command-line-flags

<!DOCTYPE html>
<html>
  <head>
    <meta charset="utf-8">
    <title>Go by Example: Command-Line Flags</title>
    <link rel=stylesheet href="site.css">
  </head>
  <script type="text/javascript">
    if (window.location.host == "gobyexample.com") {
      var _gaq = _gaq || [];
      _gaq.push(['_setAccount', 'UA-34996217-1']);
      _gaq.push(['_trackPageview']);
      (function() {
        var ga = document.createElement('script'); ga.type = 'text/javascript'; ga.async = true;
        ga.src = ('https:' == document.location.protocol ? 'https://ssl' : 'http://www') + '.google-analytics.com/ga.js';
        var s = document.getElementsByTagName('script')[0]; s.parentNode.insertBefore(ga, s);
      })();
    }
  </script>
  <body>
    <div class="example" id="command-line-flags">
      <h2><a href="./">Go by Example</a>: Command-Line Flags</h2>

      <table>

        <tr>
          <td class="docs">
            <p><a href="http://en.wikipedia.org/wiki/Command-line_interface#Command-line_option"><em>Command-line flags</em></a>
are a common way to specify options for command-line
programs. For example, in <code>wc -l</code> the <code>-l</code> is a
command-line flag.</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/NASEOq2R3n"><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">
            <p>Go provides a <code>flag</code> package supporting basic
command-line flag parsing. We&rsquo;ll use this package to
implement our example command-line program.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;flag&quot;</span>
<span class="kn">import</span> <span class="s">&quot;fmt&quot;</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>Basic flag declarations are available for string,
integer, and boolean options. Here we declare a
string flag <code>word</code> with a default value <code>&quot;foo&quot;</code>
and a short description. This <code>flag.String</code> function
returns a string pointer (not a string value);
we&rsquo;ll see how to use this pointer below.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre>    <span class="nx">wordPtr</span> <span class="o">:=</span> <span class="nx">flag</span><span class="p">.</span><span class="nx">String</span><span class="p">(</span><span class="s">&quot;word&quot;</span><span class="p">,</span> <span class="s">&quot;foo&quot;</span><span class="p">,</span> <span class="s">&quot;a string&quot;</span><span class="p">)</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>This declares <code>numb</code> and <code>fork</code> flags, using a
similar approach to the <code>word</code> flag.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre>    <span class="nx">numbPtr</span> <span class="o">:=</span> <span class="nx">flag</span><span class="p">.</span><span class="nx">Int</span><span class="p">(</span><span class="s">&quot;numb&quot;</span><span class="p">,</span> <span class="mi">42</span><span class="p">,</span> <span class="s">&quot;an int&quot;</span><span class="p">)</span>
    <span class="nx">boolPtr</span> <span class="o">:=</span> <span class="nx">flag</span><span class="p">.</span><span class="nx">Bool</span><span class="p">(</span><span class="s">&quot;fork&quot;</span><span class="p">,</span> <span class="kc">false</span><span class="p">,</span> <span class="s">&quot;a bool&quot;</span><span class="p">)</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>It&rsquo;s also possible to declare an option that uses an
existing var declared elsewhere in the program.
Note that we need to pass in a pointer to the flag
declaration function.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre>    <span class="kd">var</span> <span class="nx">svar</span> <span class="kt">string</span>
    <span class="nx">flag</span><span class="p">.</span><span class="nx">StringVar</span><span class="p">(</span><span class="o">&amp;</span><span class="nx">svar</span><span class="p">,</span> <span class="s">&quot;svar&quot;</span><span class="p">,</span> <span class="s">&quot;bar&quot;</span><span class="p">,</span> <span class="s">&quot;a string var&quot;</span><span class="p">)</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>Once all flags are declared, call <code>flag.Parse()</code>
to execute the command-line parsing.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre>    <span class="nx">flag</span><span class="p">.</span><span class="nx">Parse</span><span class="p">()</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>Here we&rsquo;ll just dump out the parsed options and
any trailing positional arguments. Note that we
need to dereference the pointers with e.g. <code>*wordPtr</code>
to get the actual option values.</p>

          </td>
          <td class="code">

            <div class="highlight"><pre>    <span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">&quot;word:&quot;</span><span class="p">,</span> <span class="o">*</span><span class="nx">wordPtr</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;numb:&quot;</span><span class="p">,</span> <span class="o">*</span><span class="nx">numbPtr</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;fork:&quot;</span><span class="p">,</span> <span class="o">*</span><span class="nx">boolPtr</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;svar:&quot;</span><span class="p">,</span> <span class="nx">svar</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">flag</span><span class="p">.</span><span class="nx">Args</span><span class="p">())</span>
<span class="p">}</span>
</pre></div>

          </td>
        </tr>

      </table>

      <table>

        <tr>
          <td class="docs">
            <p>To experiment with the command-line flags program it&rsquo;s
best to first compile it and then run the resulting
binary directly.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre><span class="gp">$</span> go build <span class="nb">command</span>-line-flags.go
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>Try out the built program by first giving it values for
all flags.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -word<span class="o">=</span>opt -numb<span class="o">=</span>7 -fork -svar<span class="o">=</span>flag
<span class="go">word: opt</span>
<span class="go">numb: 7</span>
<span class="go">fork: true</span>
<span class="go">svar: flag</span>
<span class="go">tail: []</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>Note that if you omit flags they automatically take
their default values.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -word<span class="o">=</span>opt
<span class="go">word: opt</span>
<span class="go">numb: 42</span>
<span class="go">fork: false</span>
<span class="go">svar: bar</span>
<span class="go">tail: []</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>Trailing positional arguments can be provided after
any flags.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -word<span class="o">=</span>opt a1 a2 a3
<span class="go">word: opt</span>
<span class="go">...</span>
<span class="go">tail: [a1 a2 a3]</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>Note that the <code>flag</code> package requires all flags to
appear before positional arguments (otherwise the flags
will be interpreted as positional arguments).</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -word<span class="o">=</span>opt a1 a2 a3 -numb<span class="o">=</span>7
<span class="go">word: opt</span>
<span class="go">numb: 42</span>
<span class="go">fork: false</span>
<span class="go">svar: bar</span>
<span class="go">tail: [a1 a2 a3 -numb=7]</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>Use <code>-h</code> or <code>--help</code> flags to get automatically
generated help text for the command-line program.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -h
<span class="go">Usage of ./command-line-flags:</span>
<span class="go">  -fork=false: a bool</span>
<span class="go">  -numb=42: an int</span>
<span class="go">  -svar=&quot;bar&quot;: a string var</span>
<span class="go">  -word=&quot;foo&quot;: a string</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>If you provide a flag that wasn&rsquo;t specified to the
<code>flag</code> package, the program will print an error message
an show the help text again.</p>

          </td>
          <td class="code leading">

            <div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -wat
<span class="go">flag provided but not defined: -wat</span>
<span class="go">Usage of ./command-line-flags:</span>
<span class="go">...</span>
</pre></div>

          </td>
        </tr>

        <tr>
          <td class="docs">
            <p>Next we&rsquo;ll look at environment variables, another common
way to parameterize programs.</p>

          </td>
          <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://twitter.com/mmcgrana">@mmcgrana</a> | <a href="mailto:mmcgrana@gmail.com">feedback</a> | <a href="https://github.com/mmcgrana/gobyexample/blob/master/examples/command-line-flags">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
      </p>
    </div>
  </body>
</html>