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