291 lines
12 KiB
Plaintext
Generated
291 lines
12 KiB
Plaintext
Generated
<!DOCTYPE html>
|
|
<html>
|
|
<head>
|
|
<meta charset="utf-8">
|
|
<title>Go by Example: Writing Files</title>
|
|
<link rel=stylesheet href="site.css">
|
|
</head>
|
|
<script>
|
|
onkeydown = (e) => {
|
|
|
|
if (e.key == "ArrowLeft") {
|
|
window.location.href = 'reading-files';
|
|
}
|
|
|
|
|
|
if (e.key == "ArrowRight") {
|
|
window.location.href = 'line-filters';
|
|
}
|
|
|
|
}
|
|
</script>
|
|
<body>
|
|
<div class="example" id="writing-files">
|
|
<h2><a href="./">Go by Example</a>: Writing Files</h2>
|
|
|
|
<table>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Writing files in Go follows similar patterns to the
|
|
ones we saw earlier for reading.</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/Y12O-L_zFS1"><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">
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma"><span class="kn">import</span> <span class="p">(</span>
|
|
<span class="s">"bufio"</span>
|
|
<span class="s">"fmt"</span>
|
|
<span class="s">"os"</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">check</span><span class="p">(</span><span class="nx">e</span> <span class="kt">error</span><span class="p">)</span> <span class="p">{</span>
|
|
<span class="k">if</span> <span class="nx">e</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">e</span><span class="p">)</span>
|
|
<span class="p">}</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>To start, here’s how to dump a string (or just
|
|
bytes) into a file.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="nx">d1</span> <span class="o">:=</span> <span class="p">[]</span><span class="nb">byte</span><span class="p">(</span><span class="s">"hello\ngo\n"</span><span class="p">)</span>
|
|
<span class="nx">err</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nf">WriteFile</span><span class="p">(</span><span class="s">"/tmp/dat1"</span><span class="p">,</span> <span class="nx">d1</span><span class="p">,</span> <span class="mo">0644</span><span class="p">)</span>
|
|
<span class="nf">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>For more granular writes, open a file for writing.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="nx">f</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">os</span><span class="p">.</span><span class="nf">Create</span><span class="p">(</span><span class="s">"/tmp/dat2"</span><span class="p">)</span>
|
|
<span class="nf">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>It’s idiomatic to defer a <code>Close</code> immediately
|
|
after opening a file.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="k">defer</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Close</span><span class="p">()</span>
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>You can <code>Write</code> byte slices as you’d expect.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="nx">d2</span> <span class="o">:=</span> <span class="p">[]</span><span class="kt">byte</span><span class="p">{</span><span class="mi">115</span><span class="p">,</span> <span class="mi">111</span><span class="p">,</span> <span class="mi">109</span><span class="p">,</span> <span class="mi">101</span><span class="p">,</span> <span class="mi">10</span><span class="p">}</span>
|
|
<span class="nx">n2</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">Write</span><span class="p">(</span><span class="nx">d2</span><span class="p">)</span>
|
|
<span class="nf">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
|
|
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"wrote %d bytes\n"</span><span class="p">,</span> <span class="nx">n2</span><span class="p">)</span>
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>A <code>WriteString</code> is also available.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="nx">n3</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">f</span><span class="p">.</span><span class="nf">WriteString</span><span class="p">(</span><span class="s">"writes\n"</span><span class="p">)</span>
|
|
<span class="nf">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
|
|
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"wrote %d bytes\n"</span><span class="p">,</span> <span class="nx">n3</span><span class="p">)</span>
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Issue a <code>Sync</code> to flush writes to stable storage.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="nx">f</span><span class="p">.</span><span class="nf">Sync</span><span class="p">()</span>
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p><code>bufio</code> provides buffered writers in addition
|
|
to the buffered readers we saw earlier.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="nx">w</span> <span class="o">:=</span> <span class="nx">bufio</span><span class="p">.</span><span class="nf">NewWriter</span><span class="p">(</span><span class="nx">f</span><span class="p">)</span>
|
|
<span class="nx">n4</span><span class="p">,</span> <span class="nx">err</span> <span class="o">:=</span> <span class="nx">w</span><span class="p">.</span><span class="nf">WriteString</span><span class="p">(</span><span class="s">"buffered\n"</span><span class="p">)</span>
|
|
<span class="nf">check</span><span class="p">(</span><span class="nx">err</span><span class="p">)</span>
|
|
<span class="nx">fmt</span><span class="p">.</span><span class="nf">Printf</span><span class="p">(</span><span class="s">"wrote %d bytes\n"</span><span class="p">,</span> <span class="nx">n4</span><span class="p">)</span>
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Use <code>Flush</code> to ensure all buffered operations have
|
|
been applied to the underlying writer.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="nx">w</span><span class="p">.</span><span class="nf">Flush</span><span class="p">()</span>
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
|
|
</td>
|
|
<td class="code">
|
|
|
|
<pre class="chroma"><span class="p">}</span>
|
|
</pre>
|
|
</td>
|
|
</tr>
|
|
|
|
</table>
|
|
|
|
<table>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Try running the file-writing code.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="gp">$</span> go run writing-files.go
|
|
<span class="go">wrote 5 bytes
|
|
</span><span class="go">wrote 7 bytes
|
|
</span><span class="go">wrote 9 bytes</span></pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Then check the contents of the written files.</p>
|
|
|
|
</td>
|
|
<td class="code leading">
|
|
|
|
<pre class="chroma">
|
|
<span class="gp">$</span> cat /tmp/dat1
|
|
<span class="go">hello
|
|
</span><span class="go">go
|
|
</span><span class="go"></span><span class="gp">$</span> cat /tmp/dat2
|
|
<span class="go">some
|
|
</span><span class="go">writes
|
|
</span><span class="go">buffered</span></pre>
|
|
</td>
|
|
</tr>
|
|
|
|
<tr>
|
|
<td class="docs">
|
|
<p>Next we’ll look at applying some of the file I/O ideas
|
|
we’ve just seen to the <code>stdin</code> and <code>stdout</code> streams.</p>
|
|
|
|
</td>
|
|
<td class="code empty">
|
|
|
|
|
|
</td>
|
|
</tr>
|
|
|
|
</table>
|
|
|
|
|
|
<p class="next">
|
|
Next example: <a href="line-filters">Line Filters</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>
|
|
<script>
|
|
var codeLines = [];
|
|
codeLines.push('');codeLines.push('package main\u000A');codeLines.push('import (\u000A \"bufio\"\u000A \"fmt\"\u000A \"os\"\u000A)\u000A');codeLines.push('func check(e error) {\u000A if e !\u003D nil {\u000A panic(e)\u000A }\u000A}\u000A');codeLines.push('func main() {\u000A');codeLines.push(' d1 :\u003D []byte(\"hello\\ngo\\n\")\u000A err :\u003D os.WriteFile(\"/tmp/dat1\", d1, 0644)\u000A check(err)\u000A');codeLines.push(' f, err :\u003D os.Create(\"/tmp/dat2\")\u000A check(err)\u000A');codeLines.push(' defer f.Close()\u000A');codeLines.push(' d2 :\u003D []byte{115, 111, 109, 101, 10}\u000A n2, err :\u003D f.Write(d2)\u000A check(err)\u000A fmt.Printf(\"wrote %d bytes\\n\", n2)\u000A');codeLines.push(' n3, err :\u003D f.WriteString(\"writes\\n\")\u000A check(err)\u000A fmt.Printf(\"wrote %d bytes\\n\", n3)\u000A');codeLines.push(' f.Sync()\u000A');codeLines.push(' w :\u003D bufio.NewWriter(f)\u000A n4, err :\u003D w.WriteString(\"buffered\\n\")\u000A check(err)\u000A fmt.Printf(\"wrote %d bytes\\n\", n4)\u000A');codeLines.push(' w.Flush()\u000A');codeLines.push('}\u000A');codeLines.push('');codeLines.push('');codeLines.push('');
|
|
</script>
|
|
<script src="site.js" async></script>
|
|
</body>
|
|
</html>
|