pierre/RustBook
1
0
Fork 0
Code Issues Pull Requests Projects Releases Wiki Activity
master
RustBook/ch12-06-writing-to-stderr-instead-of-stdout.html

313 lines
31 KiB
HTML
Raw Permalink Normal View History

Initial commit
2020-01-06 20:57:15 +00:00
<!DOCTYPE HTML>
<html lang="en" class="sidebar-visible no-js light">
<head>
<!-- Book generated using mdBook -->
<meta charset="UTF-8">
<title>Writing Error Messages to Standard Error Instead of Standard Output - 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="#writing-error-messages-to-standard-error-instead-of-standard-output" id="writing-error-messages-to-standard-error-instead-of-standard-output">Writing Error Messages to Standard Error Instead of Standard Output</a></h2>
<p>At the moment, we’re writing all of our output to the terminal using the
<code>println!</code> function. Most terminals provide two kinds of output: <em>standard
output</em> (<code>stdout</code>) for general information and <em>standard error</em> (<code>stderr</code>)
for error messages. This distinction enables users to choose to direct the
successful output of a program to a file but still print error messages to the
screen.</p>
<p>The <code>println!</code> function is only capable of printing to standard output, so we
have to use something else to print to standard error.</p>
<h3><a class="header" href="#checking-where-errors-are-written" id="checking-where-errors-are-written">Checking Where Errors Are Written</a></h3>
<p>First, let’s observe how the content printed by <code>minigrep</code> is currently being
written to standard output, including any error messages we want to write to
standard error instead. We’ll do that by redirecting the standard output stream
to a file while also intentionally causing an error. We won’t redirect the
standard error stream, so any content sent to standard error will continue to
display on the screen.</p>
<p>Command line programs are expected to send error messages to the standard error
stream so we can still see error messages on the screen even if we redirect the
standard output stream to a file. Our program is not currently well-behaved:
we’re about to see that it saves the error message output to a file instead!</p>
<p>The way to demonstrate this behavior is by running the program with <code>&gt;</code> and the
filename, <em>output.txt</em>, that we want to redirect the standard output stream to.
We won’t pass any arguments, which should cause an error:</p>
<pre><code class="language-text">$ cargo run &gt; output.txt
</code></pre>
<p>The <code>&gt;</code> syntax tells the shell to write the contents of standard output to
<em>output.txt</em> instead of the screen. We didn’t see the error message we were
expecting printed to the screen, so that means it must have ended up in the
file. This is what <em>output.txt</em> contains:</p>
<pre><code class="language-text">Problem parsing arguments: not enough arguments
</code></pre>
<p>Yup, our error message is being printed to standard output. It’s much more
useful for error messages like this to be printed to standard error so only
data from a successful run ends up in the file. We’ll change that.</p>
<h3><a class="header" href="#printing-errors-to-standard-error" id="printing-errors-to-standard-error">Printing Errors to Standard Error</a></h3>
<p>We’ll use the code in Listing 12-24 to change how error messages are printed.
Because of the refactoring we did earlier in this chapter, all the code that
prints error messages is in one function, <code>main</code>. The standard library provides
the <code>eprintln!</code> macro that prints to the standard error stream, so let’s change
the two places we were calling <code>println!</code> to print errors to use <code>eprintln!</code>
instead.</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><code class="language-rust ignore">fn main() {
let args: Vec&lt;String&gt; = env::args().collect();
let config = Config::new(&amp;args).unwrap_or_else(|err| {
eprintln!(&quot;Problem parsing arguments: {}&quot;, err);
process::exit(1);
});
if let Err(e) = minigrep::run(config) {
eprintln!(&quot;Application error: {}&quot;, e);
process::exit(1);
}
}
</code></pre>
<p><span class="caption">Listing 12-24: Writing error messages to standard error
instead of standard output using <code>eprintln!</code></span></p>
<p>After changing <code>println!</code> to <code>eprintln!</code>, let’s run the program again in the
same way, without any arguments and redirecting standard output with <code>&gt;</code>:</p>
<pre><code class="language-text">$ cargo run &gt; output.txt
Problem parsing arguments: not enough arguments
</code></pre>
<p>Now we see the error onscreen and <em>output.txt</em> contains nothing, which is the
behavior we expect of command line programs.</p>
<p>Let’s run the program again with arguments that don’t cause an error but still
redirect standard output to a file, like so:</p>
<pre><code class="language-text">$ cargo run to poem.txt &gt; output.txt
</code></pre>
<p>We won’t see any output to the terminal, and <em>output.txt</em> will contain our
results:</p>
<p><span class="filename">Filename: output.txt</span></p>
<pre><code class="language-text">Are you nobody, too?
How dreary to be somebody!
</code></pre>
<p>This demonstrates that we’re now using standard output for successful output
and standard error for error output as appropriate.</p>
<h2><a class="header" href="#summary" id="summary">Summary</a></h2>
<p>This chapter recapped some of the major concepts you’ve learned so far and
covered how to perform common I/O operations in Rust. By using command line
arguments, files, environment variables, and the <code>eprintln!</code> macro for printing
errors, you’re now prepared to write command line applications. By using the
concepts in previous chapters, your code will be well organized, store data
effectively in the appropriate data structures, handle errors nicely, and be
well tested.</p>
<p>Next, we’ll explore some Rust features that were influenced by functional
languages: closures and iterators.</p>
</main>
<nav class="nav-wrapper" aria-label="Page navigation">
<!-- Mobile navigation buttons -->
<a rel="prev" href="ch12-05-working-with-environment-variables.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="ch13-00-functional-features.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="ch12-05-working-with-environment-variables.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="ch13-00-functional-features.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>
Reference in New Issue Copy Permalink