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<main>
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<h1 id="重定向错误信息的输出"><a class="header" href="#重定向错误信息的输出">重定向错误信息的输出</a></h1>
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<p>迄今为止，所有的输出信息，无论 debug 还是 error 类型，都是通过 <code>println!</code> 宏输出到终端的标准输出( <code>stdout</code> )，但是对于程序来说，错误信息更适合输出到标准错误输出(stderr)。</p>
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<p>这样修改后，用户就可以选择将普通的日志类信息输出到日志文件 1，然后将错误信息输出到日志文件 2，甚至还可以输出到终端命令行。</p>
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<h2 id="目前的错误输出位置"><a class="header" href="#目前的错误输出位置">目前的错误输出位置</a></h2>
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<p>我们先来观察下，目前的输出信息包括错误，是否是如上面所说，都写到标准错误输出。</p>
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<p>测试方式很简单，将标准错误输出的内容重定向到文件中，看看是否包含故意生成的错误信息即可。</p>
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<pre><code class="language-shell">$ cargo run > output.txt
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</code></pre>
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<p>首先，这里的运行没有带任何参数，因此会报出类如文件不存在的错误，其次，通过 <code>></code> 操作符，标准输出上的内容被重定向到文件 <code>output.txt</code> 中，不再打印到控制上。</p>
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<p>大家先观察下控制台，然后再看看 <code>output.txt</code>，是否发现如下的错误信息已经如期被写入到文件中？</p>
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<pre><code class="language-shell">Problem parsing arguments: not enough arguments
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</code></pre>
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<p>所以，可以得出一个结论，如果错信息输出到标准输出，那么它们将跟普通的日志信息混在一起，难以分辨，因此我们需要将错误信息进行单独输出。</p>
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<h2 id="标准错误输出-stderr"><a class="header" href="#标准错误输出-stderr">标准错误输出 stderr</a></h2>
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<p>将错误信息重定向到 <code>stderr</code> 很简单，只需在打印错误的地方，将 <code>println!</code> 宏替换为 <code>eprintln!</code>即可。</p>
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<pre><pre class="playground"><code class="language-rust edition2021">fn main() {
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let args: Vec<String> = env::args().collect();
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let config = Config::build(&args).unwrap_or_else(|err| {
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eprintln!("Problem parsing arguments: {err}");
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process::exit(1);
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});
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if let Err(e) = minigrep::run(config) {
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eprintln!("Application error: {e}");
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process::exit(1);
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}
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}
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</code></pre></pre>
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<p>接下来，还是同样的运行命令：</p>
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<pre><code class="language-shell">$ cargo run > output.txt
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Problem parsing arguments: not enough arguments
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</code></pre>
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<p>可以看到，日志信息成功的重定向到 <code>output.txt</code> 文件中，而错误信息由于 <code>eprintln!</code> 的使用，被写入到标准错误输出中，默认还是输出在控制台中。</p>
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<p>再来试试没有错误的情况:</p>
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<pre><code class="language-shell">$ cargo run -- to poem.txt > output.txt
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</code></pre>
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<p>这次运行参数很正确，因此也没有任何错误信息产生，同时由于我们重定向了标准输出，因此相应的输出日志会写入到 <code>output.txt</code> 中，打开可以看到如下内容：</p>
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<pre><code class="language-shell">Are you nobody, too?
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How dreary to be somebody!
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</code></pre>
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<p>至此，简易搜索程序 <code>minigrep</code> 已经基本完成，下一章节将使用迭代器进行部分改进，请大家在看完<a href="https://course.rs/advance/functional-programing/iterator.html">迭代器章节</a>后，再回头阅读。</p>
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