On Linux you can use cat to view a file.

daniel@CShark:/mnt/d$ cat index.html
<html lang="en">
<meta charset="utf-8" />
    <title>Hello World!</title>
    <link rel="stylesheet" href="hw.css">
<div class="mainBox">
    <div class="textBox">
        <h1>Hello World!</h1>
    <img src="hw.jpeg" />

With the -n flag you can view that file with line numbers.

daniel@CShark:/mnt/d$ cat -n index.html
     1  <html lang="en">
     2  <head>
     3  <meta charset="utf-8" />
     4      <title>Hello World!</title>
     5      <link rel="stylesheet" href="hw.css">
     6  </head>
     7  <body>
     8  <div class="mainBox">
     9      <div class="textBox">
    10          <h1>Hello World!</h1>
    11      </div>
    12      <img src="hw.jpeg" />
    13  </div>
    14  </body>