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<div class="example" id="spawning-processes">
<h2><a href="./">Go by Example</a>: Spawning Processes</h2>
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<p>Sometimes our Go programs need to spawn other, non-Go
processes. For example, the syntax highlighting on this
site is <a href="https://github.com/mmcgrana/gobyexample/blob/master/tools/generate.go">implemented</a>
by spawning a <a href="http://pygments.org/"><code>pygmentize</code></a>
process from a Go program. Let&rsquo;s look at a few examples
of spawning processes from Go.</p>
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<a href="http://play.golang.org/p/y6SB6Mf2VQ"><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>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">&quot;fmt&quot;</span>
<span class="kn">import</span> <span class="s">&quot;io/ioutil&quot;</span>
<span class="kn">import</span> <span class="s">&quot;os/exec&quot;</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>We&rsquo;ll start with a simple command that takes no
arguments or input and just prints something to
stdout. The <code>exec.Command</code> helper creates an object
to represent this external process.</p>
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<div class="highlight"><pre> <span class="nx">dateCmd</span> <span class="o">:=</span> <span class="nx">exec</span><span class="p">.</span><span class="nx">Command</span><span class="p">(</span><span class="s">&quot;date&quot;</span><span class="p">)</span>
</pre></div>
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<p><code>.Output</code> is another helper that handles the common
case of running a command, waiting for it to finish,
and collecting its output. If there were no errors,
<code>dateOut</code> will hold bytes with the date info.</p>
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<div class="highlight"><pre> <span class="nx">dateOut</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">dateCmd</span><span class="p">.</span><span class="nx">Output</span><span class="p">()</span>
<span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nb">panic</span><span class="p">(</span><span class="nx">err</span><span class="p">)</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;&gt; date&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="nb">string</span><span class="p">(</span><span class="nx">dateOut</span><span class="p">))</span>
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<p>Next we&rsquo;ll look at a slightly more involved case
where we pipe data to the external process on its
<code>stdin</code> and collect the results from its <code>stdout</code>.</p>
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<div class="highlight"><pre> <span class="nx">grepCmd</span> <span class="o">:=</span> <span class="nx">exec</span><span class="p">.</span><span class="nx">Command</span><span class="p">(</span><span class="s">&quot;grep&quot;</span><span class="p">,</span> <span class="s">&quot;hello&quot;</span><span class="p">)</span>
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<p>Here we explicitly grab input/output pipes, start
the process, write some input to it, read the
resulting output, and finally wait for the process
to exit.</p>
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<div class="highlight"><pre> <span class="nx">grepIn</span><span class="p">,</span> <span class="nx">_</span> <span class="o">:=</span> <span class="nx">grepCmd</span><span class="p">.</span><span class="nx">StdinPipe</span><span class="p">()</span>
<span class="nx">grepOut</span><span class="p">,</span> <span class="nx">_</span> <span class="o">:=</span> <span class="nx">grepCmd</span><span class="p">.</span><span class="nx">StdoutPipe</span><span class="p">()</span>
<span class="nx">grepCmd</span><span class="p">.</span><span class="nx">Start</span><span class="p">()</span>
<span class="nx">grepIn</span><span class="p">.</span><span class="nx">Write</span><span class="p">([]</span><span class="nb">byte</span><span class="p">(</span><span class="s">&quot;hello grep\ngoodbye grep&quot;</span><span class="p">))</span>
<span class="nx">grepIn</span><span class="p">.</span><span class="nx">Close</span><span class="p">()</span>
<span class="nx">grepBytes</span><span class="p">,</span> <span class="nx">_</span> <span class="o">:=</span> <span class="nx">ioutil</span><span class="p">.</span><span class="nx">ReadAll</span><span class="p">(</span><span class="nx">grepOut</span><span class="p">)</span>
<span class="nx">grepCmd</span><span class="p">.</span><span class="nx">Wait</span><span class="p">()</span>
</pre></div>
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<p>We ommited error checks in the above example, but
you could use the usual <code>if err != nil</code> pattern for
all of them. We also only collect the <code>StdoutPipe</code>
results, but you could collect the <code>StderrPipe</code> in
exactly the same way.</p>
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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">&quot;&gt; grep hello&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="nb">string</span><span class="p">(</span><span class="nx">grepBytes</span><span class="p">))</span>
</pre></div>
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<p>Note that when spawning commands we need to
provide an explicitly delineated command and
argument array, vs. being able to just pass in one
command-line string. If you want to spawn a full
command with a string, you can use <code>bash</code>&rsquo;s <code>-c</code>
option:</p>
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<div class="highlight"><pre> <span class="nx">lsCmd</span> <span class="o">:=</span> <span class="nx">exec</span><span class="p">.</span><span class="nx">Command</span><span class="p">(</span><span class="s">&quot;bash&quot;</span><span class="p">,</span> <span class="s">&quot;-c&quot;</span><span class="p">,</span> <span class="s">&quot;ls -a -l -h&quot;</span><span class="p">)</span>
<span class="nx">lsOut</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">lsCmd</span><span class="p">.</span><span class="nx">Output</span><span class="p">()</span>
<span class="k">if</span> <span class="nx">err</span> <span class="o">!=</span> <span class="kc">nil</span> <span class="p">{</span>
<span class="nb">panic</span><span class="p">(</span><span class="nx">err</span><span class="p">)</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;&gt; ls -a -l -h&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="nb">string</span><span class="p">(</span><span class="nx">lsOut</span><span class="p">))</span>
<span class="p">}</span>
</pre></div>
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<p>The spawned programs return output that is the same
as if we had run them directly from the command-line.</p>
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<div class="highlight"><pre><span class="gp">$</span> go run spawning-processes.go
<span class="gp">&gt;</span> date
<span class="go">Wed Oct 10 09:53:11 PDT 2012</span>
</pre></div>
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<div class="highlight"><pre><span class="gp">&gt;</span> grep hello
<span class="go">hello grep</span>
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<div class="highlight"><pre><span class="gp">&gt;</span> ls -a -l -h
<span class="go">drwxr-xr-x 4 mark 136B Oct 3 16:29 .</span>
<span class="go">drwxr-xr-x 91 mark 3.0K Oct 3 12:50 ..</span>
<span class="go">-rw-r--r-- 1 mark 1.3K Oct 3 16:28 spawning-processes.go</span>
</pre></div>
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<p class="next">
Next example: <a href="execing-processes">Exec'ing Processes</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/spawning-processes">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
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