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# 使用环境变量来增强程序
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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 `panic` 后的栈展开？ Rust 提供的解决方案是通过命令行参数来控制:
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```shell
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RUST_BACKTRACE=1 cargo run
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```
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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:
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```shell
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IGNORE_CASE=1 cargo run -- to poem.txt
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```
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既然有了目标，那么一起来看看该如何实现吧。
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## 编写大小写不敏感的测试用例
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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 `search_case_insensitive`。
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还记得 TDD 的测试步骤嘛？首先编写一个注定失败的用例:
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```rust
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// in src/lib.rs
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn case_sensitive() {
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let query = "duct";
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let contents = "\
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Rust:
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safe, fast, productive.
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Pick three.
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Duct tape.";
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assert_eq!(vec!["safe, fast, productive."], search(query, contents));
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}
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#[test]
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fn case_insensitive() {
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let query = "rUsT";
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let contents = "\
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Rust:
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safe, fast, productive.
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Pick three.
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Trust me.";
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assert_eq!(
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vec!["Rust:", "Trust me."],
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search_case_insensitive(query, contents)
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);
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}
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}
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```
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可以看到，这里新增了一个 `case_insensitive` 测试用例，并对 `search_case_insensitive` 进行了测试，结果显而易见，函数都没有实现，自然会失败。
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接着来实现这个大小写不敏感的搜索函数:
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```rust
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pub fn search_case_insensitive<'a>(
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query: &str,
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contents: &'a str,
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) -> Vec<&'a str> {
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let query = query.to_lowercase();
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let mut results = Vec::new();
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for line in contents.lines() {
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if line.to_lowercase().contains(&query) {
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results.push(line);
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}
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}
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results
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}
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```
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跟之前一样，但是引入了一个新的方法 `to_lowercase`，它会将 `line` 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。
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还要注意的是 `query` 现在是 `String` 类型，而不是之前的 `&str`，因为 `to_lowercase` 返回的是 `String`。
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修改后，再来跑一次测试，看能否通过。
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```shell
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$ cargo test
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Compiling minigrep v0.1.0 (file:///projects/minigrep)
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Finished test [unoptimized + debuginfo] target(s) in 1.33s
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Running unittests src/lib.rs (target/debug/deps/minigrep-9cd200e5fac0fc94)
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running 2 tests
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test tests::case_insensitive ... ok
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test tests::case_sensitive ... ok
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test result: ok. 2 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
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Running unittests src/main.rs (target/debug/deps/minigrep-9cd200e5fac0fc94)
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running 0 tests
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test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
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Doc-tests minigrep
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running 0 tests
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test result: ok. 0 passed; 0 failed; 0 ignored; 0 measured; 0 filtered out; finished in 0.00s
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```
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Ok，TDD的第二步也完成了，测试通过，接下来就是最后一步，在 `run` 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。
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```rust
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// in lib.rs
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pub struct Config {
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pub query: String,
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pub file_path: String,
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pub ignore_case: bool,
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}
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```
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接下来就是检查该字段，来判断是否启动大小写敏感：
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```rust
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pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
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let contents = fs::read_to_string(config.file_path)?;
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let results = if config.ignore_case {
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search_case_insensitive(&config.query, &contents)
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} else {
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search(&config.query, &contents)
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};
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for line in results {
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println!("{line}");
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}
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Ok(())
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}
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```
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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 `env` 包提供了相应的方法。
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```rust
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use std::env;
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// --snip--
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impl Config {
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pub fn build(args: &[String]) -> Result<Config, &'static str> {
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if args.len() < 3 {
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return Err("not enough arguments");
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}
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let query = args[1].clone();
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let file_path = args[2].clone();
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let ignore_case = env::var("IGNORE_CASE").is_ok();
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Ok(Config {
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query,
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file_path,
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ignore_case,
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})
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}
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}
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```
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`env::var` 没啥好说的，倒是 `is_ok` 值得说道下。该方法是 `Result` 提供的，用于检查是否有值，有就返回 `true`，没有则返回 `false`，刚好完美符合我们的使用场景，因为我们并不关心 `Ok<T>` 中具体的值。
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运行下试试：
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```shell
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$ cargo run -- to poem.txt
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Compiling minigrep v0.1.0 (file:///projects/minigrep)
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Finished dev [unoptimized + debuginfo] target(s) in 0.0s
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Running `target/debug/minigrep to poem.txt`
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Are you nobody, too?
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How dreary to be somebody!
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```
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看起来没有问题，接下来测试下大小写不敏感:
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```shell
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$ IGNORE_CASE=1 cargo run -- to poem.txt
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```
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```shell
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Are you nobody, too?
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How dreary to be somebody!
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To tell your name the livelong day
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To an admiring bog!
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```
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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。
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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。
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