PatternEncoder::new("{d} - {m}{n}")))
    .build("logs/business.log")
    .unwrap();

let stdout = ConsoleAppender::builder().build();

let config = Config::builder()
    .appender(Appender::builder().build("stdout", Box::new(stdout)))
    .appender(Appender::builder().build("sys", Box::new(sys_file)))
    .appender(Appender::builder().build("business", Box::new(business_file)))
    .logger(
        Logger::builder()
            .appender("sys")
            .build("syslog", LevelFilter::Info),
    )
    .logger(
        Logger::builder()
            .appender("business")
            .build("businesslog", LevelFilter::Info),
    )
    .build(
        Root::builder()
            .appender("stdout")
            .appender("file_out")
            .build(LevelFilter::Info),
    )
    .unwrap();

let _ = log4rs::init_config(config).unwrap();

Output logs in program

Define the uselog command with two sub‑commands to input system and business logs.

Code location:

src/cmd/cmdusedifflogger.rs

pub fn new_use_sys_log_cmd() -> Command<'static> {
    clap::Command::new("syslog").about("append to syslog")
}

pub fn new_use_business_log_cmd() -> Command<'static> {
    clap::Command::new("businesslog").about("append to business log")
}

Parse command and output logs

Code location:

src/cmd/rootcmd.rs

if let Some(ref log) = matches.subcommand_matches("uselog") {
    println!("use log");
    if let Some(_) = log.subcommand_matches("syslog") {
        log::info!(target:"syslog","Input sys log");
    }
    if let Some(_) = log.subcommand_matches("businesslog") {
        log::info!(target:"businesslog","Input business log");
    }
}

When outputting, the target distinguishes which logger receives the message.

GitHub repository: https://github.com/jiashiwen/interactcli-rs