321 lines
12 KiB
Plaintext
321 lines
12 KiB
Plaintext
<!DOCTYPE html>
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<html>
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<head>
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<meta charset="utf-8">
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<title>Go by Example: Command-Line Flags</title>
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<div class="example" id="command-line-flags">
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<h2><a href="./">Go by Example</a>: Command-Line Flags</h2>
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<table>
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<tr>
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<td class="docs">
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<p><a href="http://en.wikipedia.org/wiki/Command-line_interface#Command-line_option"><em>Command-line flags</em></a>
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are a common way to specify options for command-line
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programs. For example, in <code>wc -l</code> the <code>-l</code> is a
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command-line flag.</p>
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</td>
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<td class="code empty leading">
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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<a href="http://play.golang.org/p/NASEOq2R3n"><img title="Run code" src="play.png" class="run" /></a>
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<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Go provides a <code>flag</code> package supporting basic
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command-line flag parsing. We’ll use this package to
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implement our example command-line program.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">"flag"</span>
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<span class="kn">import</span> <span class="s">"fmt"</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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</td>
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<td class="code leading">
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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>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Basic flag declarations are available for string,
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integer, and boolean options. Here we declare a
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string flag <code>word</code> with a default value <code>"foo"</code>
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and a short description. This <code>flag.String</code> function
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returns a string pointer (not a string value);
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we’ll see how to use this pointer below.</p>
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</td>
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<td class="code leading">
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<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">"word"</span><span class="p">,</span> <span class="s">"foo"</span><span class="p">,</span> <span class="s">"a string"</span><span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>This declares <code>numb</code> and <code>fork</code> flags, using a
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similar approach to the <code>word</code> flag.</p>
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</td>
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<td class="code leading">
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<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">"numb"</span><span class="p">,</span> <span class="mi">42</span><span class="p">,</span> <span class="s">"an int"</span><span class="p">)</span>
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<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">"fork"</span><span class="p">,</span> <span class="kc">false</span><span class="p">,</span> <span class="s">"a bool"</span><span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>It’s also possible to declare an option that uses an
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existing var declared elsewhere in the program.
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Note that we need to pass in a pointer to the flag
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declaration function.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="kd">var</span> <span class="nx">svar</span> <span class="kt">string</span>
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<span class="nx">flag</span><span class="p">.</span><span class="nx">StringVar</span><span class="p">(</span><span class="o">&</span><span class="nx">svar</span><span class="p">,</span> <span class="s">"svar"</span><span class="p">,</span> <span class="s">"bar"</span><span class="p">,</span> <span class="s">"a string var"</span><span class="p">)</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Once all flags are declared, call <code>flag.Parse()</code>
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to execute the command-line parsing.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre> <span class="nx">flag</span><span class="p">.</span><span class="nx">Parse</span><span class="p">()</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Here we’ll just dump out the parsed options and
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any trailing positional arguments. Note that we
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need to dereference the pointers with e.g. <code>*wordPtr</code>
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to get the actual option values.</p>
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</td>
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<td class="code">
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<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">"word:"</span><span class="p">,</span> <span class="o">*</span><span class="nx">wordPtr</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"numb:"</span><span class="p">,</span> <span class="o">*</span><span class="nx">numbPtr</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"fork:"</span><span class="p">,</span> <span class="o">*</span><span class="nx">boolPtr</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"svar:"</span><span class="p">,</span> <span class="nx">svar</span><span class="p">)</span>
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<span class="nx">fmt</span><span class="p">.</span><span class="nx">Println</span><span class="p">(</span><span class="s">"tail:"</span><span class="p">,</span> <span class="nx">flag</span><span class="p">.</span><span class="nx">Args</span><span class="p">())</span>
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<span class="p">}</span>
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</pre></div>
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</td>
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</tr>
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</table>
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<table>
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<tr>
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<td class="docs">
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<p>To experiment with the command-line flags program it’s
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best to first compile it and then run the resulting
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binary directly.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> go build command-line-flags.go
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Try out the built program by first giving it values for
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all flags.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -word<span class="o">=</span>opt -numb<span class="o">=</span><span class="m">7</span> -fork -svar<span class="o">=</span>flag
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<span class="go">word: opt</span>
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<span class="go">numb: 7</span>
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<span class="go">fork: true</span>
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<span class="go">svar: flag</span>
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<span class="go">tail: []</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Note that if you omit flags they automatically take
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their default values.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -word<span class="o">=</span>opt
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<span class="go">word: opt</span>
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<span class="go">numb: 42</span>
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<span class="go">fork: false</span>
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<span class="go">svar: bar</span>
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<span class="go">tail: []</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Trailing positional arguments can be provided after
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any flags.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -word<span class="o">=</span>opt a1 a2 a3
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<span class="go">word: opt</span>
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<span class="go">...</span>
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<span class="go">tail: [a1 a2 a3]</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Note that the <code>flag</code> package requires all flags to
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appear before positional arguments (otherwise the flags
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will be interpreted as positional arguments).</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -word<span class="o">=</span>opt a1 a2 a3 -numb<span class="o">=</span>7
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<span class="go">word: opt</span>
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<span class="go">numb: 42</span>
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<span class="go">fork: false</span>
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<span class="go">svar: bar</span>
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<span class="go">tail: [a1 a2 a3 -numb=7]</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Use <code>-h</code> or <code>--help</code> flags to get automatically
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generated help text for the command-line program.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -h
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<span class="go">Usage of ./command-line-flags:</span>
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<span class="go"> -fork=false: a bool</span>
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<span class="go"> -numb=42: an int</span>
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<span class="go"> -svar="bar": a string var</span>
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<span class="go"> -word="foo": a string</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>If you provide a flag that wasn’t specified to the
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<code>flag</code> package, the program will print an error message
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and show the help text again.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="gp">$</span> ./command-line-flags -wat
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<span class="go">flag provided but not defined: -wat</span>
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<span class="go">Usage of ./command-line-flags:</span>
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<span class="go">...</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>Next we’ll look at environment variables, another common
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way to parameterize programs.</p>
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</td>
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<td class="code empty">
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</td>
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</tr>
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</table>
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<p class="next">
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Next example: <a href="environment-variables">Environment Variables</a>.
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</p>
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<p class="footer">
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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>
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</p>
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</div>
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