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Hana 9e216da9ef go.mod: add go.mod and move pygments to third_party 
After go1.16, go will use module mode by default,
even when the repository is checked out under GOPATH
or in a one-off directory. Add go.mod, go.sum to keep
this repo buildable without opting out of the module
mode.

> go mod init github.com/mmcgrana/gobyexample
> go mod tidy
> go mod vendor

In module mode, the 'vendor' directory is special
and its contents will be actively maintained by the
go command. pygments aren't the dependency the go will
know about, so it will delete the contents from vendor
directory. Move it to `third_party` directory now.

And, vendor the blackfriday package.

Note: the tutorial contents are not affected by the
change in go1.16 because all the examples in this
tutorial ask users to run the go command with the
explicit list of files to be compiled (e.g.
`go run hello-world.go` or `go build command-line-arguments.go`).
When the source list is provided, the go command does
not have to compute the build list and whether it's
running in GOPATH mode or module mode becomes irrelevant.
2021-02-15 16:45:26 -05:00

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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/m2CpSlHPEVq"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<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="p">(</span>
<span class="s">&quot;fmt&quot;</span>
<span class="s">&quot;io/ioutil&quot;</span>
<span class="s">&quot;os/exec&quot;</span>
<span class="p">)</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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<td class="code leading">
<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>
</td>
<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>
</td>
<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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<td class="docs">
<p>We omitted 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>
</td>
<td class="code leading">
<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>
</td>
<td class="code">
<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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</table>
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<td class="docs">
<p>The spawned programs return output that is the same
as if we had run them directly from the command-line.</p>
</td>
<td class="code leading">
<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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<td class="code leading">
<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="docs">
</td>
<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>
</div>
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