496 lines
39 KiB
HTML
496 lines
39 KiB
HTML
|
<!DOCTYPE HTML>
|
|||
|
<html lang="en" class="sidebar-visible no-js light">
|
|||
|
<head>
|
|||
|
<!-- Book generated using mdBook -->
|
|||
|
<meta charset="UTF-8">
|
|||
|
<title>Working with Environment Variables - The Rust Programming Language</title>
|
|||
|
|
|||
|
|
|||
|
<meta content="text/html; charset=utf-8" http-equiv="Content-Type">
|
|||
|
<meta name="description" content="">
|
|||
|
<meta name="viewport" content="width=device-width, initial-scale=1">
|
|||
|
<meta name="theme-color" content="#ffffff" />
|
|||
|
|
|||
|
<link rel="shortcut icon" href="favicon.png">
|
|||
|
<link rel="stylesheet" href="css/variables.css">
|
|||
|
<link rel="stylesheet" href="css/general.css">
|
|||
|
<link rel="stylesheet" href="css/chrome.css">
|
|||
|
<link rel="stylesheet" href="css/print.css" media="print">
|
|||
|
|
|||
|
<!-- Fonts -->
|
|||
|
<link rel="stylesheet" href="FontAwesome/css/font-awesome.css">
|
|||
|
<link href="googleFonts/css.css" rel="stylesheet" type="text/css">
|
|||
|
|
|||
|
<!-- Highlight.js Stylesheets -->
|
|||
|
<link rel="stylesheet" href="highlight.css">
|
|||
|
<link rel="stylesheet" href="tomorrow-night.css">
|
|||
|
<link rel="stylesheet" href="ayu-highlight.css">
|
|||
|
|
|||
|
<!-- Custom theme stylesheets -->
|
|||
|
|
|||
|
<link rel="stylesheet" href="ferris.css">
|
|||
|
|
|||
|
<link rel="stylesheet" href="theme/2018-edition.css">
|
|||
|
|
|||
|
|
|||
|
|
|||
|
</head>
|
|||
|
<body>
|
|||
|
<!-- Provide site root to javascript -->
|
|||
|
<script type="text/javascript">
|
|||
|
var path_to_root = "";
|
|||
|
var default_theme = window.matchMedia("(prefers-color-scheme: dark)").matches ? "light" : "light";
|
|||
|
</script>
|
|||
|
|
|||
|
<!-- Work around some values being stored in localStorage wrapped in quotes -->
|
|||
|
<script type="text/javascript">
|
|||
|
try {
|
|||
|
var theme = localStorage.getItem('mdbook-theme');
|
|||
|
var sidebar = localStorage.getItem('mdbook-sidebar');
|
|||
|
|
|||
|
if (theme.startsWith('"') && theme.endsWith('"')) {
|
|||
|
localStorage.setItem('mdbook-theme', theme.slice(1, theme.length - 1));
|
|||
|
}
|
|||
|
|
|||
|
if (sidebar.startsWith('"') && sidebar.endsWith('"')) {
|
|||
|
localStorage.setItem('mdbook-sidebar', sidebar.slice(1, sidebar.length - 1));
|
|||
|
}
|
|||
|
} catch (e) { }
|
|||
|
</script>
|
|||
|
|
|||
|
<!-- Set the theme before any content is loaded, prevents flash -->
|
|||
|
<script type="text/javascript">
|
|||
|
var theme;
|
|||
|
try { theme = localStorage.getItem('mdbook-theme'); } catch(e) { }
|
|||
|
if (theme === null || theme === undefined) { theme = default_theme; }
|
|||
|
var html = document.querySelector('html');
|
|||
|
html.classList.remove('no-js')
|
|||
|
html.classList.remove('light')
|
|||
|
html.classList.add(theme);
|
|||
|
html.classList.add('js');
|
|||
|
</script>
|
|||
|
|
|||
|
<!-- Hide / unhide sidebar before it is displayed -->
|
|||
|
<script type="text/javascript">
|
|||
|
var html = document.querySelector('html');
|
|||
|
var sidebar = 'hidden';
|
|||
|
if (document.body.clientWidth >= 1080) {
|
|||
|
try { sidebar = localStorage.getItem('mdbook-sidebar'); } catch(e) { }
|
|||
|
sidebar = sidebar || 'visible';
|
|||
|
}
|
|||
|
html.classList.remove('sidebar-visible');
|
|||
|
html.classList.add("sidebar-" + sidebar);
|
|||
|
</script>
|
|||
|
|
|||
|
<nav id="sidebar" class="sidebar" aria-label="Table of contents">
|
|||
|
<div id="sidebar-scrollbox" class="sidebar-scrollbox">
|
|||
|
<ol class="chapter"><li class="expanded affix "><a href="title-page.html">The Rust Programming Language</a></li><li class="expanded affix "><a href="foreword.html">Foreword</a></li><li class="expanded affix "><a href="ch00-00-introduction.html">Introduction</a></li><li class="expanded "><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li class="expanded "><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li class="expanded "><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li class="expanded "><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li class="expanded "><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li class="expanded "><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li class="expanded "><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li class="expanded "><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li class="expanded "><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> Functions</a></li><li class="expanded "><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li class="expanded "><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li class="expanded "><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li class="expanded "><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li class="expanded "><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References and Borrowing</a></li><li class="expanded "><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> The Slice Type</a></li></ol></li><li class="expanded "><a href="ch05-00-structs.html"><strong aria-hidden="true">5.</strong> Using Structs to Structure Related Data</a></li><li><ol class="section"><li class="expanded "><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li class="expanded "><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li class="expanded "><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li class="expanded "><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li class="expanded "><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li class="expanded "><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li class="expanded "><a href="ch06-03-if-let.html"><strong aria-hidden="true">6.3.</strong> Concise Control Flow with if let</a></li></ol></li><li class="expanded "><a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html"><strong aria-hidden="true">7.</strong> Managing Growing Projects with Packages, Crates, and Modules</a></li><li><ol class="section"><li class="expanded "><a href="ch07-01-packages-and-crates.html"><strong aria-hidden="true">7.1.</strong> Packages and Crates</a></li><li class="expanded "><a href="ch07-02-defining-modules-to-control-scope-and-privacy.html"><strong aria-hidden="true">7.2.</strong> Defining Modules to Control Scope and Privacy</a></li><li class="expanded "><a href="ch07-03-paths-for-referring-to-an-item-in-the-module-tree.html"><
|
|||
|
</div>
|
|||
|
<div id="sidebar-resize-handle" class="sidebar-resize-handle"></div>
|
|||
|
</nav>
|
|||
|
|
|||
|
<div id="page-wrapper" class="page-wrapper">
|
|||
|
|
|||
|
<div class="page">
|
|||
|
|
|||
|
<div id="menu-bar" class="menu-bar">
|
|||
|
<div id="menu-bar-sticky-container">
|
|||
|
<div class="left-buttons">
|
|||
|
<button id="sidebar-toggle" class="icon-button" type="button" title="Toggle Table of Contents" aria-label="Toggle Table of Contents" aria-controls="sidebar">
|
|||
|
<i class="fa fa-bars"></i>
|
|||
|
</button>
|
|||
|
<button id="theme-toggle" class="icon-button" type="button" title="Change theme" aria-label="Change theme" aria-haspopup="true" aria-expanded="false" aria-controls="theme-list">
|
|||
|
<i class="fa fa-paint-brush"></i>
|
|||
|
</button>
|
|||
|
<ul id="theme-list" class="theme-popup" aria-label="Themes" role="menu">
|
|||
|
<li role="none"><button role="menuitem" class="theme" id="light">Light (default)</button></li>
|
|||
|
<li role="none"><button role="menuitem" class="theme" id="rust">Rust</button></li>
|
|||
|
<li role="none"><button role="menuitem" class="theme" id="coal">Coal</button></li>
|
|||
|
<li role="none"><button role="menuitem" class="theme" id="navy">Navy</button></li>
|
|||
|
<li role="none"><button role="menuitem" class="theme" id="ayu">Ayu</button></li>
|
|||
|
</ul>
|
|||
|
|
|||
|
<button id="search-toggle" class="icon-button" type="button" title="Search. (Shortkey: s)" aria-label="Toggle Searchbar" aria-expanded="false" aria-keyshortcuts="S" aria-controls="searchbar">
|
|||
|
<i class="fa fa-search"></i>
|
|||
|
</button>
|
|||
|
|
|||
|
</div>
|
|||
|
|
|||
|
<h1 class="menu-title">The Rust Programming Language</h1>
|
|||
|
|
|||
|
<div class="right-buttons">
|
|||
|
<a href="print.html" title="Print this book" aria-label="Print this book">
|
|||
|
<i id="print-button" class="fa fa-print"></i>
|
|||
|
</a>
|
|||
|
|
|||
|
</div>
|
|||
|
</div>
|
|||
|
</div>
|
|||
|
|
|||
|
|
|||
|
<div id="search-wrapper" class="hidden">
|
|||
|
<form id="searchbar-outer" class="searchbar-outer">
|
|||
|
<input type="search" name="search" id="searchbar" name="searchbar" placeholder="Search this book ..." aria-controls="searchresults-outer" aria-describedby="searchresults-header">
|
|||
|
</form>
|
|||
|
<div id="searchresults-outer" class="searchresults-outer hidden">
|
|||
|
<div id="searchresults-header" class="searchresults-header"></div>
|
|||
|
<ul id="searchresults">
|
|||
|
</ul>
|
|||
|
</div>
|
|||
|
</div>
|
|||
|
|
|||
|
|
|||
|
<!-- Apply ARIA attributes after the sidebar and the sidebar toggle button are added to the DOM -->
|
|||
|
<script type="text/javascript">
|
|||
|
document.getElementById('sidebar-toggle').setAttribute('aria-expanded', sidebar === 'visible');
|
|||
|
document.getElementById('sidebar').setAttribute('aria-hidden', sidebar !== 'visible');
|
|||
|
Array.from(document.querySelectorAll('#sidebar a')).forEach(function(link) {
|
|||
|
link.setAttribute('tabIndex', sidebar === 'visible' ? 0 : -1);
|
|||
|
});
|
|||
|
</script>
|
|||
|
|
|||
|
<div id="content" class="content">
|
|||
|
<main>
|
|||
|
<h2><a class="header" href="#working-with-environment-variables" id="working-with-environment-variables">Working with Environment Variables</a></h2>
|
|||
|
<p>We’ll improve <code>minigrep</code> by adding an extra feature: an option for
|
|||
|
case-insensitive searching that the user can turn on via an environment
|
|||
|
variable. We could make this feature a command line option and require that
|
|||
|
users enter it each time they want it to apply, but instead we’ll use an
|
|||
|
environment variable. Doing so allows our users to set the environment variable
|
|||
|
once and have all their searches be case insensitive in that terminal session.</p>
|
|||
|
<h3><a class="header" href="#writing-a-failing-test-for-the-case-insensitive-search-function" id="writing-a-failing-test-for-the-case-insensitive-search-function">Writing a Failing Test for the Case-Insensitive <code>search</code> Function</a></h3>
|
|||
|
<p>We want to add a new <code>search_case_insensitive</code> function that we’ll call when
|
|||
|
the environment variable is on. We’ll continue to follow the TDD process, so
|
|||
|
the first step is again to write a failing test. We’ll add a new test for the
|
|||
|
new <code>search_case_insensitive</code> function and rename our old test from
|
|||
|
<code>one_result</code> to <code>case_sensitive</code> to clarify the differences between the two
|
|||
|
tests, as shown in Listing 12-20.</p>
|
|||
|
<p><span class="filename">Filename: src/lib.rs</span></p>
|
|||
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
</span><span class="boring">fn main() {
|
|||
|
</span>#[cfg(test)]
|
|||
|
mod tests {
|
|||
|
use super::*;
|
|||
|
|
|||
|
#[test]
|
|||
|
fn case_sensitive() {
|
|||
|
let query = "duct";
|
|||
|
let contents = "\
|
|||
|
Rust:
|
|||
|
safe, fast, productive.
|
|||
|
Pick three.
|
|||
|
Duct tape.";
|
|||
|
|
|||
|
assert_eq!(
|
|||
|
vec!["safe, fast, productive."],
|
|||
|
search(query, contents)
|
|||
|
);
|
|||
|
}
|
|||
|
|
|||
|
#[test]
|
|||
|
fn case_insensitive() {
|
|||
|
let query = "rUsT";
|
|||
|
let contents = "\
|
|||
|
Rust:
|
|||
|
safe, fast, productive.
|
|||
|
Pick three.
|
|||
|
Trust me.";
|
|||
|
|
|||
|
assert_eq!(
|
|||
|
vec!["Rust:", "Trust me."],
|
|||
|
search_case_insensitive(query, contents)
|
|||
|
);
|
|||
|
}
|
|||
|
}
|
|||
|
<span class="boring">}
|
|||
|
</span></code></pre></pre>
|
|||
|
<p><span class="caption">Listing 12-20: Adding a new failing test for the
|
|||
|
case-insensitive function we’re about to add</span></p>
|
|||
|
<p>Note that we’ve edited the old test’s <code>contents</code> too. We’ve added a new line
|
|||
|
with the text <code>"Duct tape."</code> using a capital D that shouldn’t match the query
|
|||
|
<code>"duct"</code> when we’re searching in a case-sensitive manner. Changing the old test
|
|||
|
in this way helps ensure that we don’t accidentally break the case-sensitive
|
|||
|
search functionality that we’ve already implemented. This test should pass now
|
|||
|
and should continue to pass as we work on the case-insensitive search.</p>
|
|||
|
<p>The new test for the case-<em>insensitive</em> search uses <code>"rUsT"</code> as its query. In
|
|||
|
the <code>search_case_insensitive</code> function we’re about to add, the query <code>"rUsT"</code>
|
|||
|
should match the line containing <code>"Rust:"</code> with a capital R and match the 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
|
|||
|
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>
|
|||
|
<h3><a class="header" href="#implementing-the-search_case_insensitive-function" id="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 whatever the case of the input arguments,
|
|||
|
they’ll be the same case when we check whether the line contains the query.</p>
|
|||
|
<p><span class="filename">Filename: src/lib.rs</span></p>
|
|||
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
</span><span class="boring">fn main() {
|
|||
|
</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></code></pre></pre>
|
|||
|
<p><span class="caption">Listing 12-21: Defining the <code>search_case_insensitive</code>
|
|||
|
function to lowercase the query and the line before comparing them</span></p>
|
|||
|
<p>First, we lowercase the <code>query</code> string and store it in a shadowed variable with
|
|||
|
the same name. Calling <code>to_lowercase</code> on the query is necessary so 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.</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> before we check whether it
|
|||
|
contains <code>query</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-text">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
|
|||
|
</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/lib.rs</span></p>
|
|||
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
</span><span class="boring">fn main() {
|
|||
|
</span>pub struct Config {
|
|||
|
pub query: String,
|
|||
|
pub filename: String,
|
|||
|
pub case_sensitive: bool,
|
|||
|
}
|
|||
|
<span class="boring">}
|
|||
|
</span></code></pre></pre>
|
|||
|
<p>Note that we added the <code>case_sensitive</code> field that holds a Boolean. Next, we
|
|||
|
need the <code>run</code> function to check the <code>case_sensitive</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. Note this still
|
|||
|
won’t compile yet.</p>
|
|||
|
<p><span class="filename">Filename: src/lib.rs</span></p>
|
|||
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
</span><span class="boring">fn main() {
|
|||
|
</span><span class="boring">use std::error::Error;
|
|||
|
</span><span class="boring">use std::fs::{self, File};
|
|||
|
</span><span class="boring">use std::io::prelude::*;
|
|||
|
</span><span class="boring">
|
|||
|
</span><span class="boring">pub fn search<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
|||
|
</span><span class="boring"> vec![]
|
|||
|
</span><span class="boring">}
|
|||
|
</span><span class="boring">
|
|||
|
</span><span class="boring">pub fn search_case_insensitive<'a>(query: &str, contents: &'a str) -> Vec<&'a str> {
|
|||
|
</span><span class="boring"> vec![]
|
|||
|
</span><span class="boring">}
|
|||
|
</span><span class="boring">
|
|||
|
</span><span class="boring">pub struct Config {
|
|||
|
</span><span class="boring"> query: String,
|
|||
|
</span><span class="boring"> filename: String,
|
|||
|
</span><span class="boring"> case_sensitive: bool,
|
|||
|
</span><span class="boring">}
|
|||
|
</span><span class="boring">
|
|||
|
</span>pub fn run(config: Config) -> Result<(), Box<dyn Error>> {
|
|||
|
let contents = fs::read_to_string(config.filename)?;
|
|||
|
|
|||
|
let results = if config.case_sensitive {
|
|||
|
search(&config.query, &contents)
|
|||
|
} else {
|
|||
|
search_case_insensitive(&config.query, &contents)
|
|||
|
};
|
|||
|
|
|||
|
for line in results {
|
|||
|
println!("{}", line);
|
|||
|
}
|
|||
|
|
|||
|
Ok(())
|
|||
|
}
|
|||
|
<span class="boring">}
|
|||
|
</span></code></pre></pre>
|
|||
|
<p><span class="caption">Listing 12-22: Calling either <code>search</code> or
|
|||
|
<code>search_case_insensitive</code> based on the value in <code>config.case_sensitive</code></span></p>
|
|||
|
<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, so we want to bring that module into scope with a <code>use std::env;</code> line
|
|||
|
at the top of <em>src/lib.rs</em>. Then we’ll use the <code>var</code> function from the <code>env</code>
|
|||
|
module to check for an environment variable named <code>CASE_INSENSITIVE</code>, as shown
|
|||
|
in Listing 12-23.</p>
|
|||
|
<p><span class="filename">Filename: src/lib.rs</span></p>
|
|||
|
<pre><pre class="playpen"><code class="language-rust">
|
|||
|
<span class="boring">#![allow(unused_variables)]
|
|||
|
</span><span class="boring">fn main() {
|
|||
|
</span>use std::env;
|
|||
|
<span class="boring">struct Config {
|
|||
|
</span><span class="boring"> query: String,
|
|||
|
</span><span class="boring"> filename: String,
|
|||
|
</span><span class="boring"> case_sensitive: bool,
|
|||
|
</span><span class="boring">}
|
|||
|
</span>
|
|||
|
// --snip--
|
|||
|
|
|||
|
impl Config {
|
|||
|
pub fn new(args: &[String]) -> Result<Config, &'static str> {
|
|||
|
if args.len() < 3 {
|
|||
|
return Err("not enough arguments");
|
|||
|
}
|
|||
|
|
|||
|
let query = args[1].clone();
|
|||
|
let filename = args[2].clone();
|
|||
|
|
|||
|
let case_sensitive = env::var("CASE_INSENSITIVE").is_err();
|
|||
|
|
|||
|
Ok(Config { query, filename, case_sensitive })
|
|||
|
}
|
|||
|
}
|
|||
|
<span class="boring">}
|
|||
|
</span></code></pre></pre>
|
|||
|
<p><span class="caption">Listing 12-23: Checking for an environment variable named
|
|||
|
<code>CASE_INSENSITIVE</code></span></p>
|
|||
|
<p>Here, we create a new variable <code>case_sensitive</code>. To set its value, we call the
|
|||
|
<code>env::var</code> function and pass it the name of the <code>CASE_INSENSITIVE</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. It will return the <code>Err</code> variant if the
|
|||
|
environment variable is not set.</p>
|
|||
|
<p>We’re using the <code>is_err</code> method on the <code>Result</code> to check whether it’s an error
|
|||
|
and therefore unset, which means it <em>should</em> do a case-sensitive search. If the
|
|||
|
<code>CASE_INSENSITIVE</code> environment variable is set to anything, <code>is_err</code> will
|
|||
|
return false and the program will perform a case-insensitive 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_err</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>case_sensitive</code> variable to the <code>Config</code> instance so
|
|||
|
the <code>run</code> function can read that value and decide whether to call <code>search</code> or
|
|||
|
<code>search_case_insensitive</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 “to” in all lowercase:</p>
|
|||
|
<pre><code class="language-text">$ cargo run to poem.txt
|
|||
|
Compiling minigrep v0.1.0 (file:///projects/minigrep)
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
|
|||
|
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>CASE_INSENSITIVE</code>
|
|||
|
set to <code>1</code> but with the same query <code>to</code>.</p>
|
|||
|
<p>If you’re using PowerShell, you will need to set the environment variable and
|
|||
|
run the program in two commands rather than one:</p>
|
|||
|
<pre><code class="language-text">$ $env:CASE_INSENSITIVE=1
|
|||
|
$ cargo run to poem.txt
|
|||
|
</code></pre>
|
|||
|
<p>We should get lines that contain “to” that might have uppercase letters:</p>
|
|||
|
<pre><code class="language-text">$ CASE_INSENSITIVE=1 cargo run to poem.txt
|
|||
|
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
|
|||
|
Running `target/debug/minigrep to poem.txt`
|
|||
|
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 “To”! 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
|
|||
|
insensitivity 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 case insensitive.</p>
|
|||
|
<p>The <code>std::env</code> module contains many more useful features for dealing with
|
|||
|
environment variables: check out its documentation to see what is available.</p>
|
|||
|
|
|||
|
</main>
|
|||
|
|
|||
|
<nav class="nav-wrapper" aria-label="Page navigation">
|
|||
|
<!-- Mobile navigation buttons -->
|
|||
|
|
|||
|
<a rel="prev" href="ch12-04-testing-the-librarys-functionality.html" class="mobile-nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|||
|
<i class="fa fa-angle-left"></i>
|
|||
|
</a>
|
|||
|
|
|||
|
|
|||
|
|
|||
|
<a rel="next" href="ch12-06-writing-to-stderr-instead-of-stdout.html" class="mobile-nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|||
|
<i class="fa fa-angle-right"></i>
|
|||
|
</a>
|
|||
|
|
|||
|
|
|||
|
<div style="clear: both"></div>
|
|||
|
</nav>
|
|||
|
</div>
|
|||
|
</div>
|
|||
|
|
|||
|
<nav class="nav-wide-wrapper" aria-label="Page navigation">
|
|||
|
|
|||
|
<a href="ch12-04-testing-the-librarys-functionality.html" class="nav-chapters previous" title="Previous chapter" aria-label="Previous chapter" aria-keyshortcuts="Left">
|
|||
|
<i class="fa fa-angle-left"></i>
|
|||
|
</a>
|
|||
|
|
|||
|
|
|||
|
|
|||
|
<a href="ch12-06-writing-to-stderr-instead-of-stdout.html" class="nav-chapters next" title="Next chapter" aria-label="Next chapter" aria-keyshortcuts="Right">
|
|||
|
<i class="fa fa-angle-right"></i>
|
|||
|
</a>
|
|||
|
|
|||
|
</nav>
|
|||
|
|
|||
|
</div>
|
|||
|
|
|||
|
|
|||
|
|
|||
|
|
|||
|
|
|||
|
|
|||
|
|
|||
|
|
|||
|
<script type="text/javascript">
|
|||
|
window.playpen_copyable = true;
|
|||
|
</script>
|
|||
|
|
|||
|
|
|||
|
|
|||
|
|
|||
|
|
|||
|
<script src="elasticlunr.min.js" type="text/javascript" charset="utf-8"></script>
|
|||
|
<script src="mark.min.js" type="text/javascript" charset="utf-8"></script>
|
|||
|
<script src="searcher.js" type="text/javascript" charset="utf-8"></script>
|
|||
|
|
|||
|
|
|||
|
<script src="clipboard.min.js" type="text/javascript" charset="utf-8"></script>
|
|||
|
<script src="highlight.js" type="text/javascript" charset="utf-8"></script>
|
|||
|
<script src="book.js" type="text/javascript" charset="utf-8"></script>
|
|||
|
|
|||
|
<!-- Custom JS scripts -->
|
|||
|
|
|||
|
<script type="text/javascript" src="ferris.js"></script>
|
|||
|
|
|||
|
|
|||
|
|
|||
|
|
|||
|
</body>
|
|||
|
</html>
|