185 lines
7.0 KiB
Plaintext
185 lines
7.0 KiB
Plaintext
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<html>
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<head>
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<meta http-eqiv="content-type" content="text/html;charset=utf-8">
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<title>Go by Example: Environment Variables</title>
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<div class="example" id="environment-variables">
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<h2><a href="./">Go by Example</a>: Environment Variables</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/Environment_variable">Environment variables</a>
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are a universal mechanism for <a href="http://www.12factor.net/config">conveying configuration
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information to Unix programs</a>.
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Let’s look at how to set, get, and list environment variables.</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/kfqLhpmEpw"><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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</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">"os"</span>
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<span class="kn">import</span> <span class="s">"strings"</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>To set a key/value pair, use <code>os.Setenv</code>. To get a
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value for a key, use <code>os.Getenv</code>. This will return
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an empty string if the key isn’t present in the
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environment.</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">os</span><span class="p">.</span><span class="nx">Setenv</span><span class="p">(</span><span class="s">"FOO"</span><span class="p">,</span> <span class="s">"1"</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">"FOO:"</span><span class="p">,</span> <span class="nx">os</span><span class="p">.</span><span class="nx">Getenv</span><span class="p">(</span><span class="s">"FOO"</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">"BAR:"</span><span class="p">,</span> <span class="nx">os</span><span class="p">.</span><span class="nx">Getenv</span><span class="p">(</span><span class="s">"BAR"</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>Use <code>os.Environ</code> to list all key/value pairs in the
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environment. This returns a slice of strings in the
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form <code>KEY=value</code>. You can <code>strings.Split</code> them to
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get the key and value. Here we print all the keys.</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>
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<span class="k">for</span> <span class="nx">_</span><span class="p">,</span> <span class="nx">e</span> <span class="o">:=</span> <span class="k">range</span> <span class="nx">os</span><span class="p">.</span><span class="nx">Environ</span><span class="p">()</span> <span class="p">{</span>
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<span class="nx">pair</span> <span class="o">:=</span> <span class="nx">strings</span><span class="p">.</span><span class="nx">Split</span><span class="p">(</span><span class="nx">e</span><span class="p">,</span> <span class="s">"="</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="nx">pair</span><span class="p">[</span><span class="mi">0</span><span class="p">])</span>
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<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>Running the program shows that we pick up the value
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for <code>FOO</code> that we set in the program, but that
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<code>BAR</code> is empty.</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">$</span> go run environment-variables.go
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<span class="go">FOO: 1</span>
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<span class="go">BAR: </span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>The list of keys in the environment will depend on your
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particular machine.</p>
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</td>
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<td class="code leading">
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<div class="highlight"><pre><span class="go">TERM_PROGRAM</span>
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<span class="go">PATH</span>
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<span class="go">SHELL</span>
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<span class="go">...</span>
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</pre></div>
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</td>
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</tr>
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<tr>
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<td class="docs">
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<p>If we set <code>BAR</code> in the environment first, the running
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program picks that value up.</p>
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</td>
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<td class="code">
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<div class="highlight"><pre><span class="gp">$</span> <span class="nv">BAR</span><span class="o">=</span>2 go run environment-variables.go
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<span class="go">FOO: 1</span>
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<span class="go">BAR: 2</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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</table>
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<p class="next">
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Next example: <a href="spawning-processes">Spawning Processes</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/environment-variables">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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