env_logger

, ,

A logging implementation for log which is configured via an environment variable.

大多数应用都需要日志通过控制台来输出当前程序运行的状态信息，在Rust中，常用的为env_logger，配合着标准库中log包来使用。可以说env_logger把log的功能扩展了，所以在log使用前需要将 env_logger加载到内存中

1. 安装

创建项目后添加依赖

如下两种方式添加至项目

1. Cargo.toml

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[dependencies]
log = "0.4.0"
env_logger = "0.9.0"

2. Terminal

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cargo add log env_logger

2. 使用

1. 基础使用

文档[^1]例子如下

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use log::{debug, error, log_enabled, info, Level};

env_logger::init();

debug!("this is a debug {}", "message");
error!("this is printed by default");

if log_enabled!(Level::Info) {
    let x = 3 * 4; // expensive computation
    info!("the answer was: {}", x);
}

运行结果：

ShotScreen 2023-10-03 at 13.30.39@2x

只是输出了ERROR级别的log，那么如何输出debug或者info？

类似其他语言一直，日志登记默认为Error

而env_logger通过环境变量RUST_LOG控制日志等级输出

当更改环境变量是，执行结果如下所示：

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RUST_LOG=error cargo run
    Finished dev [unoptimized + debuginfo] target(s) in 0.18s
     Running `target/debug/mylog`
[2023-10-03T05:34:28Z ERROR mylog] this is printed by default

do it:
RUST_LOG=debug cargo run

output:
    Finished dev [unoptimized + debuginfo] target(s) in 0.03s
     Running `target/debug/mylog`
[2023-10-03T05:34:43Z DEBUG mylog] this is a debug message
[2023-10-03T05:34:43Z ERROR mylog] this is printed by default
[2023-10-03T05:34:43Z INFO  mylog] the answer was: 12

2. 控制输出等级

Log levels are controlled on a per-module basis, and by default all logging is disabled except for the error level.

日志等级是按照每个模块控制的，默认情况下所有的日志被禁止除了error等级的输出

在模块中控制日志等级，使用env_logger::build创建log的实例

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mod mylog;

use log::{debug, error, info, log_enabled, Level, LevelFilter};

fn main() {
    env_logger::builder()
        .filter_level(LevelFilter::Debug)
        .init();

    debug!("this is a debug {}", "message");
    error!("this is printed by default");

    if log_enabled!(Level::Info) {
        let x = 3 * 4; // expensive computation
        info!("the answer was: {}", x);
    }
}

将日志输出等级设置为Debug，那么在Debug之上的等级会全部输出在console中

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➜  log git:(master) ✗ cargo run
   Compiling mylog v0.1.0 (/Users/yother/WorkSpace/BackEnd/Rust/log)
    Finished dev [unoptimized + debuginfo] target(s) in 1.19s
     Running `target/debug/mylog`
[2023-10-03T05:48:46Z DEBUG mylog] this is a debug message
[2023-10-03T05:48:46Z ERROR mylog] this is printed by default
[2023-10-03T05:48:46Z INFO  mylog] the answer was: 12

使用build可以自定义日志输出格式，例如颜色，时间戳，输出日之所在位置等

3. 自定义输出格式

env_logger::build

3. 引用

[^1]: crates env_logger addr：https://crates.io/crates/env_logger
[^2]: env_logger doc: https://docs.rs/env_logger/0.10.0/env_logger