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Oleg Butuzov 646c8b39da bugfix (mmcgrana/gobyexample#294) sha1 sum calculator 
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<title>Go by Example: Command-Line Subcommands</title>
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<div class="example" id="command-line-subcommands">
<h2><a href="./">Go by Example</a>: Command-Line Subcommands</h2>
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<td class="docs">
<p>Some command-line tools, like the <code>go</code> tool or <code>git</code>
have many <em>subcommands</em>, each with its own set of
flags. For example, <code>go build</code> and <code>go get</code> are two
different subcommands of the <code>go</code> tool.
The <code>flag</code> package lets us easily define simple
subcommands that have their own flags.</p>
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<td class="code leading">
<a href="http://play.golang.org/p/DkvdHKK-XCv"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
</pre></div>
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<td class="code leading">
<div class="highlight"><pre><span class="kn">import</span> <span class="p">(</span>
<span class="s">&quot;flag&quot;</span>
<span class="s">&quot;fmt&quot;</span>
<span class="s">&quot;os&quot;</span>
<span class="p">)</span>
</pre></div>
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<td class="docs">
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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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<td class="docs">
<p>We declare a subcommand using the <code>NewFlagSet</code>
function, and proceed to define new flags specific
for this subcommand.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="nx">fooCmd</span> <span class="o">:=</span> <span class="nx">flag</span><span class="p">.</span><span class="nx">NewFlagSet</span><span class="p">(</span><span class="s">&quot;foo&quot;</span><span class="p">,</span> <span class="nx">flag</span><span class="p">.</span><span class="nx">ExitOnError</span><span class="p">)</span>
<span class="nx">fooEnable</span> <span class="o">:=</span> <span class="nx">fooCmd</span><span class="p">.</span><span class="nx">Bool</span><span class="p">(</span><span class="s">&quot;enable&quot;</span><span class="p">,</span> <span class="kc">false</span><span class="p">,</span> <span class="s">&quot;enable&quot;</span><span class="p">)</span>
<span class="nx">fooName</span> <span class="o">:=</span> <span class="nx">fooCmd</span><span class="p">.</span><span class="nx">String</span><span class="p">(</span><span class="s">&quot;name&quot;</span><span class="p">,</span> <span class="s">&quot;&quot;</span><span class="p">,</span> <span class="s">&quot;name&quot;</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>For a different subcommand we can define different
supported flags.</p>
</td>
<td class="code leading">
<div class="highlight"><pre> <span class="nx">barCmd</span> <span class="o">:=</span> <span class="nx">flag</span><span class="p">.</span><span class="nx">NewFlagSet</span><span class="p">(</span><span class="s">&quot;bar&quot;</span><span class="p">,</span> <span class="nx">flag</span><span class="p">.</span><span class="nx">ExitOnError</span><span class="p">)</span>
<span class="nx">barLevel</span> <span class="o">:=</span> <span class="nx">barCmd</span><span class="p">.</span><span class="nx">Int</span><span class="p">(</span><span class="s">&quot;level&quot;</span><span class="p">,</span> <span class="mi">0</span><span class="p">,</span> <span class="s">&quot;level&quot;</span><span class="p">)</span>
</pre></div>
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<td class="docs">
<p>The subcommand is expected as the first argument
to the program.</p>
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<td class="code leading">
<div class="highlight"><pre> <span class="k">if</span> <span class="nb">len</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Args</span><span class="p">)</span> <span class="p">&lt;</span> <span class="mi">2</span> <span class="p">{</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;expected &#39;foo&#39; or &#39;bar&#39; subcommands&quot;</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>Check which subcommand is invoked.</p>
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<div class="highlight"><pre> <span class="k">switch</span> <span class="nx">os</span><span class="p">.</span><span class="nx">Args</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>For every subcommand, we parse its own flags and
have access to trailing positional arguments.</p>
</td>
<td class="code">
<div class="highlight"><pre> <span class="k">case</span> <span class="s">&quot;foo&quot;</span><span class="p">:</span>
<span class="nx">fooCmd</span><span class="p">.</span><span class="nx">Parse</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Args</span><span class="p">[</span><span class="mi">2</span><span class="p">:])</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;subcommand &#39;foo&#39;&quot;</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot; enable:&quot;</span><span class="p">,</span> <span class="o">*</span><span class="nx">fooEnable</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot; name:&quot;</span><span class="p">,</span> <span class="o">*</span><span class="nx">fooName</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot; tail:&quot;</span><span class="p">,</span> <span class="nx">fooCmd</span><span class="p">.</span><span class="nx">Args</span><span class="p">())</span>
<span class="k">case</span> <span class="s">&quot;bar&quot;</span><span class="p">:</span>
<span class="nx">barCmd</span><span class="p">.</span><span class="nx">Parse</span><span class="p">(</span><span class="nx">os</span><span class="p">.</span><span class="nx">Args</span><span class="p">[</span><span class="mi">2</span><span class="p">:])</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;subcommand &#39;bar&#39;&quot;</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot; level:&quot;</span><span class="p">,</span> <span class="o">*</span><span class="nx">barLevel</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot; tail:&quot;</span><span class="p">,</span> <span class="nx">barCmd</span><span class="p">.</span><span class="nx">Args</span><span class="p">())</span>
<span class="k">default</span><span class="p">:</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;expected &#39;foo&#39; or &#39;bar&#39; subcommands&quot;</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>
</pre></div>
</td>
</tr>
</table>
<table>
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<td class="docs">
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> go build command-line-subcommands.go
</pre></div>
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<td class="docs">
<p>First invoke the foo subcommand.</p>
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<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> ./command-line-subcommands foo -enable -name<span class="o">=</span>joe a1 a2
<span class="go">subcommand &#39;foo&#39;</span>
<span class="go"> enable: true</span>
<span class="go"> name: joe</span>
<span class="go"> tail: [a1 a2]</span>
</pre></div>
</td>
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<td class="docs">
<p>Now try bar.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> ./command-line-subcommands bar -level <span class="m">8</span> a1
<span class="go">subcommand &#39;bar&#39;</span>
<span class="go"> level: 8</span>
<span class="go"> tail: [a1]</span>
</pre></div>
</td>
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<td class="docs">
<p>But bar won&rsquo;t accept foo&rsquo;s flags.</p>
</td>
<td class="code leading">
<div class="highlight"><pre><span class="gp">$</span> ./command-line-subcommands bar -enable a1
<span class="go">flag provided but not defined: -enable</span>
<span class="go">Usage of bar:</span>
<span class="go"> -level int</span>
<span class="go"> level</span>
</pre></div>
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<td class="docs">
<p>Next we&rsquo;ll look at environment variables, another common
way to parameterize programs.</p>
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<td class="code empty">
</td>
</tr>
</table>
<p class="next">
Next example: <a href="environment-variables">Environment Variables</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/command-line-subcommands">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
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