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writing-files
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<!DOCTYPE html>
<html>
<head>
<meta http-eqiv="content-type" content="text/html;charset=utf-8">
<title>Go by Example: Writing Files</title>
<link rel=stylesheet href="site.css">
</head>
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<body>
<div class="example" id="writing-files">
<h2><a href="./">Go by Example</a>: Writing Files</h2>
<table>
<tr>
<td class="docs">
<p>Writing files in Go follows similar patterns to the
ones we saw earlier for reading.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="http://play.golang.org/p/5usz1Oc-xY"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
<span class="s">"bufio"</span>
<span class="s">"fmt"</span>
<span class="s">"io/ioutil"</span>
<span class="s">"os"</span>
<span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">check</span><span class="p">(</span><span class="nx">e</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
<span class="k">if</span> <span class="nx">e</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nb">panic</span><span class="p">(</span><span class="nx">e</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>To start, here’s how to dump a string (or just
bytes) into a file.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">d1</span> <span class="o">:=</span> <span class="p">[]</span><span class="nb">byte</span><span class="p">(</span><span class="s">"hello\ngo\n"</span><span class="p">)</span>
<span class="nx">err</span> <span class="o">:=</span> <span class="nx">ioutil</span><span class="p">.</span><span class="nx">WriteFile</span><span class="p">(</span><span class="s">"/tmp/dat1"</span><span class="p">,</span> <span class="nx">d1</span><span class="p">,</span> <span class="mo">0644</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>For more granular writes, open a file for writing.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">f</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nx">Create</span><span class="p">(</span><span class="s">"/tmp/dat2"</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>It’s idiomatic to defer a <code>Close</code> immediately
after opening a file.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="k">defer</span> <span class="nx">f</span><span class="p">.</span><span class="nx">Close</span><span class="p">()</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>You can <code>Write</code> byte slices as you’d expect.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">d2</span> <span class="o">:=</span> <span class="p">[]</span><span class="kt">byte</span><span class="p">{</span><span class="mi">115</span><span class="p">,</span> <span class="mi">111</span><span class="p">,</span> <span class="mi">109</span><span class="p">,</span> <span class="mi">101</span><span class="p">,</span> <span class="mi">10</span><span class="p">}</span>
<span class="nx">n2</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nx">Write</span><span class="p">(</span><span class="nx">d2</span><span class="p">)</span>
<span class="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">"wrote %d bytes\n"</span><span class="p">,</span> <span class="nx">n2</span><span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>A <code>WriteString</code> is also available.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">n3</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nx">WriteString</span><span class="p">(</span><span class="s">"writes\n"</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">"wrote %d bytes\n"</span><span class="p">,</span> <span class="nx">n3</span><span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Issue a <code>Sync</code> to flush writes to stable storage.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">f</span><span class="p">.</span><span class="nx">Sync</span><span class="p">()</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p><code>bufio</code> provides buffered writers in addition
to the buffered readers we saw earlier.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">w</span> <span class="o">:=</span> <span class="nx">bufio</span><span class="p">.</span><span class="nx">NewWriter</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
<span class="nx">n4</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">w</span><span class="p">.</span><span class="nx">WriteString</span><span class="p">(</span><span class="s">"buffered\n"</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Printf</span><span class="p">(</span><span class="s">"wrote %d bytes\n"</span><span class="p">,</span> <span class="nx">n4</span><span class="p">)</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Use <code>Flush</code> to ensure all buffered operations have
been applied to the underlying writer.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">w</span><span class="p">.</span><span class="nx">Flush</span><span class="p">()</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code">
<div class="highlight"><pre><span class="p">}</span>
</pre></div>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
<p>Try running the file-writing code.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> go run writing-files.go
<span class="go">wrote 5 bytes</span>
<span class="go">wrote 7 bytes</span>
<span class="go">wrote 9 bytes</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Then check the contents of the written files.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> cat /tmp/dat1
<span class="go">hello</span>
<span class="go">go</span>
<span class="gp">$</span> cat /tmp/dat2
<span class="go">some</span>
<span class="go">writes</span>
<span class="go">buffered</span>
</pre></div>
</td>
</tr>
<tr>
<td class="docs">
<p>Next we’ll look at applying some of the file I/O ideas
we’ve just seen to the <code>stdin</code> and <code>stdout</code> streams.</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="line-filters">Line Filters</a>.
</p>
<p class="footer">
<a href="https://twitter.com/gobyexample">@gobyexample</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/writing-files">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
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