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<div class="example" id="execing-processes">
<h2><a href="./">Go by Example</a>: Exec'ing Processes</h2>
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<p>In the previous example we looked at
<a href="spawning-processes">spawning external processes</a>. We
do this when we need an external process accessible to
a running Go process. Sometimes we just want to
completely replace the current Go process with another
(perhaps non-Go) one. To do this we’ll use Go’s
implementation of the classic
<a href="http://en.wikipedia.org/wiki/Exec_(operating_system)"><code>exec</code></a>
function.</p>
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<a href="http://play.golang.org/p/neqdJ51KLN"><img title="Run code" src="play.png" class="run" /></a>
<div class="highlight"><pre><span class="kn">package</span> <span class="nx">main</span>
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<div class="highlight"><pre><span class="kn">import</span> <span class="s">"syscall"</span>
<span class="kn">import</span> <span class="s">"os"</span>
<span class="kn">import</span> <span class="s">"os/exec"</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>
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<p>For our example we’ll exec <code>ls</code>. Go requires an
absolute path to the binary we want to execute, so
we’ll use <code>exec.LookPath</code> to find it (probably
<code>/bin/ls</code>).</p>
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<div class="highlight"><pre> <span class="nx">binary</span><span class="p">,</span> <span class="nx">lookErr</span> <span class="o">:=</span> <span class="nx">exec</span><span class="p">.</span><span class="nx">LookPath</span><span class="p">(</span><span class="s">"ls"</span><span class="p">)</span>
<span class="k">if</span> <span class="nx">lookErr</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">lookErr</span><span class="p">)</span>
<span class="p">}</span>
</pre></div>
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<p><code>Exec</code> requires arguments in slice form (as
apposed to one big string). We’ll give <code>ls</code> a few
common arguments. Note that the first argument should
be the program name.</p>
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<div class="highlight"><pre> <span class="nx">args</span> <span class="o">:=</span> <span class="p">[]</span><span class="kt">string</span><span class="p">{</span><span class="s">"ls"</span><span class="p">,</span> <span class="s">"-a"</span><span class="p">,</span> <span class="s">"-l"</span><span class="p">,</span> <span class="s">"-h"</span><span class="p">}</span>
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<p><code>Exec</code> also needs a set of <a href="environment-variables">environment variables</a>
to use. Here we just provide our current
environment.</p>
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<div class="highlight"><pre> <span class="nx">env</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nx">Environ</span><span class="p">()</span>
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<p>Here’s the actual <code>syscall.Exec</code> call. If this call is
successful, the execution of our process will end
here and be replaced by the <code>/bin/ls -a -l -h</code>
process. If there is an error we’ll get a return
value.</p>
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<div class="highlight"><pre> <span class="nx">execErr</span> <span class="o">:=</span> <span class="nx">syscall</span><span class="p">.</span><span class="nx">Exec</span><span class="p">(</span><span class="nx">binary</span><span class="p">,</span> <span class="nx">args</span><span class="p">,</span> <span class="nx">env</span><span class="p">)</span>
<span class="k">if</span> <span class="nx">execErr</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">execErr</span><span class="p">)</span>
<span class="p">}</span>
<span class="p">}</span>
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<p>When we run our program it is replaced by <code>ls</code>.</p>
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<div class="highlight"><pre><span class="gp">$</span> go run execing-processes.go
<span class="go">total 16</span>
<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 execing-processes.go</span>
</pre></div>
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<p>Note that Go does not offer a classic Unix <code>fork</code>
function. Usually this isn’t an issue though, since
starting goroutines, spawning processes, and exec’ing
processes covers most use cases for <code>fork</code>.</p>
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<p class="next">
Next example: <a href="signals">Signals</a>.
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