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<div class="example" id="reading-files">
<h2><a href="./">Go by Example</a>: Reading Files</h2>
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<p>Reading and writing files are basic tasks needed for
many Go programs. First we&rsquo;ll look at some examples of
reading files.</p>
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<a href="http://play.golang.org/p/VcLqpSwnkN"><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>
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<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
<span class="s">&quot;bufio&quot;</span>
<span class="s">&quot;fmt&quot;</span>
<span class="s">&quot;io&quot;</span>
<span class="s">&quot;io/ioutil&quot;</span>
<span class="s">&quot;os&quot;</span>
<span class="p">)</span>
</pre></div>
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<p>Reading files requires checking most calls for errors.
This helper will streamline our error checks below.</p>
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<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>
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<div class="highlight"><pre><span class="kd">func</span> <span class="nx">main</span><span class="p">()</span> <span class="p">{</span>
</pre></div>
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<p>Perhaps the most basic file reading task is
slurping a file&rsquo;s entire contents into memory.</p>
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<div class="highlight"><pre> <span class="nx">dat</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">ReadFile</span><span class="p">(</span><span class="s">&quot;/tmp/dat&quot;</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">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></div>
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<p>You&rsquo;ll often want more control over how and what
parts of a file are read. For these tasks, start
by <code>Open</code>ing a file to obtain an <code>os.File</code> value.</p>
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<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">Open</span><span class="p">(</span><span class="s">&quot;/tmp/dat&quot;</span><span class="p">)</span>
</pre></div>
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<p>Read some bytes from the beginning of the file.
Allow up to 5 to be read but also note how many
actually were read.</p>
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<div class="highlight"><pre> <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="nx">Read</span><span class="p">(</span><span class="nx">b1</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">&quot;%d bytes: %s\n&quot;</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>
</pre></div>
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<p>You can also <code>Seek</code> to a known location in the file
and <code>Read</code> from there.</p>
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<div class="highlight"><pre> <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="nx">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="nx">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="nx">Read</span><span class="p">(</span><span class="nx">b2</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">&quot;%d bytes @ %d: %s\n&quot;</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="nb">string</span><span class="p">(</span><span class="nx">b2</span><span class="p">))</span>
</pre></div>
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<p>The <code>io</code> package provides some functions that may
be helpful for file reading. For example, reads
like the ones above can be more robustly
implemented with <code>ReadAtLeast</code>.</p>
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<td class="code leading">
<div class="highlight"><pre> <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="nx">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="nx">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="nx">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="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">&quot;%d bytes @ %d: %s\n&quot;</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></div>
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<p>There is no built-in rewind, but <code>Seek(0, 0)</code>
accomplishes this.</p>
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<td class="code leading">
<div class="highlight"><pre> <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="nx">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="nx">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
</pre></div>
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<p>The <code>bufio</code> package implements a buffered
reader that may be useful both for its efficiency
with many small reads and because of the additional
reading methods it provides.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">r4</span> <span class="o">:=</span> <span class="nx">bufio</span><span class="p">.</span><span class="nx">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="nx">Peek</span><span class="p">(</span><span class="mi">5</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">&quot;5 bytes: %s\n&quot;</span><span class="p">,</span> <span class="nb">string</span><span class="p">(</span><span class="nx">b4</span><span class="p">))</span>
</pre></div>
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<p>Close the file when you&rsquo;re done (usually this would
be scheduled immediately after <code>Open</code>ing with
<code>defer</code>).</p>
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<div class="highlight"><pre> <span class="nx">f</span><span class="p">.</span><span class="nx">Close</span><span class="p">()</span>
</pre></div>
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<div class="highlight"><pre><span class="p">}</span>
</pre></div>
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<div class="highlight"><pre><span class="gp">$</span> <span class="nb">echo</span> <span class="s2">&quot;hello&quot;</span> &gt; /tmp/dat
<span class="gp">$</span> <span class="nb">echo</span> <span class="s2">&quot;go&quot;</span> &gt;&gt; /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></div>
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<p>Next we&rsquo;ll look at writing files.</p>
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<p class="next">
Next example: <a href="writing-files">Writing Files</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/reading-files">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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