深入Nodejs模块fs - 文件系统操作

2019/04/10 10:10
阅读数 10

<section id="nice" data-tool="mdnice编辑器" data-website="https://www.mdnice.com" style="font-size: 16px; color: black; padding: 10px; line-height: 1.6; word-spacing: 0px; letter-spacing: 0px; word-break: break-word; word-wrap: break-word; text-align: left; font-family: Optima-Regular, Optima, PingFangSC-light, PingFangTC-light, 'PingFang SC', Cambria, Cochin, Georgia, Times, 'Times New Roman', serif;"><figure data-tool="mdnice编辑器" style="margin: 0; margin-top: 10px; margin-bottom: 10px;"><img src="https://tva1.sinaimg.cn/large/006tNbRwly1gb0qoh8vpwj30xc0gomxw.jpg" alt style="display: block; margin: 0 auto; width: 100%;"></figure> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">node 的<code style="font-size: 14px; word-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0 2px; background-color: rgba(27,31,35,.05); font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; word-break: break-all; color: rgb(239, 112, 96);">fs</code>文档密密麻麻的 api 非常多,毕竟全面支持对文件系统的操作。文档组织的很好,操作基本分为文件操作、目录操作、文件信息、流这个大方面,编程方式也支持同步、异步和 Promise。</p> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">本文记录了几个文档中没详细描写的问题,可以更好地串联<code style="font-size: 14px; word-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0 2px; background-color: rgba(27,31,35,.05); font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; word-break: break-all; color: rgb(239, 112, 96);">fs</code>文档思路:</p> <ul data-tool="mdnice编辑器" style="margin-top: 8px; margin-bottom: 8px; padding-left: 25px; color: black; list-style-type: disc;"> <li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;">文件描述符</section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;">同步、异步与 Promise</section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;">目录与目录项</section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;">文件信息</section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;">stream</section></li></ul> <h2 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 24px; margin: 10px auto; height: 40px; background-color: rgb(251, 251, 251); border-bottom: 1px solid rgb(246, 246, 246); overflow: hidden; box-sizing: border-box;"><span style="margin-left: -10px; display: inline-block; width: auto; height: 40px; background-color: rgb(33, 33, 34); border-bottom-right-radius: 100px; color: rgb(255, 255, 255); padding-right: 30px; padding-left: 30px; line-height: 40px; font-size: 16px;">文件描述符</span></h2> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">文件描述符是一个非负整数。它是一个索引值,操作系统可以根据它来找到对应的文件。</p> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">在 fs 的很多底层 api 中,需要用到文件描述符。在文档中,描述符通常用<code style="font-size: 14px; word-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0 2px; background-color: rgba(27,31,35,.05); font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; word-break: break-all; color: rgb(239, 112, 96);">fd</code>来代表。例如:<code style="font-size: 14px; word-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0 2px; background-color: rgba(27,31,35,.05); font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; word-break: break-all; color: rgb(239, 112, 96);">fs.read(fd, buffer, offset, length, position, callback)</code>。与这个 api 相对应的是:<code style="font-size: 14px; word-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0 2px; background-color: rgba(27,31,35,.05); font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; word-break: break-all; color: rgb(239, 112, 96);">fs.readFile(path[, options], callback)</code>。</p> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">因为操作系统对文件描述符的数量有限制,因此在结束文件操作后,别忘记 close:</p> <pre class="custom" data-tool="mdnice编辑器" style="margin-top: 10px; margin-bottom: 10px;"><code class="hljs" style="overflow-x: auto; padding: 16px; color: #333; background: #f8f8f8; display: -webkit-box; font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; border-radius: 0px; font-size: 12px; -webkit-overflow-scrolling: touch;"><span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">const</span> fs = <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">require</span>(<span class="hljs-string" style="color: #d14; line-height: 26px;">"fs"</span>);

fs.open(<span class="hljs-string" style="color: #d14; line-height: 26px;">"./db.json"</span>, <span class="hljs-string" style="color: #d14; line-height: 26px;">"r"</span>, (err, fd) => { <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">if</span> (err) <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">throw</span> err; <span class="hljs-comment" style="color: #998; font-style: italic; line-height: 26px;">// 文件操作...</span> <span class="hljs-comment" style="color: #998; font-style: italic; line-height: 26px;">// 完成操作后,关闭文件</span> fs.close(fd, err => { <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">if</span> (err) <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">throw</span> err; }); }); </code></pre>

<h2 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 24px; margin: 10px auto; height: 40px; background-color: rgb(251, 251, 251); border-bottom: 1px solid rgb(246, 246, 246); overflow: hidden; box-sizing: border-box;"><span style="margin-left: -10px; display: inline-block; width: auto; height: 40px; background-color: rgb(33, 33, 34); border-bottom-right-radius: 100px; color: rgb(255, 255, 255); padding-right: 30px; padding-left: 30px; line-height: 40px; font-size: 16px;">同步、异步与 Promise</span></h2> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">所有文件系统的 api 都有同步和异步两种形式。</p> <h3 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 20px; margin: 20px auto 5px; border-top: 1px solid rgb(221, 221, 221); box-sizing: border-box;"><span style="margin-top: -1px; padding-top: 6px; padding-right: 5px; padding-left: 5px; font-size: 17px; border-top: 2px solid rgb(33, 33, 34); display: inline-block; line-height: 1.1;">同步写法</span></h3> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;"><strong style="font-weight: bold; color: black;">不推荐使用同步 api,会阻塞线程</strong>。</p> <pre class="custom" data-tool="mdnice编辑器" style="margin-top: 10px; margin-bottom: 10px;"><code class="hljs" style="overflow-x: auto; padding: 16px; color: #333; background: #f8f8f8; display: -webkit-box; font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; border-radius: 0px; font-size: 12px; -webkit-overflow-scrolling: touch;"><span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">try</span> { <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">const</span> buf = fs.readFileSync(<span class="hljs-string" style="color: #d14; line-height: 26px;">"./package.json"</span>); <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.log(buf.toString(<span class="hljs-string" style="color: #d14; line-height: 26px;">"utf8"</span>)); } <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">catch</span> (error) { <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.log(error.message); } </code></pre> <h3 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 20px; margin: 20px auto 5px; border-top: 1px solid rgb(221, 221, 221); box-sizing: border-box;"><span style="margin-top: -1px; padding-top: 6px; padding-right: 5px; padding-left: 5px; font-size: 17px; border-top: 2px solid rgb(33, 33, 34); display: inline-block; line-height: 1.1;">异步写法</span></h3> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">异步写法写起来容易进入回调地狱。</p> <pre class="custom" data-tool="mdnice编辑器" style="margin-top: 10px; margin-bottom: 10px;"><code class="hljs" style="overflow-x: auto; padding: 16px; color: #333; background: #f8f8f8; display: -webkit-box; font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; border-radius: 0px; font-size: 12px; -webkit-overflow-scrolling: touch;">fs.readFile(<span class="hljs-string" style="color: #d14; line-height: 26px;">"./package.json"</span>, (err, data) =&gt; { <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">if</span> (err) <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">throw</span> err; <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.log(data.toString(<span class="hljs-string" style="color: #d14; line-height: 26px;">"utf8"</span>)); }); </code></pre> <h3 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 20px; margin: 20px auto 5px; border-top: 1px solid rgb(221, 221, 221); box-sizing: border-box;"><span style="margin-top: -1px; padding-top: 6px; padding-right: 5px; padding-left: 5px; font-size: 17px; border-top: 2px solid rgb(33, 33, 34); display: inline-block; line-height: 1.1;">(推荐)Promise 写法</span></h3> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">在 node v12 之前,需要自己借助 promise 封装:</p> <pre class="custom" data-tool="mdnice编辑器" style="margin-top: 10px; margin-bottom: 10px;"><code class="hljs" style="overflow-x: auto; padding: 16px; color: #333; background: #f8f8f8; display: -webkit-box; font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; border-radius: 0px; font-size: 12px; -webkit-overflow-scrolling: touch;"><span class="hljs-function" style="line-height: 26px;"><span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">function</span> <span class="hljs-title" style="color: #900; font-weight: bold; line-height: 26px;">readFilePromise</span>(<span class="hljs-params" style="line-height: 26px;">path, encoding = <span class="hljs-string" style="color: #d14; line-height: 26px;">"utf8"</span></span>) </span>{ <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">const</span> promise = <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">new</span> <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">Promise</span>(<span class="hljs-function" style="line-height: 26px;">(<span class="hljs-params" style="line-height: 26px;">resolve, reject</span>) =&gt;</span> { fs.readFile(path, (err, data) =&gt; { <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">if</span> (err) <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">return</span> reject(err); <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">return</span> resolve(data.toString(encoding)); }); }); <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">return</span> promise; }

readFilePromise(<span class="hljs-string" style="color: #d14; line-height: 26px;">"./package.json"</span>).then(<span class="hljs-function" style="line-height: 26px;"><span class="hljs-params" style="line-height: 26px;">res</span> =></span> <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.log(res)); </code></pre>

<p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">在 node v12 中,引入了 fs Promise api。它们返回 Promise 对象而不是使用回调。 API 可通过 <code style="font-size: 14px; word-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0 2px; background-color: rgba(27,31,35,.05); font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; word-break: break-all; color: rgb(239, 112, 96);">require('fs').promises</code> 访问。如此一来,开发成本更低了。</p> <pre class="custom" data-tool="mdnice编辑器" style="margin-top: 10px; margin-bottom: 10px;"><code class="hljs" style="overflow-x: auto; padding: 16px; color: #333; background: #f8f8f8; display: -webkit-box; font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; border-radius: 0px; font-size: 12px; -webkit-overflow-scrolling: touch;"><span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">const</span> fsPromises = <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">require</span>(<span class="hljs-string" style="color: #d14; line-height: 26px;">"fs"</span>).promises;

fsPromises .readFile(<span class="hljs-string" style="color: #d14; line-height: 26px;">"./package.json"</span>, { <span class="hljs-attr" style="line-height: 26px;">encoding</span>: <span class="hljs-string" style="color: #d14; line-height: 26px;">"utf8"</span>, <span class="hljs-attr" style="line-height: 26px;">flag</span>: <span class="hljs-string" style="color: #d14; line-height: 26px;">"r"</span> }) .then(<span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.log) .catch(<span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.error); </code></pre>

<h2 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 24px; margin: 10px auto; height: 40px; background-color: rgb(251, 251, 251); border-bottom: 1px solid rgb(246, 246, 246); overflow: hidden; box-sizing: border-box;"><span style="margin-left: -10px; display: inline-block; width: auto; height: 40px; background-color: rgb(33, 33, 34); border-bottom-right-radius: 100px; color: rgb(255, 255, 255); padding-right: 30px; padding-left: 30px; line-height: 40px; font-size: 16px;">目录与目录项</span></h2> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">fs.Dir 类:封装了和文件目录相关的操作</p> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">fs.Dirent 类:封装了目录项的相关操作。例如判断设备类型(字符、块、FIFO 等)。</p> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">它们之间的关系,通过代码展示:</p> <pre class="custom" data-tool="mdnice编辑器" style="margin-top: 10px; margin-bottom: 10px;"><code class="hljs" style="overflow-x: auto; padding: 16px; color: #333; background: #f8f8f8; display: -webkit-box; font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; border-radius: 0px; font-size: 12px; -webkit-overflow-scrolling: touch;"><span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">const</span> fsPromises = <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">require</span>(<span class="hljs-string" style="color: #d14; line-height: 26px;">"fs"</span>).promises;

<span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">async</span> <span class="hljs-function" style="line-height: 26px;"><span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">function</span> <span class="hljs-title" style="color: #900; font-weight: bold; line-height: 26px;">main</span>(<span class="hljs-params" style="line-height: 26px;"></span>) </span>{ <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">const</span> dir = <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">await</span> fsPromises.opendir(<span class="hljs-string" style="color: #d14; line-height: 26px;">"."</span>); <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">let</span> dirent = <span class="hljs-literal" style="color: #008080; line-height: 26px;">null</span>; <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">while</span> ((dirent = <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">await</span> dir.read()) !== <span class="hljs-literal" style="color: #008080; line-height: 26px;">null</span>) { <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.log(dirent.name); } }

main(); </code></pre>

<h2 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 24px; margin: 10px auto; height: 40px; background-color: rgb(251, 251, 251); border-bottom: 1px solid rgb(246, 246, 246); overflow: hidden; box-sizing: border-box;"><span style="margin-left: -10px; display: inline-block; width: auto; height: 40px; background-color: rgb(33, 33, 34); border-bottom-right-radius: 100px; color: rgb(255, 255, 255); padding-right: 30px; padding-left: 30px; line-height: 40px; font-size: 16px;">文件信息</span></h2> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">fs.Stats 类:封装了文件信息相关的操作。它在<code style="font-size: 14px; word-wrap: break-word; padding: 2px 4px; border-radius: 4px; margin: 0 2px; background-color: rgba(27,31,35,.05); font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; word-break: break-all; color: rgb(239, 112, 96);">fs.stat()</code>的回调函数中返回。</p> <pre class="custom" data-tool="mdnice编辑器" style="margin-top: 10px; margin-bottom: 10px;"><code class="hljs" style="overflow-x: auto; padding: 16px; color: #333; background: #f8f8f8; display: -webkit-box; font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; border-radius: 0px; font-size: 12px; -webkit-overflow-scrolling: touch;">fs.stat(<span class="hljs-string" style="color: #d14; line-height: 26px;">"./package.json"</span>, (err, stats) =&gt; { <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">if</span> (err) <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">throw</span> err; <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.log(stats); }); </code></pre> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">注意,关于检查文件是否存在:</p> <ul data-tool="mdnice编辑器" style="margin-top: 8px; margin-bottom: 8px; padding-left: 25px; color: black; list-style-type: disc;"> <li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;">不建议在调用 fs.open()、 fs.readFile() 或 fs.writeFile() 之前使用 fs.stat() 检查文件是否存在。<strong style="font-weight: bold; color: black;">而是应该直接打开、读取或写入文件,如果文件不可用则处理引发的错误</strong>。</section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;">要检查文件是否存在但随后并不对其进行操作,则建议使用 fs.access()。</section></li></ul> <h2 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 24px; margin: 10px auto; height: 40px; background-color: rgb(251, 251, 251); border-bottom: 1px solid rgb(246, 246, 246); overflow: hidden; box-sizing: border-box;"><span style="margin-left: -10px; display: inline-block; width: auto; height: 40px; background-color: rgb(33, 33, 34); border-bottom-right-radius: 100px; color: rgb(255, 255, 255); padding-right: 30px; padding-left: 30px; line-height: 40px; font-size: 16px;">ReadStream 与 WriteStream</span></h2> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">在 nodejs 中,stream 是个非常重要的库。很多库的 api 都是基于 stream 来封装的。例如下面要说的 fs 中的 ReadStream 和 WriteStream。</p> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">fs 本身提供了 readFile 和 writeFile,它们好用的代价就是性能有问题,会将内容一次全部载入内存。但是对于几 GB 的大文件,显然会有问题。</p> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">那么针对大文件的解决方案自然是:一点点读出来。这就需要用到 stream 了。以 readStream 为例,代码如下:</p> <pre class="custom" data-tool="mdnice编辑器" style="margin-top: 10px; margin-bottom: 10px;"><code class="hljs" style="overflow-x: auto; padding: 16px; color: #333; background: #f8f8f8; display: -webkit-box; font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; border-radius: 0px; font-size: 12px; -webkit-overflow-scrolling: touch;"><span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">const</span> rs = fs.createReadStream(<span class="hljs-string" style="color: #d14; line-height: 26px;">"./package.json"</span>); <span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">let</span> content = <span class="hljs-string" style="color: #d14; line-height: 26px;">""</span>;

rs.on(<span class="hljs-string" style="color: #d14; line-height: 26px;">"open"</span>, () => { <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.log(<span class="hljs-string" style="color: #d14; line-height: 26px;">"start to read"</span>); });

rs.on(<span class="hljs-string" style="color: #d14; line-height: 26px;">"data"</span>, chunk => { content += chunk.toString(<span class="hljs-string" style="color: #d14; line-height: 26px;">"utf8"</span>); });

rs.on(<span class="hljs-string" style="color: #d14; line-height: 26px;">"close"</span>, () => { <span class="hljs-built_in" style="color: #0086b3; line-height: 26px;">console</span>.log(<span class="hljs-string" style="color: #d14; line-height: 26px;">"finish read, content is:\n"</span>, content); }); </code></pre>

<p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">借助 stream 的 pipe,一行快速封装一个大文件的拷贝函数:</p> <pre class="custom" data-tool="mdnice编辑器" style="margin-top: 10px; margin-bottom: 10px;"><code class="hljs" style="overflow-x: auto; padding: 16px; color: #333; background: #f8f8f8; display: -webkit-box; font-family: Operator Mono, Consolas, Monaco, Menlo, monospace; border-radius: 0px; font-size: 12px; -webkit-overflow-scrolling: touch;"><span class="hljs-function" style="line-height: 26px;"><span class="hljs-keyword" style="color: #333; font-weight: bold; line-height: 26px;">function</span> <span class="hljs-title" style="color: #900; font-weight: bold; line-height: 26px;">copyBigFile</span>(<span class="hljs-params" style="line-height: 26px;">src, target</span>) </span>{ fs.createReadStream(src).pipe(fs.createWriteStream(target)); } </code></pre> <h2 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 24px; margin: 10px auto; height: 40px; background-color: rgb(251, 251, 251); border-bottom: 1px solid rgb(246, 246, 246); overflow: hidden; box-sizing: border-box;"><span style="margin-left: -10px; display: inline-block; width: auto; height: 40px; background-color: rgb(33, 33, 34); border-bottom-right-radius: 100px; color: rgb(255, 255, 255); padding-right: 30px; padding-left: 30px; line-height: 40px; font-size: 16px;">参考链接</span></h2> <ul data-tool="mdnice编辑器" style="margin-top: 8px; margin-bottom: 8px; padding-left: 25px; color: black; list-style-type: disc;"> <li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;"><a href="https://baike.baidu.com/item/%E6%96%87%E4%BB%B6%E6%8F%8F%E8%BF%B0%E7%AC%A6" style="text-decoration: none; word-wrap: break-word; font-weight: bold; color: rgb(239, 112, 96); border-bottom: 1px solid rgb(239, 112, 96);">文件描述符</a></section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;"><a href="https://baike.baidu.com/item/%E5%A5%97%E6%8E%A5%E5%AD%97" style="text-decoration: none; word-wrap: break-word; font-weight: bold; color: rgb(239, 112, 96); border-bottom: 1px solid rgb(239, 112, 96);">Socket 套接字</a></section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;"><a href="https://www.cnblogs.com/chyingp/p/nodejs-learning-stream.html" style="text-decoration: none; word-wrap: break-word; font-weight: bold; color: rgb(239, 112, 96); border-bottom: 1px solid rgb(239, 112, 96);">Nodejs 基础:stream 模块入门介绍与使用</a></section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;"><a href="https://stackoverflow.com/questions/11293857/fastest-way-to-copy-file-in-node-js" style="text-decoration: none; word-wrap: break-word; font-weight: bold; color: rgb(239, 112, 96); border-bottom: 1px solid rgb(239, 112, 96);">Fastest way to copy file in node.js</a></section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;"><a href="https://itnext.io/using-node-js-to-read-really-really-large-files-pt-1-d2057fe76b33" style="text-decoration: none; word-wrap: break-word; font-weight: bold; color: rgb(239, 112, 96); border-bottom: 1px solid rgb(239, 112, 96);">Using Node.js to Read Really, Really Large Datasets &amp; Files (Pt 1)</a></section></li></ul> <h2 data-tool="mdnice编辑器" style="margin-top: 40px; margin-bottom: 20px; font-weight: bold; color: black; font-size: 24px; margin: 10px auto; height: 40px; background-color: rgb(251, 251, 251); border-bottom: 1px solid rgb(246, 246, 246); overflow: hidden; box-sizing: border-box;"><span style="margin-left: -10px; display: inline-block; width: auto; height: 40px; background-color: rgb(33, 33, 34); border-bottom-right-radius: 100px; color: rgb(255, 255, 255); padding-right: 30px; padding-left: 30px; line-height: 40px; font-size: 16px;">最后</span></h2> <ol data-tool="mdnice编辑器" style="margin-top: 8px; margin-bottom: 8px; padding-left: 25px; color: black; list-style-type: decimal;"> <li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;">觉得不错,<strong style="font-weight: bold; color: black;">帮忙点个推荐呗</strong>,您的支持是对我最大的激励</section></li><li><section style="margin-top: 5px; margin-bottom: 5px; line-height: 26px; text-align: left; color: rgb(1,1,1); font-weight: 500; font-size: 15px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif;">欢迎我的公众号:<strong style="font-weight: bold; color: black;">「心谭博客」</strong>,只专注于<strong style="font-weight: bold; color: black;">前端</strong> + <strong style="font-weight: bold; color: black;">算法</strong>的原创分享</section></li></ol> <p data-tool="mdnice编辑器" style="padding-top: 8px; padding-bottom: 8px; margin: 0; color: black; box-sizing: border-box; margin-bottom: 16px; font-family: 'Helvetica Neue', Helvetica, 'Segoe UI', Arial, freesans, sans-serif; font-size: 15px; text-align: start; white-space: normal; text-size-adjust: auto; line-height: 1.75em;">由于个人精力有限,很多系列和历史文章没有即时同步,<strong style="font-weight: bold; color: black;">请前往<a href="https://xxoo521.com/frontend/" style="text-decoration: none; word-wrap: break-word; font-weight: bold; color: rgb(239, 112, 96); border-bottom: 1px solid rgb(239, 112, 96);">「前端图谱」</a>&amp;<a href="https://xxoo521.com/algorithm/" style="text-decoration: none; word-wrap: break-word; font-weight: bold; color: rgb(239, 112, 96); border-bottom: 1px solid rgb(239, 112, 96);">「算法题解」</a></strong>,保证您有所收获。</p> </section>

原文出处:https://www.cnblogs.com/geyouneihan/p/12239865.html

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