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<title>Accepting Command Line Arguments - The Rust Programming Language</title>
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<ol class="chapter"><li class="affix"><a href="foreword.html">Foreword</a></li><li class="affix"><a href="ch00-00-introduction.html">Introduction</a></li><li><a href="ch01-00-getting-started.html"><strong aria-hidden="true">1.</strong> Getting Started</a></li><li><ol class="section"><li><a href="ch01-01-installation.html"><strong aria-hidden="true">1.1.</strong> Installation</a></li><li><a href="ch01-02-hello-world.html"><strong aria-hidden="true">1.2.</strong> Hello, World!</a></li><li><a href="ch01-03-hello-cargo.html"><strong aria-hidden="true">1.3.</strong> Hello, Cargo!</a></li></ol></li><li><a href="ch02-00-guessing-game-tutorial.html"><strong aria-hidden="true">2.</strong> Programming a Guessing Game</a></li><li><a href="ch03-00-common-programming-concepts.html"><strong aria-hidden="true">3.</strong> Common Programming Concepts</a></li><li><ol class="section"><li><a href="ch03-01-variables-and-mutability.html"><strong aria-hidden="true">3.1.</strong> Variables and Mutability</a></li><li><a href="ch03-02-data-types.html"><strong aria-hidden="true">3.2.</strong> Data Types</a></li><li><a href="ch03-03-how-functions-work.html"><strong aria-hidden="true">3.3.</strong> How Functions Work</a></li><li><a href="ch03-04-comments.html"><strong aria-hidden="true">3.4.</strong> Comments</a></li><li><a href="ch03-05-control-flow.html"><strong aria-hidden="true">3.5.</strong> Control Flow</a></li></ol></li><li><a href="ch04-00-understanding-ownership.html"><strong aria-hidden="true">4.</strong> Understanding Ownership</a></li><li><ol class="section"><li><a href="ch04-01-what-is-ownership.html"><strong aria-hidden="true">4.1.</strong> What is Ownership?</a></li><li><a href="ch04-02-references-and-borrowing.html"><strong aria-hidden="true">4.2.</strong> References & Borrowing</a></li><li><a href="ch04-03-slices.html"><strong aria-hidden="true">4.3.</strong> Slices</a></li></ol></li><li><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><a href="ch05-01-defining-structs.html"><strong aria-hidden="true">5.1.</strong> Defining and Instantiating Structs</a></li><li><a href="ch05-02-example-structs.html"><strong aria-hidden="true">5.2.</strong> An Example Program Using Structs</a></li><li><a href="ch05-03-method-syntax.html"><strong aria-hidden="true">5.3.</strong> Method Syntax</a></li></ol></li><li><a href="ch06-00-enums.html"><strong aria-hidden="true">6.</strong> Enums and Pattern Matching</a></li><li><ol class="section"><li><a href="ch06-01-defining-an-enum.html"><strong aria-hidden="true">6.1.</strong> Defining an Enum</a></li><li><a href="ch06-02-match.html"><strong aria-hidden="true">6.2.</strong> The match Control Flow Operator</a></li><li><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><a href="ch07-00-modules.html"><strong aria-hidden="true">7.</strong> Modules</a></li><li><ol class="section"><li><a href="ch07-01-mod-and-the-filesystem.html"><strong aria-hidden="true">7.1.</strong> mod and the Filesystem</a></li><li><a href="ch07-02-controlling-visibility-with-pub.html"><strong aria-hidden="true">7.2.</strong> Controlling Visibility with pub</a></li><li><a href="ch07-03-importing-names-with-use.html"><strong aria-hidden="true">7.3.</strong> Referring to Names in Different Modules</a></li></ol></li><li><a href="ch08-00-common-collections.html"><strong aria-hidden="true">8.</strong> Common Collections</a></li><li><ol class="section"><li><a href="ch08-01-vectors.html"><strong aria-hidden="true">8.1.</strong> Vectors</a></li><li><a href="ch08-02-strings.html"><strong aria-hidden="true">8.2.</strong> Strings</a></li><li><a href="ch08-03-hash-maps.html"><strong aria-hidden="true">8.3.</strong> Hash Maps</a></li></ol></li><li><a href="ch09-00-error-handling.html"><strong aria-hidden="true">9.</strong> Error Handling</a></li><li><ol class="section"><li><a href="ch09-01-unrecoverable-errors-with-panic.html"><strong aria-hidden="true">9.1.</strong> Unrecoverable Errors with panic!</a></li><li><a href="ch09-02-recoverable-errors-with-result.html"><strong aria-hidden="true">9.2.</strong> Recoverable Errors with Result</a></li><li><a href="ch09-03-to-panic-or-not-to-panic.html"><strong aria-hidden="true">9.3.</strong> To panic! or Not To panic!</a></li></ol></li><li><a href="ch10-00-generics.html"><strong aria-hidden="true">10.</strong> Generic Types, Traits, and Lifetimes</a></li><li><ol class="section"><li><a href="ch10-01-syntax.html"><strong aria-hidden="true">10.1.</strong> Generic Data Types</a></li><li><a href="ch10-02-traits.html"><strong aria-hidden="true">10.2.</strong> Traits: Defining Shared Behavior</a></li><li><a href="ch10-03-lifetime-syntax.html"><strong aria-hidden="true">10.3.</strong> Validating References with Lifetimes</a></li></ol></li><li><a href="ch11-00-testing.html"><strong aria-hidden="true">11.</strong> Testing</a></li><li><ol class="section"><li><a href="ch11-01-writing-tests.html"><strong aria-hidden="true">11.1.</strong> Writing tests</a></li><li><a href="ch11-02-running-tests.html"><strong aria-hidden="true">11.2.</strong> Running tests</a></li><li><a href="ch11-03-test-organization.html"><strong aria-hidden="true">11.3.</strong> Test Organization</a></li></ol></li><li><a href="ch12-00-an-io-project.html"><strong aria-hidden="true">12.</strong> An I/O Project: Building a Command Line Program</a></li><li><ol class="section"><li><a href="ch12-01-accepting-command-line-arguments.html" class="active"><strong aria-hidden="true">12.1.</strong> Accepting Command Line Arguments</a></li><li><a href="ch12-02-reading-a-file.html"><strong aria-hidden="true">12.2.</strong> Reading a File</a></li><li><a href="ch12-03-improving-error-handling-and-modularity.html"><strong aria-hidden="true">12.3.</strong> Refactoring to Improve Modularity and Error Handling</a></li><li><a href="ch12-04-testing-the-librarys-functionality.html"><strong aria-hidden="true">12.4.</strong> Developing the Library’s Functionality with Test Driven Development</a></li><li><a href="ch12-05-working-with-environment-variables.html"><strong aria-hidden="true">12.5.</strong> Working with Environment Variables</a></li><li><a href="ch12-06-writing-to-stderr-instead-of-stdout.html"><strong aria-hidden="true">12.6.</strong> Writing Error Messages to Standard Error Instead of Standard Output</a></li></ol></li><li><a href="ch13-00-functional-features.html"><strong aria-hidden="true">13.</strong> Functional Language Features: Iterators and Closures</a></li><li><ol class="section"><li><a href="ch13-01-closures.html"><strong aria-hidden="true">13.1.</strong> Closures: Anonymous Functions that Can Capture Their Environment</a></li><li><a href="ch13-02-iterators.html"><strong aria-hidden="true">13.2.</strong> Processing a Series of Items with Iterators</a></li><li><a href="ch13-03-improving-our-io-project.html"><strong aria-hidden="true">13.3.</strong> Improving Our I/O Project</a></li><li><a href="ch13-04-performance.html"><strong aria-hidden="true">13.4.</strong> Comparing Performance: Loops vs. Iterators</a></li></ol></li><li><a href="ch14-00-more-about-cargo.html"><strong aria-hidden="true">14.</strong> More about Cargo and Crates.io</a></li><li><ol class="section"><li><a href="ch14-01-release-profiles.html"><strong aria-hidden="true">14.1.</strong> Customizing Builds with Release Profiles</a></li><li><a href="ch14-02-publishing-to-crates-io.html"><strong aria-hidden="true">14.2.</strong> Publishing a Crate to Crates.io</a></li><li><a href="ch14-03-cargo-workspaces.html"><strong aria-hidden="true">14.3.</strong> Cargo Workspaces</a></li><li><a href="ch14-04-installing-binaries.html"><strong aria-hidden="true">14.4.</strong> Installing Binaries from Crates.io with cargo install</a></li><li><a href="ch14-05-extending-cargo.html"><strong aria-hidden="true">14.5.</strong> Extending Cargo with Custom Commands</a></li></ol></li><li><a href="ch15-00-smart-pointers.html"><strong aria-hidden="true">15.</strong> Smart Pointers</a></li><li><ol class="section"><li><a href="ch15-01-box.html"><strong aria-hidden="true">15.1.</strong> Box<T> Points to Data on the Heap and Has a Known Size</a></li><li><a href="ch15-02-deref.html"><strong aria-hidden="true">15.2.</strong> The Deref Trait Allows Access to the Data Through a Reference</a></li><li><a href="ch15-03-drop.html"><strong aria-hidden="true">15.3.</strong> The Drop Trait Runs Code on Cleanup</a></li><li><a href="ch15-04-rc.html"><strong aria-hidden="true">15.4.</strong> Rc<T>, the Reference Counted Smart Pointer</a></li><li><a href="ch15-05-interior-mutability.html"><strong aria-hidden="true">15.5.</strong> RefCell<T> and the Interior Mutability Pattern</a></li><li><a href="ch15-06-reference-cycles.html"><strong aria-hidden="true">15.6.</strong> Creating Reference Cycles and Leaking Memory is Safe</a></li></ol></li><li><a href="ch16-00-concurrency.html"><strong aria-hidden="true">16.</strong> Fearless Concurrency</a></li><li><ol class="section"><li><a href="ch16-01-threads.html"><strong aria-hidden="true">16.1.</strong> Threads</a></li><li><a href="ch16-02-message-passing.html"><strong aria-hidden="true">16.2.</strong> Message Passing</a></li><li><a href="ch16-03-shared-state.html"><strong aria-hidden="true">16.3.</strong> Shared State</a></li><li><a href="ch16-04-extensible-concurrency-sync-and-send.html"><strong aria-hidden="true">16.4.</strong> Extensible Concurrency: Sync and Send</a></li></ol></li><li><a href="ch17-00-oop.html"><strong aria-hidden="true">17.</strong> Object Oriented Programming Features of Rust</a></li><li><ol class="section"><li><a href="ch17-01-what-is-oo.html"><strong aria-hidden="true">17.1.</strong> Characteristics of Object-Oriented Languages</a></li><li><a href="ch17-02-trait-objects.html"><strong aria-hidden="true">17.2.</strong> Using Trait Objects that Allow for Values of Different Types</a></li><li><a href="ch17-03-oo-design-patterns.html"><strong aria-hidden="true">17.3.</strong> Implementing an Object-Oriented Design Pattern</a></li></ol></li><li><a href="ch18-00-patterns.html"><strong aria-hidden="true">18.</strong> Patterns Match the Structure of Values</a></li><li><ol class="section"><li><a href="ch18-01-all-the-places-for-patterns.html"><strong aria-hidden="true">18.1.</strong> All the Places Patterns May be Used</a></li><li><a href="ch18-02-refutability.html"><strong aria-hidden="true">18.2.</strong> Refutability: Whether a Pattern Might Fail to Match</a></li><li><a href="ch18-03-pattern-syntax.html"><strong aria-hidden="true">18.3.</strong> All the Pattern Syntax</a></li></ol></li><li><a href="ch19-00-advanced-features.html"><strong aria-hidden="true">19.</strong> Advanced Features</a></li><li><ol class="section"><li><a href="ch19-01-unsafe-rust.html"><strong aria-hidden="true">19.1.</strong> Unsafe Rust</a></li><li><a href="ch19-02-advanced-lifetimes.html"><strong aria-hidden="true">19.2.</strong> Advanced Lifetimes</a></li><li><a href="ch19-03-advanced-traits.html"><strong aria-hidden="true">19.3.</strong> Advanced Traits</a></li><li><a href="ch19-04-advanced-types.html"><strong aria-hidden="true">19.4.</strong> Advanced Types</a></li><li><a href="ch19-05-advanced-functions-and-closures.html"><strong aria-hidden="true">19.5.</strong> Advanced Functions & Closures</a></li></ol></li><li><a href="ch20-00-final-project-a-web-server.html"><strong aria-hidden="true">20.</strong> Final Project: Building a Multithreaded Web Server</a></li><li><ol class="section"><li><a href="ch20-01-single-threaded.html"><strong aria-hidden="true">20.1.</strong> A Single Threaded Web Server</a></li><li><a href="ch20-02-multithreaded.html"><strong aria-hidden="true">20.2.</strong> Turning our Single Threaded Server into a Multithreaded Server</a></li><li><a href="ch20-03-graceful-shutdown-and-cleanup.html"><strong aria-hidden="true">20.3.</strong> Graceful Shutdown and Cleanup</a></li></ol></li><li><a href="appendix-00.html"><strong aria-hidden="true">21.</strong> Appendix</a></li><li><ol class="section"><li><a href="appendix-01-keywords.html"><strong aria-hidden="true">21.1.</strong> A - Keywords</a></li><li><a href="appendix-02-operators.html"><strong aria-hidden="true">21.2.</strong> B - Operators and Symbols</a></li><li><a href="appendix-03-derivable-traits.html"><strong aria-hidden="true">21.3.</strong> C - Derivable Traits</a></li><li><a href="appendix-04-macros.html"><strong aria-hidden="true">21.4.</strong> D - Macros</a></li><li><a href="appendix-05-translation.html"><strong aria-hidden="true">21.5.</strong> E - Translations</a></li><li><a href="appendix-06-nightly-rust.html"><strong aria-hidden="true">21.6.</strong> F - How Rust is Made and “Nightly Rust”</a></li></ol></li></ol>
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<a class="header" href="ch12-01-accepting-command-line-arguments.html#accepting-command-line-arguments" id="accepting-command-line-arguments"><h2>Accepting Command Line Arguments</h2></a>
<p>Let’s create a new project with, as always, <code>cargo new</code>. We’ll call our project
<code>minigrep</code> to distinguish it from the <code>grep</code> tool that you might already have
on your system.</p>
<pre><code class="language-text">$ cargo new minigrep
Created binary (application) `minigrep` project
$ cd minigrep
</code></pre>
<p>The first task is to make <code>minigrep</code> accept its two command line arguments: the
filename and a string to search for. That is, we want to be able to run our
program with <code>cargo run</code>, a string to search for, and a path to a file to
search in, like so:</p>
<pre><code class="language-text">$ cargo run searchstring example-filename.txt
</code></pre>
<p>Right now, the program generated by <code>cargo new</code> cannot process arguments we
give it. Some existing libraries on <a href="https://crates.io/">Crates.io</a> can help
with writing a program that accepts command line arguments, but because you’re
just learning this concept, let’s implement this capability ourselves.</p>
<a class="header" href="ch12-01-accepting-command-line-arguments.html#reading-the-argument-values" id="reading-the-argument-values"><h3>Reading the Argument Values</h3></a>
<p>To enable <code>minigrep</code> to read the values of command line arguments we pass to
it, we’ll need a function provided in Rust’s standard library, which is
<code>std::env::args</code>. This function returns an <em>iterator</em> of the command line
arguments that were given to <code>minigrep</code>. We haven’t discussed iterators yet
(we’ll cover them fully in Chapter 13), but for now, you only need to know two
details about iterators: iterators produce a series of values, and we can call
the <code>collect</code> method on an iterator to turn it into a collection, such as a
vector, containing all the elements the iterator produces.</p>
<p>Use the code in Listing 12-1 to allow your <code>minigrep</code> program to read any
command line arguments passed to it and then collect the values into a vector:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust">use std::env;
fn main() {
let args: Vec<String> = env::args().collect();
println!("{:?}", args);
}
</code></pre></pre>
<p><span class="caption">Listing 12-1: Collecting the command line arguments into
a vector and printing them</span></p>
<p>First, we bring the <code>std::env</code> module into scope with a <code>use</code> statement so we
can use its <code>args</code> function. Notice that the <code>std::env::args</code> function is
nested in two levels of modules. As we discussed in Chapter 7, in cases where
the desired function is nested in more than one module, it’s conventional to
bring the parent module into scope rather than the function. By doing so, we
can easily use other functions from <code>std::env</code>. It’s also less ambiguous than
adding <code>use std::env::args</code> and then calling the function with just <code>args</code>,
because <code>args</code> might easily be mistaken for a function that’s defined in the
current module.</p>
<blockquote>
<a class="header" href="ch12-01-accepting-command-line-arguments.html#the-args-function-and-invalid-unicode" id="the-args-function-and-invalid-unicode"><h3>The <code>args</code> Function and Invalid Unicode</h3></a>
<p>Note that <code>std::env::args</code> will panic if any argument contains invalid
Unicode. If your program needs to accept arguments containing invalid
Unicode, use <code>std::env::args_os</code> instead. That function returns an iterator
that produces <code>OsString</code> values instead of <code>String</code> values. We’ve chosen to
use <code>std::env::args</code> here for simplicity, because <code>OsString</code> values differ
per platform and are more complex to work with than <code>String</code> values.</p>
</blockquote>
<p>On the first line of <code>main</code>, we call <code>env::args</code>, and we immediately use
<code>collect</code> to turn the iterator into a vector containing all the values produced
by the iterator. We can use the <code>collect</code> function to create many kinds of
collections, so we explicitly annotate the type of <code>args</code> to specify that we
want a vector of strings. Although we very rarely need to annotate types in
Rust, <code>collect</code> is one function you do often need to annotate because Rust
isn’t able to infer the kind of collection you want.</p>
<p>Finally, we print the vector using the debug formatter, <code>:?</code>. Let’s try running
the code first with no arguments and then with two arguments:</p>
<pre><code class="language-text">$ cargo run
--snip--
["target/debug/minigrep"]
$ cargo run needle haystack
--snip--
["target/debug/minigrep", "needle", "haystack"]
</code></pre>
<p>Notice that the first value in the vector is <code>"target/debug/minigrep"</code>, which
is the name of our binary. This matches the behavior of the arguments list in
C, letting programs use the name by which they were invoked in their execution.
It’s often convenient to have access to the program name in case you want to
print it in messages or change behavior of the program based on what command
line alias was used to invoke the program. But for the purposes of this
chapter, we’ll ignore it and save only the two arguments we need.</p>
<a class="header" href="ch12-01-accepting-command-line-arguments.html#saving-the-argument-values-in-variables" id="saving-the-argument-values-in-variables"><h3>Saving the Argument Values in Variables</h3></a>
<p>Printing the value of the vector of arguments illustrated that the program is
able to access the values specified as command line arguments. Now we need to
save the values of the two arguments in variables so we can use the values
throughout the rest of the program. We do that in Listing 12-2:</p>
<p><span class="filename">Filename: src/main.rs</span></p>
<pre><pre class="playpen"><code class="language-rust should_panic">use std::env;
fn main() {
let args: Vec<String> = env::args().collect();
let query = &args[1];
let filename = &args[2];
println!("Searching for {}", query);
println!("In file {}", filename);
}
</code></pre></pre>
<p><span class="caption">Listing 12-2: Creating variables to hold the query
argument and filename argument</span></p>
<p>As we saw when we printed the vector, the program’s name takes up the first
value in the vector at <code>args[0]</code>, so we’re starting at index <code>1</code>. The first
argument <code>minigrep</code> takes is the string we’re searching for, so we put a
reference to the first argument in the variable <code>query</code>. The second argument
will be the filename, so we put a reference to the second argument in the
variable <code>filename</code>.</p>
<p>We temporarily print the values of these variables to prove that the code is
working as we intend. Let’s run this program again with the arguments <code>test</code>
and <code>sample.txt</code>:</p>
<pre><code class="language-text">$ cargo run test sample.txt
Compiling minigrep v0.1.0 (file:///projects/minigrep)
Finished dev [unoptimized + debuginfo] target(s) in 0.0 secs
Running `target/debug/minigrep test sample.txt`
Searching for test
In file sample.txt
</code></pre>
<p>Great, the program is working! The values of the arguments we need are being
saved into the right variables. Later we’ll add some error handling to deal
with certain potential erroneous situations, such as when the user provides no
arguments; for now, we’ll ignore that situation and work on adding file-reading
capabilities instead.</p>
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