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command-line-flags
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<!DOCTYPE html>
<html>
<head>
<meta charset="utf-8">
<title>Go by Example: Command-Line Flags</title>
<link rel=stylesheet href="site.css">
</head>
<script>
onkeydown = (e) => {
if (e.key == "ArrowLeft") {
window.location.href = 'command-line-arguments';
}
if (e.key == "ArrowRight") {
window.location.href = 'command-line-subcommands';
}
}
</script>
<body>
<div class="example" id="command-line-flags">
<h2><a href="./">Go by Example</a>: Command-Line Flags</h2>
<table>
<tr>
<td class="docs">
<p><a href="http://en.wikipedia.org/wiki/Command-line_interface#Command-line_option"><em>Command-line flags</em></a>
are a common way to specify options for command-line
programs. For example, in <code>wc -l</code> the <code>-l</code> is a
command-line flag.</p>
</td>
<td class="code empty leading">
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<a href="http://play.golang.org/p/-zzqphwtdJq"><img title="Run code" src="play.png" class="run" /></a><img title="Copy code" src="clipboard.png" class="copy" />
<pre class="chroma"><span class="kn">package</span> <span class="nx">main</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Go provides a <code>flag</code> package supporting basic
command-line flag parsing. We’ll use this package to
implement our example command-line program.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kn">import</span> <span class="p">(</span>
<span class="s">"flag"</span>
<span class="s">"fmt"</span>
<span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
</td>
<td class="code leading">
<pre class="chroma"><span class="kd">func</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Basic flag declarations are available for string,
integer, and boolean options. Here we declare a
string flag <code>word</code> with a default value <code>"foo"</code>
and a short description. This <code>flag.String</code> function
returns a string pointer (not a string value);
we’ll see how to use this pointer below.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">wordPtr</span> <span class="o">:=</span> <span class="nx">flag</span><span class="p">.</span><span class="nf">String</span><span class="p">(</span><span class="s">"word"</span><span class="p">,</span> <span class="s">"foo"</span><span class="p">,</span> <span class="s">"a string"</span><span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>This declares <code>numb</code> and <code>fork</code> flags, using a
similar approach to the <code>word</code> flag.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">numbPtr</span> <span class="o">:=</span> <span class="nx">flag</span><span class="p">.</span><span class="nf">Int</span><span class="p">(</span><span class="s">"numb"</span><span class="p">,</span> <span class="mi">42</span><span class="p">,</span> <span class="s">"an int"</span><span class="p">)</span>
<span class="nx">forkPtr</span> <span class="o">:=</span> <span class="nx">flag</span><span class="p">.</span><span class="nf">Bool</span><span class="p">(</span><span class="s">"fork"</span><span class="p">,</span> <span class="kc">false</span><span class="p">,</span> <span class="s">"a bool"</span><span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>It’s also possible to declare an option that uses an
existing var declared elsewhere in the program.
Note that we need to pass in a pointer to the flag
declaration function.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="kd">var</span> <span class="nx">svar</span> <span class="kt">string</span>
<span class="nx">flag</span><span class="p">.</span><span class="nf">StringVar</span><span class="p">(</span><span class="o">&</span><span class="nx">svar</span><span class="p">,</span> <span class="s">"svar"</span><span class="p">,</span> <span class="s">"bar"</span><span class="p">,</span> <span class="s">"a string var"</span><span class="p">)</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Once all flags are declared, call <code>flag.Parse()</code>
to execute the command-line parsing.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="nx">flag</span><span class="p">.</span><span class="nf">Parse</span><span class="p">()</span>
</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Here we’ll just dump out the parsed options and
any trailing positional arguments. Note that we
need to dereference the pointers with e.g. <code>*wordPtr</code>
to get the actual option values.</p>
</td>
<td class="code">
<pre class="chroma">
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"word:"</span><span class="p">,</span> <span class="o">*</span><span class="nx">wordPtr</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"numb:"</span><span class="p">,</span> <span class="o">*</span><span class="nx">numbPtr</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"fork:"</span><span class="p">,</span> <span class="o">*</span><span class="nx">forkPtr</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"svar:"</span><span class="p">,</span> <span class="nx">svar</span><span class="p">)</span>
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Println</span><span class="p">(</span><span class="s">"tail:"</span><span class="p">,</span> <span class="nx">flag</span><span class="p">.</span><span class="nf">Args</span><span class="p">())</span>
<span class="p">}</span>
</pre>
</td>
</tr>
</table>
<table>
<tr>
<td class="docs">
<p>To experiment with the command-line flags program it’s
best to first compile it and then run the resulting
binary directly.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="gp">$</span> go build command-line-flags.go</pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Try out the built program by first giving it values for
all flags.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="gp">$</span> ./command-line-flags -word=opt -numb=7 -fork -svar=flag
<span class="go">word: opt
</span><span class="go">numb: 7
</span><span class="go">fork: true
</span><span class="go">svar: flag
</span><span class="go">tail: []</span></pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Note that if you omit flags they automatically take
their default values.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="gp">$</span> ./command-line-flags -word=opt
<span class="go">word: opt
</span><span class="go">numb: 42
</span><span class="go">fork: false
</span><span class="go">svar: bar
</span><span class="go">tail: []</span></pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Trailing positional arguments can be provided after
any flags.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="gp">$</span> ./command-line-flags -word=opt a1 a2 a3
<span class="go">word: opt
</span><span class="go">...
</span><span class="go">tail: [a1 a2 a3]</span></pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Note that the <code>flag</code> package requires all flags to
appear before positional arguments (otherwise the flags
will be interpreted as positional arguments).</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="gp">$</span> ./command-line-flags -word=opt a1 a2 a3 -numb=7
<span class="go">word: opt
</span><span class="go">numb: 42
</span><span class="go">fork: false
</span><span class="go">svar: bar
</span><span class="go">tail: [a1 a2 a3 -numb=7]</span></pre>
</td>
</tr>
<tr>
<td class="docs">
<p>Use <code>-h</code> or <code>--help</code> flags to get automatically
generated help text for the command-line program.</p>
</td>
<td class="code leading">
<pre class="chroma">
<span class="gp">$</span> ./command-line-flags -h
<span class="go">Usage of ./command-line-flags:
</span><span class="go"> -fork=false: a bool
</span><span class="go"> -numb=42: an int
</span><span class="go"> -svar="bar": a string var
</span><span class="go"> -word="foo": a string</span></pre>
</td>
</tr>
<tr>
<td class="docs">
<p>If you provide a flag that wasn’t specified to the
<code>flag</code> package, the program will print an error message
and show the help text again.</p>
</td>
<td class="code">
<pre class="chroma">
<span class="gp">$</span> ./command-line-flags -wat
<span class="go">flag provided but not defined: -wat
</span><span class="go">Usage of ./command-line-flags:
</span><span class="go">...</span></pre>
</td>
</tr>
</table>
<p class="next">
Next example: <a href="command-line-subcommands">Command-Line Subcommands</a>.
</p>
<p class="footer">
by <a href="https://markmcgranaghan.com">Mark McGranaghan</a> and <a href="https://eli.thegreenplace.net">Eli Bendersky</a> | <a href="https://github.com/mmcgrana/gobyexample">source</a> | <a href="https://github.com/mmcgrana/gobyexample#license">license</a>
</p>
</div>
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