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<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"><
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<main>
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<h1><a class="header" href="#an-io-project-building-a-command-line-program" id="an-io-project-building-a-command-line-program">An I/O Project: Building a Command Line Program</a></h1>
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<p>This chapter is a recap of the many skills you’ve learned so far and an
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exploration of a few more standard library features. We’ll build a command line
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tool that interacts with file and command line input/output to practice some of
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the Rust concepts you now have under your belt.</p>
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<p>Rust’s speed, safety, single binary output, and cross-platform support make it
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an ideal language for creating command line tools, so for our project, we’ll
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make our own version of the classic command line tool <code>grep</code> (<strong>g</strong>lobally
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search a <strong>r</strong>egular <strong>e</strong>xpression and <strong>p</strong>rint). In the simplest use case,
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<code>grep</code> searches a specified file for a specified string. To do so, <code>grep</code> takes
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as its arguments a filename and a string. Then it reads the file, finds lines
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in that file that contain the string argument, and prints those lines.</p>
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<p>Along the way, we’ll show how to make our command line tool use features of the
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terminal that many command line tools use. We’ll read the value of an
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environment variable to allow the user to configure the behavior of our tool.
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We’ll also print error messages to the standard error console stream (<code>stderr</code>)
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instead of standard output (<code>stdout</code>), so, for example, the user can redirect
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successful output to a file while still seeing error messages onscreen.</p>
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<p>One Rust community member, Andrew Gallant, has already created a fully
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featured, very fast version of <code>grep</code>, called <code>ripgrep</code>. By comparison, our
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version of <code>grep</code> will be fairly simple, but this chapter will give you some of
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the background knowledge you need to understand a real-world project such as
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<code>ripgrep</code>.</p>
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<p>Our <code>grep</code> project will combine a number of concepts you’ve learned so far:</p>
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<ul>
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<li>Organizing code (using what you learned about modules in <a href="ch07-00-managing-growing-projects-with-packages-crates-and-modules.html">Chapter 7</a><!--
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ignore -->)</li>
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<li>Using vectors and strings (collections, <a href="ch08-00-common-collections.html">Chapter 8</a><!-- ignore -->)</li>
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<li>Handling errors (<a href="ch09-00-error-handling.html">Chapter 9</a><!-- ignore -->)</li>
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<li>Using traits and lifetimes where appropriate (<a href="ch10-00-generics.html">Chapter 10</a><!-- ignore
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-->)</li>
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<li>Writing tests (<a href="ch11-00-testing.html">Chapter 11</a><!-- ignore -->)</li>
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</ul>
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<p>We’ll also briefly introduce closures, iterators, and trait objects, which
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Chapters <a href="ch13-00-functional-features.html">13</a><!-- ignore --> and <a href="ch17-00-oop.html">17</a><!-- ignore --> will cover in
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detail.</p>
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