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<h1 id="使用环境变量来增强程序"><a class="header" href="#使用环境变量来增强程序">使用环境变量来增强程序</a></h1>
<p>在上一章节中，留下了一个悬念，该如何实现用户控制的大小写敏感，其实答案很简单，你在其它程序中肯定也遇到过不少，例如如何控制 <code>panic</code> 后的栈展开？ Rust 提供的解决方案是通过命令行参数来控制: </p>
<pre><code class="language-shell">RUST_BACKTRACE=1 cargo run
</code></pre>
<p>与之类似，我们也可以使用环境变量来控制大小写敏感，例如: </p>
<pre><code class="language-shell">IGNORE_CASE=1 cargo run -- to poem.txt
</code></pre>
<p>既然有了目标，那么一起来看看该如何实现吧。</p>
<h2 id="编写大小写不敏感的测试用例"><a class="header" href="#编写大小写不敏感的测试用例">编写大小写不敏感的测试用例</a></h2>
<p>还是遵循之前的规则：测试驱动，这次是对一个新的大小写不敏感函数进行测试 <code>search_case_insensitive</code>。</p>
<p>还记得 TDD 的测试步骤嘛？首先编写一个注定失败的用例:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// in src/lib.rs
#[cfg(test)]
mod tests {
use super::*;
#[test]
fn case_sensitive() {
let query = &quot;duct&quot;;
let contents = &quot;\
Rust:
safe, fast, productive.
Pick three.
Duct tape.&quot;;
assert_eq!(vec![&quot;safe, fast, productive.&quot;], search(query, contents));
}
#[test]
fn case_insensitive() {
let query = &quot;rUsT&quot;;
let contents = &quot;\
Rust:
safe, fast, productive.
Pick three.
Trust me.&quot;;
assert_eq!(
vec![&quot;Rust:&quot;, &quot;Trust me.&quot;],
search_case_insensitive(query, contents)
);
}
}
<span class="boring">}</span></code></pre></pre>
<p>可以看到，这里新增了一个 <code>case_insensitive</code> 测试用例，并对 <code>search_case_insensitive</code> 进行了测试，结果显而易见，函数都没有实现，自然会失败。</p>
<p>接着来实现这个大小写不敏感的搜索函数:</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn search_case_insensitive&lt;'a&gt;(
query: &amp;str,
contents: &amp;'a str,
) -&gt; Vec&lt;&amp;'a str&gt; {
let query = query.to_lowercase();
let mut results = Vec::new();
for line in contents.lines() {
if line.to_lowercase().contains(&amp;query) {
results.push(line);
}
}
results
}
<span class="boring">}</span></code></pre></pre>
<p>跟之前一样，但是引入了一个新的方法 <code>to_lowercase</code>，它会将 <code>line</code> 转换成全小写的字符串，类似的方法在其它语言中也差不多，就不再赘述。</p>
<p>还要注意的是 <code>query</code> 现在是 <code>String</code> 类型，而不是之前的 <code>&amp;str</code>，因为 <code>to_lowercase</code> 返回的是 <code>String</code>。</p>
<p>修改后，再来跑一次测试，看能否通过。</p>
<pre><code class="language-shell">$ cargo test
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished test [unoptimized + debuginfo] target(s) in 1.33s
Running unittests src/lib.rs (target/debug/deps/minigrep-9cd200e5fac0fc94)
running 2 tests
test tests::case_insensitive ... ok
test tests::case_sensitive ... ok
test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
Running unittests src/main.rs (target/debug/deps/minigrep-9cd200e5fac0fc94)
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
Doc-tests minigrep
running 0 tests
test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
</code></pre>
<p>Ok，TDD的第二步也完成了，测试通过，接下来就是最后一步，在 <code>run</code> 中调用新的搜索函数。但是在此之前，要新增一个配置项，用于控制是否开启大小写敏感。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>// in lib.rs
pub struct Config {
pub query: String,
pub file_path: String,
pub ignore_case: bool,
}
<span class="boring">}</span></code></pre></pre>
<p>接下来就是检查该字段，来判断是否启动大小写敏感：</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>pub fn run(config: Config) -&gt; Result&lt;(), Box&lt;dyn Error&gt;&gt; {
let contents = fs::read_to_string(config.file_path)?;
let results = if config.ignore_case {
search_case_insensitive(&amp;config.query, &amp;contents)
} else {
search(&amp;config.query, &amp;contents)
};
for line in results {
println!(&quot;{line}&quot;);
}
Ok(())
}
<span class="boring">}</span></code></pre></pre>
<p>现在的问题来了，该如何控制这个配置项呢。这个就要借助于章节开头提到的环境变量，好在 Rust 的 <code>env</code> 包提供了相应的方法。</p>
<pre><pre class="playground"><code class="language-rust edition2021"><span class="boring">#![allow(unused)]
</span><span class="boring">fn main() {
</span>use std::env;
// --snip--
impl Config {
pub fn build(args: &amp;[String]) -&gt; Result&lt;Config, &amp;'static str&gt; {
if args.len() &lt; 3 {
return Err(&quot;not enough arguments&quot;);
}
let query = args[1].clone();
let file_path = args[2].clone();
let ignore_case = env::var(&quot;IGNORE_CASE&quot;).is_ok();
Ok(Config {
query,
file_path,
ignore_case,
})
}
}
<span class="boring">}</span></code></pre></pre>
<p><code>env::var</code> 没啥好说的，倒是 <code>is_ok</code> 值得说道下。该方法是 <code>Result</code> 提供的，用于检查是否有值，有就返回 <code>true</code>，没有则返回 <code>false</code>，刚好完美符合我们的使用场景，因为我们并不关心 <code>Ok&lt;T&gt;</code> 中具体的值。</p>
<p>运行下试试：</p>
<pre><code class="language-shell">$ cargo run -- to poem.txt
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.0s
Running `target/debug/minigrep to poem.txt`
Are you nobody, too?
How dreary to be somebody!
</code></pre>
<p>看起来没有问题，接下来测试下大小写不敏感: </p>
<pre><code class="language-shell">$ IGNORE_CASE=1 cargo run -- to poem.txt
</code></pre>
<pre><code class="language-shell">Are you nobody, too?
How dreary to be somebody!
To tell your name the livelong day
To an admiring bog!
</code></pre>
<p>大小写不敏感后，查询到的内容明显多了很多，也很符合我们的预期。</p>
<p>最后，给大家留一个小作业：同时使用命令行参数和环境变量的方式来控制大小写不敏感，其中环境变量的优先级更高，也就是两个都设置的情况下，优先使用环境变量的设置。</p>
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