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