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<div class="example" id="line-filters">
<h2><a href="./">Go by Example</a>: Line Filters</h2>
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<p>A <em>line filter</em> is a common type of program that reads
input on stdin, processes it, and then prints some
derived result to stdout. <code>grep</code> and <code>sed</code> are common
line filters.</p>
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<p>Here&rsquo;s an example line filter in Go that writes a
capitalized version of all input text. You can use this
pattern to write your own Go line filters.</p>
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<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;os&quot;</span>
<span class="s">&quot;strings&quot;</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>Wrapping the unbuffered <code>os.Stdin</code> with a buffered
reader gives us a convenient <code>ReadString</code> method
that we&rsquo;ll use to read input line-by-line.</p>
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<div class="highlight"><pre> <span class="nx">rdr</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">os</span><span class="p">.</span><span class="nx">Stdin</span><span class="p">)</span>
<span class="nx">out</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nx">Stdout</span>
</pre></div>
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<p><code>ReadString</code> returns the next string from the
input up to the given separator byte. We give the
newline byte <code>'\n'</code> as our separator so we&rsquo;ll get
successive input lines.</p>
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<div class="highlight"><pre> <span class="k">for</span> <span class="p">{</span>
<span class="k">switch</span> <span class="nx">line</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">rdr</span><span class="p">.</span><span class="nx">ReadString</span><span class="p">(</span><span class="sc">&#39;\n&#39;</span><span class="p">);</span> <span class="nx">err</span> <span class="p">{</span>
</pre></div>
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<p>If the read succeeded (the read <code>err</code> is nil),
write out out the uppercased line. Check for an
error on this write as we do on the read.</p>
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<div class="highlight"><pre> <span class="k">case</span> <span class="kc">nil</span><span class="p">:</span>
<span class="nx">ucl</span> <span class="o">:=</span> <span class="nx">strings</span><span class="p">.</span><span class="nx">ToUpper</span><span class="p">(</span><span class="nx">line</span><span class="p">)</span>
<span class="k">if</span> <span class="nx">_</span><span class="p">,</span> <span class="nx">err</span> <span class="p">=</span> <span class="nx">out</span><span class="p">.</span><span class="nx">WriteString</span><span class="p">(</span><span class="nx">ucl</span><span class="p">);</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Fprintln</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Stderr</span><span class="p">,</span> <span class="s">&quot;error:&quot;</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span>
<span class="nx">os</span><span class="p">.</span><span class="nx">Exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<p>The <code>EOF</code> error is expected when we reach the
end of input, so exit gracefully in that case.</p>
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<div class="highlight"><pre> <span class="k">case</span> <span class="nx">io</span><span class="p">.</span><span class="nx">EOF</span><span class="p">:</span>
<span class="nx">os</span><span class="p">.</span><span class="nx">Exit</span><span class="p">(</span><span class="mi">0</span><span class="p">)</span>
</pre></div>
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<p>Otherwise there&rsquo;s a problem; print the
error and exit with non-zero status.</p>
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<div class="highlight"><pre> <span class="k">default</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Fprintln</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Stderr</span><span class="p">,</span> <span class="s">&quot;error:&quot;</span><span class="p">,</span> <span class="nx">err</span><span class="p">)</span>
<span class="nx">os</span><span class="p">.</span><span class="nx">Exit</span><span class="p">(</span><span class="mi">1</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
<span class="p">}</span>
</pre></div>
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<p>To try out our line filter, first make a file with a few
lowercase lines.</p>
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<div class="highlight"><pre><span class="gp">$</span> <span class="nb">echo</span> <span class="s1">&#39;hello&#39;</span> &gt; /tmp/lines
<span class="gp">$</span> <span class="nb">echo</span> <span class="s1">&#39;filter&#39;</span> &gt;&gt; /tmp/lines
</pre></div>
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<p>Then use the line filter to get uppercase lines.</p>
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<div class="highlight"><pre><span class="gp">$</span> cat /tmp/lines | go run line-filters.go
<span class="go">HELLO</span>
<span class="go">FILTER</span>
</pre></div>
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<p class="next">
Next example: <a href="command-line-arguments">Command-Line Arguments</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/line-filters">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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