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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example 中文版: 读文件</title>
<link rel=stylesheet href="site.css">
</head>
<script>
onkeydown = (e) => {
if (e.key == "ArrowLeft") {
window.location.href = 'base64-encoding';
}
if (e.key == "ArrowRight") {
window.location.href = 'writing-files';
}
}
</script>
<body>
<div class="example" id="reading-files">
<h2><a href="./">Go by Example 中文版</a>: 读文件</h2>
<table>
<tr>
<td class="docs">
<p>读写文件在很多程序中都是必须的基本任务。
首先我们来看一些读文件的例子。</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="https://play.studygolang.com/p/zBe9iUpeaXJ"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>
<span class="s">"bufio"</span>
<span class="s">"fmt"</span>
<span class="s">"io"</span>
<span class="s">"os"</span>
<span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>读取文件需要经常进行错误检查，
这个帮助方法可以精简下面的错误检查过程。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">func</span> <span class="nf">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>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>最基本的文件读取任务或许就是将文件内容读取到内存中。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">dat</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="nf">ReadFile</span><span class="p">(</span><span class="s">"/tmp/dat"</span><span class="p">)</span>
<span class="nf">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="nf">Print</span><span class="p">(</span><span class="nb">string</span><span class="p">(</span><span class="nx">dat</span><span class="p">))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>您通常会希望对文件的读取方式和内容进行更多控制。
对于这个任务，首先使用 <code>Open</code> 打开一个文件，以获取一个 <code>os.File</code> 值。</p>
</td>
<td class="code leading">
<pre class="chroma">
<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="nf">Open</span><span class="p">(</span><span class="s">"/tmp/dat"</span><span class="p">)</span>
<span class="nf">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>从文件的开始位置读取一些字节。
最多允许读取 5 个字节，但还要注意实际读取了多少个。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">b1</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">byte</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span>
<span class="nx">n1</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="nf">Read</span><span class="p">(</span><span class="nx">b1</span><span class="p">)</span>
<span class="nf">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="nf">Printf</span><span class="p">(</span><span class="s">"%d bytes: %s\n"</span><span class="p">,</span> <span class="nx">n1</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b1</span><span class="p">[:</span><span class="nx">n1</span><span class="p">]))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>你也可以 <code>Seek</code> 到一个文件中已知的位置，并从这个位置开始读取。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">o2</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="nf">Seek</span><span class="p">(</span><span class="mi">6</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="nf">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<span class="nx">b2</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">byte</span><span class="p">,</span> <span class="mi">2</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="nf">Read</span><span class="p">(</span><span class="nx">b2</span><span class="p">)</span>
<span class="nf">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="nf">Printf</span><span class="p">(</span><span class="s">"%d bytes @ %d: "</span><span class="p">,</span> <span class="nx">n2</span><span class="p">,</span> <span class="nx">o2</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"%v\n"</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b2</span><span class="p">[:</span><span class="nx">n2</span><span class="p">]))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>例如，<code>io</code> 包提供了一个更健壮的实现 <code>ReadAtLeast</code>，用于读取上面那种文件。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">o3</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="nf">Seek</span><span class="p">(</span><span class="mi">6</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="nf">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
<span class="nx">b3</span> <span class="o">:=</span> <span class="nb">make</span><span class="p">([]</span><span class="kt">byte</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="nx">n3</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">io</span><span class="p">.</span><span class="nf">ReadAtLeast</span><span class="p">(</span><span class="nx">f</span><span class="p">,</span> <span class="nx">b3</span><span class="p">,</span> <span class="mi">2</span><span class="p">)</span>
<span class="nf">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="nf">Printf</span><span class="p">(</span><span class="s">"%d bytes @ %d: %s\n"</span><span class="p">,</span> <span class="nx">n3</span><span class="p">,</span> <span class="nx">o3</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b3</span><span class="p">))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>没有内建的倒带，但是 <code>Seek(0, 0)</code> 实现了这一功能。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">_</span><span class="p">,</span> <span class="nx">err</span> <span class="p">=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Seek</span><span class="p">(</span><span class="mi">0</span><span class="p">,</span> <span class="mi">0</span><span class="p">)</span>
<span class="nf">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p><code>bufio</code> 包实现了一个缓冲读取器，这可能有助于提高许多小读操作的效率，以及它提供了很多附加的读取函数。</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">r4</span> <span class="o">:=</span> <span class="nx">bufio</span><span class="p">.</span><span class="nf">NewReader</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
<span class="nx">b4</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">r4</span><span class="p">.</span><span class="nf">Peek</span><span class="p">(</span><span class="mi">5</span><span class="p">)</span>
<span class="nf">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="nf">Printf</span><span class="p">(</span><span class="s">"5 bytes: %s\n"</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b4</span><span class="p">))</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>任务结束后要关闭这个文件
（通常这个操作应该在 <code>Open</code> 操作后立即使用 <code>defer</code> 来完成）。</p>
</td>
<td class="code">
<pre class="chroma">
<span class="nx">f</span><span class="p">.</span><span class="nf">Close</span><span class="p">()</span>
<span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="gp">$</span> echo "hello" > /tmp/dat
<span class="gp">$</span> echo "go" >> /tmp/dat
<span class="gp">$</span> go run reading-files.go
<span class="go">hello
</span><span class="go">go
</span><span class="go">5 bytes: hello
</span><span class="go">2 bytes @ 6: go
</span><span class="go">2 bytes @ 6: go
</span><span class="go">5 bytes: hello</span></pre>
</td>
</tr>
<tr>
<td class="docs">
<p>下面我们来看一下写入文件。</p>
</td>
<td class="code empty">
</td>
</tr>
</table>
<p class="next">
下一个例子: <a href="writing-files">写文件</a>
</p>
<p class="footer">
<a href="https://twitter.com/mmcgrana">@mmcgrana</a> 和<a href="https://eli.thegreenplace.net">Eli Bendersky</a>编写 | <a href="https://github.com/gobyexample-cn">gobyexample-cn</a> 翻译 | <a href="https://github.com/gobyexample-cn/gobyexample/issues">反馈</a> | <a href="https://github.com/gobyexample-cn/gobyexample">源码</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a> </p>
</p>
</div>
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