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gobyexample/public/spawning-processes
Eli Bendersky baf22e84e7 Move <img> for play.golang.org to separate line in the template 
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Note that there are whitespace diffs in the generated files where there
is no image. This is because there was a stray tab in the template before,
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Fixes #235
2019-06-04 19:03:37 -07:00

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<!DOCTYPE html>
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<title>Go by Example: Spawning Processes</title>
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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/6HRWVK5gPYU">
<img title="Run code" class="run" src="data:image/png;base64,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" />
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<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>
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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>
</pre></div>
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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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<td class="code leading">
<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>
</pre></div>
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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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<td class="code leading">
<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>
</pre></div>
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<td class="code">
<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://markmcgranaghan.com">Mark McGranaghan</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>
</p>
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