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<main>
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<h2 id="working-with-environment-variables"><a class="header" href="#working-with-environment-variables">Working with Environment Variables</a></h2>
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<p>We’ll improve the <code>minigrep</code> binary by adding an extra feature: an option for
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case-insensitive searching that the user can turn on via an environment
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variable. We could make this feature a command line option and require that
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users enter it each time they want it to apply, but by instead making it an
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environment variable, we allow our users to set the environment variable once
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and have all their searches be case insensitive in that terminal session.</p>
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<!-- Old headings. Do not remove or links may break. -->
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<p><a id="writing-a-failing-test-for-the-case-insensitive-search-function"></a></p>
|
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<h3 id="writing-a-failing-test-for-case-insensitive-search"><a class="header" href="#writing-a-failing-test-for-case-insensitive-search">Writing a Failing Test for Case-Insensitive Search</a></h3>
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<p>We first add a new <code>search_case_insensitive</code> function to the <code>minigrep</code> library
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that will be called when the environment variable has a value. We’ll continue
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to follow the TDD process, so the first step is again to write a failing test.
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We’ll add a new test for the new <code>search_case_insensitive</code> function and rename
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our old test from <code>one_result</code> to <code>case_sensitive</code> to clarify the differences
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between the two tests, as shown in Listing 12-20.</p>
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<figure class="listing" id="listing-12-20">
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<span class="file-name">Filename: src/lib.rs</span>
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<pre><code class="language-rust ignore does_not_compile"><span class="boring">pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
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</span><span class="boring"> let mut results = Vec::new();
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</span><span class="boring">
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</span><span class="boring"> for line in contents.lines() {
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</span><span class="boring"> if line.contains(query) {
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</span><span class="boring"> results.push(line);
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</span><span class="boring"> }
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</span><span class="boring"> }
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</span><span class="boring">
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</span><span class="boring"> results
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</span><span class="boring">}
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</span><span class="boring">
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</span>#[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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}</code></pre>
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<figcaption><a href="#listing-12-20">Listing 12-20</a>: Adding a new failing test for the case-insensitive function we’re about to add</figcaption>
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</figure>
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<p>Note that we’ve edited the old test’s <code>contents</code> too. We’ve added a new line
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with the text <code>"Duct tape."</code> using a capital <em>D</em> that shouldn’t match the query
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<code>"duct"</code> when we’re searching in a case-sensitive manner. Changing the old test
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in this way helps ensure that we don’t accidentally break the case-sensitive
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search functionality that we’ve already implemented. This test should pass now
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and should continue to pass as we work on the case-insensitive search.</p>
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<p>The new test for the case-<em>insensitive</em> search uses <code>"rUsT"</code> as its query. In
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the <code>search_case_insensitive</code> function we’re about to add, the query <code>"rUsT"</code>
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should match the line containing <code>"Rust:"</code> with a capital <em>R</em> and match the
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line <code>"Trust me."</code> even though both have different casing from the query. This
|
||
is our failing test, and it will fail to compile because we haven’t yet defined
|
||
the <code>search_case_insensitive</code> function. Feel free to add a skeleton
|
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implementation that always returns an empty vector, similar to the way we did
|
||
for the <code>search</code> function in Listing 12-16 to see the test compile and fail.</p>
|
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<h3 id="implementing-the-search_case_insensitive-function"><a class="header" href="#implementing-the-search_case_insensitive-function">Implementing the <code>search_case_insensitive</code> Function</a></h3>
|
||
<p>The <code>search_case_insensitive</code> function, shown in Listing 12-21, will be almost
|
||
the same as the <code>search</code> function. The only difference is that we’ll lowercase
|
||
the <code>query</code> and each <code>line</code> so that whatever the case of the input arguments,
|
||
they’ll be the same case when we check whether the line contains the query.</p>
|
||
<figure class="listing" id="listing-12-21">
|
||
<span class="file-name">Filename: src/lib.rs</span>
|
||
<pre><code class="language-rust noplayground"><span class="boring">pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
||
</span><span class="boring"> let mut results = Vec::new();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for line in contents.lines() {
|
||
</span><span class="boring"> if line.contains(query) {
|
||
</span><span class="boring"> results.push(line);
|
||
</span><span class="boring"> }
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> results
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>pub fn search_case_insensitive<'a>(
|
||
query: &str,
|
||
contents: &'a str,
|
||
) -> Vec<&'a str> {
|
||
let query = query.to_lowercase();
|
||
let mut results = Vec::new();
|
||
|
||
for line in contents.lines() {
|
||
if line.to_lowercase().contains(&query) {
|
||
results.push(line);
|
||
}
|
||
}
|
||
|
||
results
|
||
}
|
||
<span class="boring">
|
||
</span><span class="boring">#[cfg(test)]
|
||
</span><span class="boring">mod tests {
|
||
</span><span class="boring"> use super::*;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> #[test]
|
||
</span><span class="boring"> fn case_sensitive() {
|
||
</span><span class="boring"> let query = "duct";
|
||
</span><span class="boring"> let contents = "\
|
||
</span><span class="boring">Rust:
|
||
</span><span class="boring">safe, fast, productive.
|
||
</span><span class="boring">Pick three.
|
||
</span><span class="boring">Duct tape.";
|
||
</span><span class="boring">
|
||
</span><span class="boring"> assert_eq!(vec!["safe, fast, productive."], search(query, contents));
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> #[test]
|
||
</span><span class="boring"> fn case_insensitive() {
|
||
</span><span class="boring"> let query = "rUsT";
|
||
</span><span class="boring"> let contents = "\
|
||
</span><span class="boring">Rust:
|
||
</span><span class="boring">safe, fast, productive.
|
||
</span><span class="boring">Pick three.
|
||
</span><span class="boring">Trust me.";
|
||
</span><span class="boring">
|
||
</span><span class="boring"> assert_eq!(
|
||
</span><span class="boring"> vec!["Rust:", "Trust me."],
|
||
</span><span class="boring"> search_case_insensitive(query, contents)
|
||
</span><span class="boring"> );
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-12-21">Listing 12-21</a>: Defining the <code>search_case_insensitive</code> function to lowercase the query and the line before comparing them</figcaption>
|
||
</figure>
|
||
<p>First, we lowercase the <code>query</code> string and store it in a new variable with the
|
||
same name, shadowing the original <code>query</code>. Calling <code>to_lowercase</code> on the query
|
||
is necessary so that no matter whether the user’s query is <code>"rust"</code>, <code>"RUST"</code>,
|
||
<code>"Rust"</code>, or <code>"rUsT"</code>, we’ll treat the query as if it were <code>"rust"</code> and be
|
||
insensitive to the case. While <code>to_lowercase</code> will handle basic Unicode, it
|
||
won’t be 100 percent accurate. If we were writing a real application, we’d want
|
||
to do a bit more work here, but this section is about environment variables,
|
||
not Unicode, so we’ll leave it at that here.</p>
|
||
<p>Note that <code>query</code> is now a <code>String</code> rather than a string slice because calling
|
||
<code>to_lowercase</code> creates new data rather than referencing existing data. Say the
|
||
query is <code>"rUsT"</code>, as an example: That string slice doesn’t contain a lowercase
|
||
<code>u</code> or <code>t</code> for us to use, so we have to allocate a new <code>String</code> containing
|
||
<code>"rust"</code>. When we pass <code>query</code> as an argument to the <code>contains</code> method now, we
|
||
need to add an ampersand because the signature of <code>contains</code> is defined to take
|
||
a string slice.</p>
|
||
<p>Next, we add a call to <code>to_lowercase</code> on each <code>line</code> to lowercase all
|
||
characters. Now that we’ve converted <code>line</code> and <code>query</code> to lowercase, we’ll
|
||
find matches no matter what the case of the query is.</p>
|
||
<p>Let’s see if this implementation passes the tests:</p>
|
||
<pre><code class="language-console">$ cargo test
|
||
Compiling minigrep v0.1.0 (file:///projects/minigrep)
|
||
Finished `test` profile [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>Great! They passed. Now let’s call the new <code>search_case_insensitive</code> function
|
||
from the <code>run</code> function. First, we’ll add a configuration option to the <code>Config</code>
|
||
struct to switch between case-sensitive and case-insensitive search. Adding
|
||
this field will cause compiler errors because we aren’t initializing this field
|
||
anywhere yet:</p>
|
||
<p><span class="filename">Filename: src/main.rs</span></p>
|
||
<pre><code class="language-rust ignore does_not_compile"><span class="boring">use std::env;
|
||
</span><span class="boring">use std::error::Error;
|
||
</span><span class="boring">use std::fs;
|
||
</span><span class="boring">use std::process;
|
||
</span><span class="boring">
|
||
</span><span class="boring">use minigrep::{search, search_case_insensitive};
|
||
</span><span class="boring">
|
||
</span><span class="boring">// --snip--
|
||
</span><span class="boring">
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> let args: Vec<String> = env::args().collect();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let config = Config::build(&args).unwrap_or_else(|err| {
|
||
</span><span class="boring"> println!("Problem parsing arguments: {err}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> });
|
||
</span><span class="boring">
|
||
</span><span class="boring"> if let Err(e) = run(config) {
|
||
</span><span class="boring"> println!("Application error: {e}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>pub struct Config {
|
||
pub query: String,
|
||
pub file_path: String,
|
||
pub ignore_case: bool,
|
||
}
|
||
<span class="boring">
|
||
</span><span class="boring">impl Config {
|
||
</span><span class="boring"> fn build(args: &[String]) -> Result<Config, &'static str> {
|
||
</span><span class="boring"> if args.len() < 3 {
|
||
</span><span class="boring"> return Err("not enough arguments");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let query = args[1].clone();
|
||
</span><span class="boring"> let file_path = args[2].clone();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(Config { query, file_path })
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
||
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let results = if config.ignore_case {
|
||
</span><span class="boring"> search_case_insensitive(&config.query, &contents)
|
||
</span><span class="boring"> } else {
|
||
</span><span class="boring"> search(&config.query, &contents)
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for line in results {
|
||
</span><span class="boring"> println!("{line}");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(())
|
||
</span><span class="boring">}</span></code></pre>
|
||
<p>We added the <code>ignore_case</code> field that holds a Boolean. Next, we need the <code>run</code>
|
||
function to check the <code>ignore_case</code> field’s value and use that to decide
|
||
whether to call the <code>search</code> function or the <code>search_case_insensitive</code>
|
||
function, as shown in Listing 12-22. This still won’t compile yet.</p>
|
||
<figure class="listing" id="listing-12-22">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore does_not_compile"><span class="boring">use std::env;
|
||
</span><span class="boring">use std::error::Error;
|
||
</span><span class="boring">use std::fs;
|
||
</span><span class="boring">use std::process;
|
||
</span><span class="boring">
|
||
</span>use minigrep::{search, search_case_insensitive};
|
||
|
||
// --snip--
|
||
|
||
<span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> let args: Vec<String> = env::args().collect();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let config = Config::build(&args).unwrap_or_else(|err| {
|
||
</span><span class="boring"> println!("Problem parsing arguments: {err}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> });
|
||
</span><span class="boring">
|
||
</span><span class="boring"> if let Err(e) = run(config) {
|
||
</span><span class="boring"> println!("Application error: {e}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">pub struct Config {
|
||
</span><span class="boring"> pub query: String,
|
||
</span><span class="boring"> pub file_path: String,
|
||
</span><span class="boring"> pub ignore_case: bool,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">impl Config {
|
||
</span><span class="boring"> fn build(args: &[String]) -> Result<Config, &'static str> {
|
||
</span><span class="boring"> if args.len() < 3 {
|
||
</span><span class="boring"> return Err("not enough arguments");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let query = args[1].clone();
|
||
</span><span class="boring"> let file_path = args[2].clone();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(Config { query, file_path })
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
||
let contents = fs::read_to_string(config.file_path)?;
|
||
|
||
let results = if config.ignore_case {
|
||
search_case_insensitive(&config.query, &contents)
|
||
} else {
|
||
search(&config.query, &contents)
|
||
};
|
||
|
||
for line in results {
|
||
println!("{line}");
|
||
}
|
||
|
||
Ok(())
|
||
}</code></pre>
|
||
<figcaption><a href="#listing-12-22">Listing 12-22</a>: Calling either <code>search</code> or <code>search_case_insensitive</code> based on the value in <code>config.ignore_case</code></figcaption>
|
||
</figure>
|
||
<p>Finally, we need to check for the environment variable. The functions for
|
||
working with environment variables are in the <code>env</code> module in the standard
|
||
library, which is already in scope at the top of <em>src/main.rs</em>. We’ll use the
|
||
<code>var</code> function from the <code>env</code> module to check to see if any value has been set
|
||
for an environment variable named <code>IGNORE_CASE</code>, as shown in Listing 12-23.</p>
|
||
<figure class="listing" id="listing-12-23">
|
||
<span class="file-name">Filename: src/main.rs</span>
|
||
<pre><code class="language-rust ignore noplayground"><span class="boring">use std::env;
|
||
</span><span class="boring">use std::error::Error;
|
||
</span><span class="boring">use std::fs;
|
||
</span><span class="boring">use std::process;
|
||
</span><span class="boring">
|
||
</span><span class="boring">use minigrep::{search, search_case_insensitive};
|
||
</span><span class="boring">
|
||
</span><span class="boring">fn main() {
|
||
</span><span class="boring"> let args: Vec<String> = env::args().collect();
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let config = Config::build(&args).unwrap_or_else(|err| {
|
||
</span><span class="boring"> println!("Problem parsing arguments: {err}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> });
|
||
</span><span class="boring">
|
||
</span><span class="boring"> if let Err(e) = run(config) {
|
||
</span><span class="boring"> println!("Application error: {e}");
|
||
</span><span class="boring"> process::exit(1);
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span><span class="boring">pub struct Config {
|
||
</span><span class="boring"> pub query: String,
|
||
</span><span class="boring"> pub file_path: String,
|
||
</span><span class="boring"> pub ignore_case: bool,
|
||
</span><span class="boring">}
|
||
</span><span class="boring">
|
||
</span>impl Config {
|
||
fn build(args: &[String]) -> Result<Config, &'static str> {
|
||
if args.len() < 3 {
|
||
return Err("not enough arguments");
|
||
}
|
||
|
||
let query = args[1].clone();
|
||
let file_path = args[2].clone();
|
||
|
||
let ignore_case = env::var("IGNORE_CASE").is_ok();
|
||
|
||
Ok(Config {
|
||
query,
|
||
file_path,
|
||
ignore_case,
|
||
})
|
||
}
|
||
}
|
||
<span class="boring">
|
||
</span><span class="boring">fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
||
</span><span class="boring"> let contents = fs::read_to_string(config.file_path)?;
|
||
</span><span class="boring">
|
||
</span><span class="boring"> let results = if config.ignore_case {
|
||
</span><span class="boring"> search_case_insensitive(&config.query, &contents)
|
||
</span><span class="boring"> } else {
|
||
</span><span class="boring"> search(&config.query, &contents)
|
||
</span><span class="boring"> };
|
||
</span><span class="boring">
|
||
</span><span class="boring"> for line in results {
|
||
</span><span class="boring"> println!("{line}");
|
||
</span><span class="boring"> }
|
||
</span><span class="boring">
|
||
</span><span class="boring"> Ok(())
|
||
</span><span class="boring">}</span></code></pre>
|
||
<figcaption><a href="#listing-12-23">Listing 12-23</a>: Checking for any value in an environment variable named <code>IGNORE_CASE</code></figcaption>
|
||
</figure>
|
||
<p>Here, we create a new variable, <code>ignore_case</code>. To set its value, we call the
|
||
<code>env::var</code> function and pass it the name of the <code>IGNORE_CASE</code> environment
|
||
variable. The <code>env::var</code> function returns a <code>Result</code> that will be the
|
||
successful <code>Ok</code> variant that contains the value of the environment variable if
|
||
the environment variable is set to any value. It will return the <code>Err</code> variant
|
||
if the environment variable is not set.</p>
|
||
<p>We’re using the <code>is_ok</code> method on the <code>Result</code> to check whether the environment
|
||
variable is set, which means the program should do a case-insensitive search.
|
||
If the <code>IGNORE_CASE</code> environment variable isn’t set to anything, <code>is_ok</code> will
|
||
return <code>false</code> and the program will perform a case-sensitive search. We don’t
|
||
care about the <em>value</em> of the environment variable, just whether it’s set or
|
||
unset, so we’re checking <code>is_ok</code> rather than using <code>unwrap</code>, <code>expect</code>, or any
|
||
of the other methods we’ve seen on <code>Result</code>.</p>
|
||
<p>We pass the value in the <code>ignore_case</code> variable to the <code>Config</code> instance so
|
||
that the <code>run</code> function can read that value and decide whether to call
|
||
<code>search_case_insensitive</code> or <code>search</code>, as we implemented in Listing 12-22.</p>
|
||
<p>Let’s give it a try! First, we’ll run our program without the environment
|
||
variable set and with the query <code>to</code>, which should match any line that contains
|
||
the word <em>to</em> in all lowercase:</p>
|
||
<pre><code class="language-console">$ cargo run -- to poem.txt
|
||
Compiling minigrep v0.1.0 (file:///projects/minigrep)
|
||
Finished `dev` profile [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>Looks like that still works! Now let’s run the program with <code>IGNORE_CASE</code> set
|
||
to <code>1</code> but with the same query <code>to</code>:</p>
|
||
<pre><code class="language-console">$ IGNORE_CASE=1 cargo run -- to poem.txt
|
||
</code></pre>
|
||
<p>If you’re using PowerShell, you will need to set the environment variable and
|
||
run the program as separate commands:</p>
|
||
<pre><code class="language-console">PS> $Env:IGNORE_CASE=1; cargo run -- to poem.txt
|
||
</code></pre>
|
||
<p>This will make <code>IGNORE_CASE</code> persist for the remainder of your shell session.
|
||
It can be unset with the <code>Remove-Item</code> cmdlet:</p>
|
||
<pre><code class="language-console">PS> Remove-Item Env:IGNORE_CASE
|
||
</code></pre>
|
||
<p>We should get lines that contain <em>to</em> that might have uppercase letters:</p>
|
||
<!-- manual-regeneration
|
||
cd listings/ch12-an-io-project/listing-12-23
|
||
IGNORE_CASE=1 cargo run -- to poem.txt
|
||
can't extract because of the environment variable
|
||
-->
|
||
<pre><code class="language-console">Are you nobody, too?
|
||
How dreary to be somebody!
|
||
To tell your name the livelong day
|
||
To an admiring bog!
|
||
</code></pre>
|
||
<p>Excellent, we also got lines containing <em>To</em>! Our <code>minigrep</code> program can now do
|
||
case-insensitive searching controlled by an environment variable. Now you know
|
||
how to manage options set using either command line arguments or environment
|
||
variables.</p>
|
||
<p>Some programs allow arguments <em>and</em> environment variables for the same
|
||
configuration. In those cases, the programs decide that one or the other takes
|
||
precedence. For another exercise on your own, try controlling case sensitivity
|
||
through either a command line argument or an environment variable. Decide
|
||
whether the command line argument or the environment variable should take
|
||
precedence if the program is run with one set to case sensitive and one set to
|
||
ignore case.</p>
|
||
<p>The <code>std::env</code> module contains many more useful features for dealing with
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environment variables: Check out its documentation to see what is available.</p>
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