<!DOCTYPE HTML>
<html lang="en">
<head>
<meta charset="UTF-8">
<title>Writing to `stderr` instead of `stdout` - 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">
<base href="">
<link rel="stylesheet" href="book.css">
<link href='https://fonts.googleapis.com/css?family=Open+Sans:300italic,400italic,600italic,700italic,800italic,400,300,600,700,800' rel='stylesheet' type='text/css'>
<link rel="shortcut icon" href="favicon.png">
<!-- Font Awesome -->
<link rel="stylesheet" href="https://maxcdn.bootstrapcdn.com/font-awesome/4.3.0/css/font-awesome.min.css">
<link rel="stylesheet" href="highlight.css">
<link rel="stylesheet" href="tomorrow-night.css">
<style>
.page-wrapper.has-warning > .nav-chapters {
/* add height for warning content & margin */
top: 120px;
}
p.warning {
background-color: rgb(242, 222, 222);
border-bottom-color: rgb(238, 211, 215);
border-bottom-left-radius: 4px;
border-bottom-right-radius: 4px;
border-bottom-style: solid;
border-bottom-width: 0.666667px;
border-image-outset: 0 0 0 0;
border-image-repeat: stretch stretch;
border-image-slice: 100% 100% 100% 100%;
border-image-source: none;
border-image-width: 1 1 1 1;
border-left-color: rgb(238, 211, 215);
border-left-style: solid;
border-left-width: 0.666667px;
border-right-color: rgb(238, 211, 215);
border-right-style: solid;
border-right-width: 0.666667px;
border-top-color: rgb(238, 211, 215);
border-top-left-radius: 4px;
border-top-right-radius: 4px;
border-top-style: solid;
border-top-width: 0.666667px;
color: rgb(185, 74, 72);
margin-bottom: 0px;
margin-left: 0px;
margin-right: 0px;
margin-top: 30px;
padding-bottom: 8px;
padding-left: 14px;
padding-right: 35px;
padding-top: 8px;
}
p.warning strong {
color: rgb(185, 74, 72)
}
p.warning a {
color: rgb(0, 136, 204)
}
</style>
<!-- MathJax -->
<script type="text/javascript" src="https://cdn.mathjax.org/mathjax/latest/MathJax.js?config=TeX-AMS-MML_HTMLorMML"></script>
<!-- Fetch JQuery from CDN but have a local fallback -->
<script src="https://code.jquery.com/jquery-2.1.4.min.js"></script>
<script>
if (typeof jQuery == 'undefined') {
document.write(unescape("%3Cscript src='jquery.js'%3E%3C/script%3E"));
}
</script>
</head>
<body class="light">
<!-- Set the theme before any content is loaded, prevents flash -->
<script type="text/javascript">
var theme = localStorage.getItem('theme');
if (theme == null) { theme = 'light'; }
$('body').removeClass().addClass(theme);
</script>
<!-- Hide / unhide sidebar before it is displayed -->
<script type="text/javascript">
var sidebar = localStorage.getItem('sidebar');
if (sidebar === "hidden") { $("html").addClass("sidebar-hidden") }
else if (sidebar === "visible") { $("html").addClass("sidebar-visible") }
</script>
<div id="sidebar" class="sidebar">
<ul class="chapter"><li><a href="ch01-00-introduction.html"><strong>1.</strong> Introduction</a></li><li><ul class="section"><li><a href="ch01-01-installation.html"><strong>1.1.</strong> Installation</a></li><li><a href="ch01-02-hello-world.html"><strong>1.2.</strong> Hello, World!</a></li></ul></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong>2.</strong> Guessing Game Tutorial</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong>3.</strong> Common Programming Concepts</a></li><li><ul class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong>3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong>3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong>3.3.</strong> How Functions Work</a></li><li><a href="ch03-04-comments.html"><strong>3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong>3.5.</strong> Control Flow</a></li></ul></li><li><a href="ch04-00-understanding-ownership.html"><strong>4.</strong> Understanding Ownership</a></li><li><ul class="section"><li><a href="ch04-01-what-is-ownership.html"><strong>4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong>4.2.</strong> References & Borrowing</a></li><li><a href="ch04-03-slices.html"><strong>4.3.</strong> Slices</a></li></ul></li><li><a href="ch05-00-structs.html"><strong>5.</strong> Using Structs to Structure Related Data</a></li><li><ul class="section"><li><a href="ch05-01-defining-structs.html"><strong>5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html"><strong>5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html"><strong>5.3.</strong> Method Syntax</a></li></ul></li><li><a href="ch06-00-enums.html"><strong>6.</strong> Enums and Pattern Matching</a></li><li><ul class="section"><li><a href="ch06-01-defining-an-enum.html"><strong>6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong>6.2.</strong> The <code>match</code> Control Flow Operator</a></li><li><a href="ch06-03-if-let.html"><strong>6.3.</strong> Concise Control Flow with <code>if let</code></a></li></ul></li><li><a href="ch07-00-modules.html"><strong>7.</strong> Modules</a></li><li><ul class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong>7.1.</strong> <code>mod</code> and the Filesystem</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong>7.2.</strong> Controlling Visibility with <code>pub</code></a></li><li><a href="ch07-03-importing-names-with-use.html"><strong>7.3.</strong> Importing Names with <code>use</code></a></li></ul></li><li><a href="ch08-00-common-collections.html"><strong>8.</strong> Common Collections</a></li><li><ul class="section"><li><a href="ch08-01-vectors.html"><strong>8.1.</strong> Vectors</a></li><li><a href="ch08-02-strings.html"><strong>8.2.</strong> Strings</a></li><li><a href="ch08-03-hash-maps.html"><strong>8.3.</strong> Hash Maps</a></li></ul></li><li><a href="ch09-00-error-handling.html"><strong>9.</strong> Error Handling</a></li><li><ul class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong>9.1.</strong> Unrecoverable Errors with <code>panic!</code></a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong>9.2.</strong> Recoverable Errors with <code>Result</code></a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong>9.3.</strong> To <code>panic!</code> or Not To <code>panic!</code></a></li></ul></li><li><a href="ch10-00-generics.html"><strong>10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ul class="section"><li><a href="ch10-01-syntax.html"><strong>10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong>10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong>10.3.</strong> Validating References with Lifetimes</a></li></ul></li><li><a href="ch11-00-testing.html"><strong>11.</strong> Testing</a></li><li><ul class="section"><li><a href="ch11-01-writing-tests.html"><strong>11.1.</strong> Writing tests</a></li><li><a href="ch11-02-running-tests.html"><strong>11.2.</strong> Running tests</a></li><li><a href="ch11-03-test-organization.html"><strong>11.3.</strong> Test Organization</a></li></ul></li><li><a href="ch12-00-an-io-project.html"><strong>12.</strong> An I/O Project</a></li><li><ul class="section"><li><a href="ch12-01-accepting-command-line-arguments.html"><strong>12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong>12.2.</strong> Reading a File</a></li><li><a href="ch12-03-improving-error-handling-and-modularity.html"><strong>12.3.</strong> Improving Error Handling and Modularity</a></li><li><a href="ch12-04-testing-the-librarys-functionality.html"><strong>12.4.</strong> Testing the Library's Functionality</a></li><li><a href="ch12-05-working-with-environment-variables.html"><strong>12.5.</strong> Working with Environment Variables</a></li><li><a href="ch12-06-writing-to-stderr-instead-of-stdout.html" class="active"><strong>12.6.</strong> Writing to <code>stderr</code> instead of <code>stdout</code></a></li></ul></li><li><a href="ch13-00-functional-features.html"><strong>13.</strong> Functional Language Features in Rust</a></li><li><ul class="section"><li><a href="ch13-01-closures.html"><strong>13.1.</strong> Closures</a></li><li><a href="ch13-02-iterators.html"><strong>13.2.</strong> Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong>13.3.</strong> Improving our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong>13.4.</strong> Performance</a></li></ul></li><li><a href="ch14-00-more-about-cargo.html"><strong>14.</strong> More about Cargo and Crates.io</a></li><li><ul class="section"><li><a href="ch14-01-release-profiles.html"><strong>14.1.</strong> Release Profiles</a></li><li><a href="ch14-02-publishing-to-crates-io.html"><strong>14.2.</strong> Publishing a Crate to Crates.io</a></li><li><a href="ch14-03-cargo-workspaces.html"><strong>14.3.</strong> Cargo Workspaces</a></li><li><a href="ch14-04-installing-binaries.html"><strong>14.4.</strong> Installing Binaries from Crates.io with <code>cargo install</code></a></li><li><a href="ch14-05-extending-cargo.html"><strong>14.5.</strong> Extending Cargo with Custom Commands</a></li></ul></li><li><a href="ch15-00-smart-pointers.html"><strong>15.</strong> Smart Pointers</a></li><li><ul class="section"><li><a href="ch15-01-box.html"><strong>15.1.</strong> <code>Box<T></code> Points to Data on the Heap and Has a Known Size</a></li><li><a href="ch15-02-deref.html"><strong>15.2.</strong> The <code>Deref</code> Trait Allows Access to the Data Through a Reference</a></li><li><a href="ch15-03-drop.html"><strong>15.3.</strong> The <code>Drop</code> Trait Runs Code on Cleanup</a></li><li><a href="ch15-04-rc.html"><strong>15.4.</strong> <code>Rc<T></code>, the Reference Counted Smart Pointer</a></li><li><a href="ch15-05-interior-mutability.html"><strong>15.5.</strong> <code>RefCell<T></code> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong>15.6.</strong> Creating Reference Cycles and Leaking Memory is Safe</a></li></ul></li><li><a href="ch16-00-concurrency.html"><strong>16.</strong> Fearless Concurrency</a></li><li><ul class="section"><li><a href="ch16-01-threads.html"><strong>16.1.</strong> Threads</a></li><li><a href="ch16-02-message-passing.html"><strong>16.2.</strong> Message Passing</a></li><li><a href="ch16-03-shared-state.html"><strong>16.3.</strong> Shared State</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong>16.4.</strong> Extensible Concurrency: <code>Sync</code> and <code>Send</code></a></li></ul></li><li><a href="ch17-00-oop.html"><strong>17.</strong> Is Rust an Object-Oriented Programming Language?</a></li><li><ul class="section"><li><a href="ch17-01-what-is-oo.html"><strong>17.1.</strong> What Does Object-Oriented Mean?</a></li><li><a href="ch17-02-trait-objects.html"><strong>17.2.</strong> Trait Objects for Using Values of Different Types</a></li><li><a href="ch17-03-oo-design-patterns.html"><strong>17.3.</strong> Object-Oriented Design Pattern Implementations</a></li></ul></li><li><a href="ch18-00-patterns.html"><strong>18.</strong> Patterns Match the Structure of Values</a></li><li><ul class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong>18.1.</strong> All the Places Patterns May be Used</a></li><li><a href="ch18-02-refutability.html"><strong>18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong>18.3.</strong> All the Pattern Syntax</a></li></ul></li><li><a href="ch19-00-advanced-features.html"><strong>19.</strong> Advanced Features</a></li><li><ul class="section"><li><a href="ch19-01-unsafe-rust.html"><strong>19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html"><strong>19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong>19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong>19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong>19.5.</strong> Advanced Functions & Closures</a></li></ul></li><li><a href="ch20-00-final-project-a-web-server.html"><strong>20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ul class="section"><li><a href="ch20-01-single-threaded.html"><strong>20.1.</strong> A Single Threaded Web Server</a></li><li><a href="ch20-02-slow-requests.html"><strong>20.2.</strong> How Slow Requests Affect Throughput</a></li><li><a href="ch20-03-designing-the-interface.html"><strong>20.3.</strong> Designing the Thread Pool Interface</a></li><li><a href="ch20-04-storing-threads.html"><strong>20.4.</strong> Creating the Thread Pool and Storing Threads</a></li><li><a href="ch20-05-sending-requests-via-channels.html"><strong>20.5.</strong> Sending Requests to Threads Via Channels</a></li><li><a href="ch20-06-graceful-shutdown-and-cleanup.html"><strong>20.6.</strong> Graceful Shutdown and Cleanup</a></li></ul></li><li><a href="appendix-00.html"><strong>21.</strong> Appendix</a></li><li><ul class="section"><li><a href="appendix-01-keywords.html"><strong>21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong>21.2.</strong> B - Operators</a></li><li><strong>21.3.</strong> C - Derivable Traits</li><li><strong>21.4.</strong> D - Nightly Rust</li><li><strong>21.5.</strong> E - Macros</li><li><strong>21.6.</strong> F - Translations</li><li><a href="appendix-07-newest-features.html"><strong>21.7.</strong> G - Newest Features</a></li></ul></li></ul>
</div>
<div id="page-wrapper" class="page-wrapper has-warning">
<div class="page">
<header><p class="warning">You are reading a <strong>draft</strong> of the next edition of TRPL. For more, go <a href="../index.html">here</a>.</p></header>
<div id="menu-bar" class="menu-bar">
<div class="left-buttons">
<i id="sidebar-toggle" class="fa fa-bars"></i>
<i id="theme-toggle" class="fa fa-paint-brush"></i>
</div>
<h1 class="menu-title">The Rust Programming Language</h1>
<div class="right-buttons">
<i id="print-button" class="fa fa-print" title="Print this book"></i>
</div>
</div>
<div id="content" class="content">
<a class="header" href="ch12-06-writing-to-stderr-instead-of-stdout.html#write-to-stderr-instead-of-stdout" id="write-to-stderr-instead-of-stdout"><h2>Write to <code>stderr</code> Instead of <code>stdout</code></h2></a>
<p>Right now, we're writing all of our output to the terminal with <code>println!</code>.
Most terminals provide two kinds of output: "standard out" for general
information, and "standard error" for error messages. This distinction is the
behavior that's expected of command line programs: it enables users to choose
to direct a program's successful output to a file but still print error
messages to the screen, for example. <code>println!</code> is only capable of printing to
standard out, though, so we have to use something else in order to print to
standard error.</p>
<p>We can verify that, the way we've written <code>greprs</code> so far, everything is being
written to standard out, including error messages that should be written to
standard error instead. We'll do that by intentionally causing an error, the
one that happens when we run the program without any arguments. We're going to
redirect standard output to a file, but not standard error. The way command
line programs are expected to work is that, because the output is an error
message, it should be shown on the screen rather than being redirected to the
file. Let's see that our program is not currently meeting this expectation by
using <code>></code> and specifying a filename, <em>output.txt</em>, that we want to redirect
standard out to:</p>
<pre><code class="language-text">$ cargo run > output.txt
</code></pre>
<!-- why do we get an error here? Was that intentional? Does that mean it can't
print stdout to a file? -->
<!-- Yes, we're intentionally causing an error here to show that errors are
currently going to the wrong place. It's showing that `println!` only prints
to standard out, even when we're printing error messages that should go
to standard error. /Carol-->
<p>The <code>></code> syntax tells the shell to write the contents of standard out to
<em>output.txt</em> instead of the screen. We didn't see the error message we were
expecting printed on the screen, so that means it must have ended up in the
file. Let's see what <em>output.txt</em> contains:</p>
<pre><code class="language-text">Problem parsing arguments: not enough arguments
</code></pre>
<!-- I don't understand why we send this output to a file to then just say we
want it to the screen, won't it do that by default? And what has this got to do
with our use of println? I'm finding the motives here hard to follow -->
<!-- The point of showing this is to demonstrate that our program is NOT doing
the correct thing by default, we need to change the places we're calling
`println!` with error messages to print to standard error instead. When to use
stdout vs. stderr, and why you might want to redirect stdout but not stderr,
is something our readers will be familiar with. /Carol -->
<p>Yup, there's our error message, which means it's being printed to standard out.
This isn't what's expected from command line programs. It's much more useful
for error messages like this to be printed to standard error, and only have
data printed to standard out from a successful run end up in the file when we
redirect standard out in this way. Let's change how error messages are printed
as shown in Listing 12-23. Because of the refactoring we did earlier in this
chapter, all of the code that prints error messages is in one place, in <code>main</code>:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><code class="language-rust ignore">extern crate greprs;
use std::env;
use std::process;
use std::io::prelude::*;
use greprs::Config;
fn main() {
let args: Vec<String> = env::args().collect();
let mut stderr = std::io::stderr();
let config = Config::new(&args).unwrap_or_else(|err| {
writeln!(
&mut stderr,
"Problem parsing arguments: {}",
err
).expect("Could not write to stderr");
process::exit(1);
});
if let Err(e) = greprs::run(config) {
writeln!(
&mut stderr,
"Application error: {}",
e
).expect("Could not write to stderr");
process::exit(1);
}
}
</code></pre>
<p><span class="caption">Listing 12-23: Writing error messages to <code>stderr</code> instead
of <code>stdout</code> using <code>writeln!</code></span></p>
<!-- Will add ghosting and wingdings in libreoffice /Carol -->
<p>Rust does not have a convenient function like <code>println!</code> for writing to
standard error. Instead, we use the <code>writeln!</code> macro, which is like <code>println!</code>
but takes an extra argument. The first thing we pass to it is what to write to.
We can acquire a handle to standard error through the <code>std::io::stderr</code>
function. We give a mutable reference to <code>stderr</code> to <code>writeln!</code>; we need it to
be mutable so we can write to it! The second and third arguments to <code>writeln!</code>
are like the first and second arguments to <code>println!</code>: a format string and any
variables we're interpolating.</p>
<p>Let's try running the program again in the same way, without any arguments and
redirecting <code>stdout</code> with <code>></code>:</p>
<pre><code class="language-text">$ cargo run > output.txt
Problem parsing arguments: not enough arguments
</code></pre>
<p>Now we see our error on the screen, and <code>output.txt</code> contains nothing, which is
the behavior that's expected of command line programs.</p>
<p>If we run the program again with arguments that don't cause an error, but still
redirecting standard out to a file:</p>
<pre><code class="language-text">$ cargo run to poem.txt > output.txt
</code></pre>
<p>We won't see any output to our terminal, and <code>output.txt</code> 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 out for successful output and
standard error for error output as appropriate.</p>
<a class="header" href="ch12-06-writing-to-stderr-instead-of-stdout.html#summary" id="summary"><h2>Summary</h2></a>
<p>In this chapter, we've recapped on some of the major concepts so far and
covered how to do common I/O operations in a Rust context. By using command
line arguments, files, environment variables, and the <code>writeln!</code> macro with
<code>stderr</code>, you're now prepared to write command line applications. By using the
concepts from previous chapters, your code will be well-organized, be able to
store data effectively in the appropriate data structures, handle errors
nicely, and be well tested.</p>
<p>Next, let's explore some functional-language influenced Rust features: closures
and iterators.</p>
</div>
<!-- Mobile navigation buttons -->
<a href="ch12-05-working-with-environment-variables.html" class="mobile-nav-chapters previous">
<i class="fa fa-angle-left"></i>
</a>
<a href="ch13-00-functional-features.html" class="mobile-nav-chapters next">
<i class="fa fa-angle-right"></i>
</a>
</div>
<a href="ch12-05-working-with-environment-variables.html" class="nav-chapters previous" title="You can navigate through the chapters using the arrow keys">
<i class="fa fa-angle-left"></i>
</a>
<a href="ch13-00-functional-features.html" class="nav-chapters next" title="You can navigate through the chapters using the arrow keys">
<i class="fa fa-angle-right"></i>
</a>
</div>
<!-- Local fallback for Font Awesome -->
<script>
if ($(".fa").css("font-family") !== "FontAwesome") {
$('<link rel="stylesheet" type="text/css" href="_FontAwesome/css/font-awesome.css">').prependTo('head');
}
</script>
<!-- Livereload script (if served using the cli tool) -->
<script src="highlight.js"></script>
<script src="book.js"></script>
</body>
</html>
distrib
>
Mageia
>
6
>
armv7hl
>
media
>
core-updates
>
by-pkgid
>
564935689ab5527f955e5449ded02799
>
files
>
273
