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  13.         Lab 8: Systemd - HackMD

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  37.     <div id="doc" class="markdown-body container-fluid comment-enabled" data-hard-breaks="true"><h1 id="Lab-8-Systemd" data-id="Lab-8-Systemd"><a class="anchor hidden-xs" href="#Lab-8-Systemd" title="Lab-8-Systemd"><span class="octicon octicon-link"></span></a><span>Lab 8: Systemd</span></h1><h2 id="Task-1-Create-a-shell-script" data-id="Task-1-Create-a-shell-script"><a class="anchor hidden-xs" href="#Task-1-Create-a-shell-script" title="Task-1-Create-a-shell-script"><span class="octicon octicon-link"></span></a><span>Task 1: Create a shell script</span></h2><ul data-original-title="" title="">

  38. <li><span>Create a custom web server with bash. The web server displays the processes running on the server by executing the </span><code>top</code><span> command.</span><pre><code>$ sudo vim /usr/bin/script.sh 

  39. </code></pre>

  40. </li>

  41. <li><span>Add the following to the file.</span><pre><code class="bash hljs"><div class="wrapper"><div class="gutter linenumber"><span></span>

  42. <span></span>

  43. <span></span>

  44. <span></span>

  45. <span></span>

  46. <span></span>

  47. <span></span>

  48. <span></span>

  49. <span></span>

  50. <span></span></div><div class="code"><span class="hljs-meta">#!/bin/bash</span>

  51. 

  52. <span class="hljs-keyword">while</span> <span class="hljs-literal">true</span>;

  53.   <span class="hljs-keyword">do</span> <span class="hljs-built_in">dd</span> <span class="hljs-keyword">if</span>=/dev/zero of=/dev/null

  54. <span class="hljs-keyword">done</span> &amp;

  55. 

  56. <span class="hljs-keyword">while</span> <span class="hljs-literal">true</span>;

  57.   <span class="hljs-keyword">do</span> <span class="hljs-built_in">echo</span> -e <span class="hljs-string">"HTTP/1.1 200 OK\n\n<span class="hljs-subst">$(top -bn1)</span>"</span> \

  58.   | nc -l -k -p 8080 -q 1;

  59. <span class="hljs-keyword">done</span>

  60. </div></div></code></pre>

  61. <blockquote>

  62. <p><code>dd</code><span> has been added for a purpose which you will see in further sections.</span><br>

  63. <span>Consider the web server as a means of remotely viewing the resource usage of </span><code>dd</code><span>.</span></p>

  64. </blockquote>

  65. </li>

  66. <li><span>Save the script and set execute permission.</span><pre><code>$ sudo chmod +x /usr/bin/script.sh 

  67. </code></pre>

  68. </li>

  69. </ul><p><span>You have to manually run this script whenever the system is restarted. We can solve this by creating a systemd service for it. This will give us more options for managing the script’s execution.</span></p><h2 id="Task-2-Create-a-systemd-file" data-id="Task-2-Create-a-systemd-file"><a class="anchor hidden-xs" href="#Task-2-Create-a-systemd-file" title="Task-2-Create-a-systemd-file"><span class="octicon octicon-link"></span></a><span>Task 2: Create a systemd file</span></h2><p><span>Next, create a systemd service file for the script on your system. This file must have </span><code>.service</code><span> extension and saved under the </span><code>/lib/systemd/system/</code><span> directory</span></p><pre><code>$ sudo vim /lib/systemd/system/shellscript.service 

  70. </code></pre><p><span>Now, add the following content and update the script filename and location. You can also change the description of the service. After that, save the file and close it.</span></p><pre><code class="hljs"><div class="wrapper"><div class="gutter linenumber"><span></span>

  71. <span></span>

  72. <span></span>

  73. <span></span>

  74. <span></span>

  75. <span></span>

  76. <span></span>

  77. <span></span></div><div class="code">[Unit]

  78. Description=My custom web service to show system processes

  79. 

  80. [Service]

  81. ExecStart=/usr/bin/script.sh

  82. 

  83. [Install]

  84. WantedBy=multi-user.target

  85. </div></div></code></pre><blockquote>

  86. <p><span>The [Unit] section describes the service, specifies the ordering dependencies, as well as conflicting units. In [Service], a sequence of custom scripts is specified to be executed during unit activation, on stop, and on reload. Finally, the [Install] section lists units that depend on the service.</span></p>

  87. </blockquote><h2 id="Task-3-Enable-the-service" data-id="Task-3-Enable-the-service"><a class="anchor hidden-xs" href="#Task-3-Enable-the-service" title="Task-3-Enable-the-service"><span class="octicon octicon-link"></span></a><span>Task 3: Enable the service</span></h2><ul>

  88. <li><span>Your systemd service has been added to your system. Let’s reload the systemctl daemon to read new files. You need to reload the configuration file of a unit each time after making changes in any </span><code>*.service</code><span> file.</span><pre><code>$ sudo systemctl daemon-reload 

  89. </code></pre>

  90. </li>

  91. <li><span>Enable the service to start on system boot and also start the service using the following commands.</span><pre><code>$ sudo systemctl enable shellscript.service 

  92. $ sudo systemctl start shellscript.service 

  93. </code></pre>

  94. <blockquote>

  95. <p><span>When you enable a service, a symbolic link of that service is created in the </span><code>/etc/systemd/system/multi-user.target.wants</code><span> directory.</span></p>

  96. </blockquote>

  97. </li>

  98. <li><span>Finally, verify that the script is up and running as a systemd service.</span><pre><code>$ sudo systemctl status shellscript.service 

  99. </code></pre>

  100. </li>

  101. <li><span>Example output when we access the web service via port 8080 is shown below.</span><br>

  102. <img src="https://i.imgur.com/GfB4DAB.jpg" alt="" loading="lazy"></li>

  103. <li><span>The </span><code>dd</code><span> process shown in the output was executed by the service we just created. This process is using 100% of the CPU. We can fix this problem by making use of systemd control groups.</span></li>

  104. </ul><h2 id="Task-4-Systemd-control-groups-cgroups" data-id="Task-4-Systemd-control-groups-cgroups"><a class="anchor hidden-xs" href="#Task-4-Systemd-control-groups-cgroups" title="Task-4-Systemd-control-groups-cgroups"><span class="octicon octicon-link"></span></a><span>Task 4: Systemd control groups (cgroups)</span></h2><p><span>Systemd control group is a mechanism that allows you to control the use of system resources by a group of processes.</span></p><ul>

  105. <li>

  106. <p><span>You can view the cgroup of a service with the </span><code>systemctl status &lt;service_name&gt;</code><span> command.</span><br>

  107. <span>Let’s view the status of our custom system service </span><code>shellscript.service</code><span> again.</span></p>

  108. <pre><code>$ systemctl status shellscript.service

  109. </code></pre>

  110. <p><span>You get an output similar to the following</span></p>

  111. <pre><code>● shellscript.service - My Shell Script

  112.    Loaded: loaded (/lib/systemd/system/shellscript.service; enabled; vendor preset: enabled)

  113.    Active: active (running) since Sun 2022-10-23 19:39:02 +04; 1s ago

  114.  Main PID: 6821 (script.sh)

  115.     Tasks: 4 (limit: 9415)

  116.    Memory: 1.0M

  117.       CPU: 1.945s

  118.    CGroup: /system.slice/shellscript.service

  119.            ├─6821 /bin/bash /usr/bin/script.sh

  120.            ├─6822 /bin/bash /usr/bin/script.sh

  121.            ├─6824 nc -l -k -p 8080 -q 1

  122.            └─6825 dd if=/dev/zero of=/dev/null

  123. </code></pre>

  124. <p><span>The output shows that </span><code>shellscript.service</code><span> is under the </span><code>system.slice</code><span> control group.</span><br>

  125. <span>The second line </span><code>6821 /bin/bash /usr/bin/script.sh</code><span> shows the process ID and the command used to start </span><code>shellscript.service</code><span>.</span><br>

  126. <span>Subsequent lines under the </span><code>CGroup</code><span> are the other commands executed in the service.</span></p>

  127. </li>

  128. <li>

  129. <p><span>View your system’s cgroup hierarchy</span></p>

  130. <pre><code>$ systemctl status

  131. </code></pre>

  132. </li>

  133. <li>

  134. <p><span>You can view the system resource usage by each cgroup</span></p>

  135. <pre><code>$ systemd-cgtop

  136. </code></pre>

  137. </li>

  138. <li>

  139. <p><span>Slices allow one to create a hierarchical structure in which relative shares of resources are defined for the entities that belong to those slices.</span><br>

  140. <span>View a list of all systemd slices</span></p>

  141. <pre><code>$ systemctl -t slice --all

  142. </code></pre>

  143. </li>

  144. <li>

  145. <p><span>Create a systemd slice at </span><code>/etc/systemd/system/testslice.slice</code><span>. Add the following to the file.</span></p>

  146. <pre><code class="hljs"><div class="wrapper"><div class="gutter linenumber"><span></span>

  147. <span></span>

  148. <span></span>

  149. <span></span>

  150. <span></span>

  151. <span></span>

  152. <span></span>

  153. <span></span>

  154. <span></span></div><div class="code">[Unit]

  155. Description=Custom systemd slice for SNA lab on systemd.

  156. Before=slices.target

  157. 

  158. [Slice]

  159. MemoryAccounting=true

  160. CPUAccounting=true

  161. MemoryMax=10%

  162. CPUQuota=10%

  163. </div></div></code></pre>

  164. <p><span>This slice will set CPU and memory usage limit to all processes running under it. As seen in the configuration, a maximum of 10% of memory and CPU resources can be used by the processes running under this control group </span><code>testslice.slice</code><span>.</span></p>

  165. </li>

  166. <li>

  167. <p><span>Let’s add our custom service </span><code>shellscript.service</code><span>to this new control group.</span><br>

  168. <span>Modify the service file </span><code>/lib/systemd/system/shellscript.service</code><span> to use this systemd slice by adding the line </span><code>Slice=testslice.slice</code><span> to the </span><code>[Service]</code><span> section as shown below:</span></p>

  169. <pre><code class="hljs"><div class="wrapper"><div class="gutter linenumber"><span></span>

  170. <span></span>

  171. <span></span>

  172. <span></span>

  173. <span></span>

  174. <span></span>

  175. <span></span>

  176. <span></span>

  177. <span></span></div><div class="code">[Unit]

  178. Description=My custom web service to show system processes

  179. 

  180. [Service]

  181. ExecStart=/usr/bin/script.sh

  182. Slice=testslice.slice

  183. 

  184. [Install]

  185. WantedBy=multi-user.target

  186. </div></div></code></pre>

  187. </li>

  188. <li>

  189. <p><span>Reload the daemon and restart the service to apply the changes</span></p>

  190. <pre><code>$ sudo systemctl daemon-reload

  191. $ sudo systemctl restart shellscript.service

  192. </code></pre>

  193. </li>

  194. <li>

  195. <p><span>Refresh the web page. The output shows that </span><code>dd</code><span> now uses less than 10% of the CPU.</span><br>

  196. <img src="https://i.imgur.com/SQ5xb0b.jpg" alt="" loading="lazy"></p>

  197. </li>

  198. <li>

  199. <p><span>Run </span><code>$ systemd-cgtop</code><span> to view resource usage by cgroups.</span></p>

  200. <pre><code>Control Group                            Tasks   %CPU   Memory  Input/s Output/s

  201. /                                          774   13,9     1.7G        -        -

  202. testslice.slice                              2    9,9   956.0K        -        -

  203. testslice.slice/shellscript.service          2    9,9   576.0K        -        -

  204. user.slice                                 489    3,1     1.5G        -        -

  205. user.slice/user-1000.slice                 489    3,4     1.3G        -        -

  206. </code></pre>

  207. </li>

  208. <li>

  209. <p><span>View the hierarchy and other artifacts about the cgroup.</span></p>

  210. <pre><code>$ systemctl -t slice --all

  211. $ systemctl status

  212. </code></pre>

  213. </li>

  214. <li>

  215. <p><span>View systemd log for your service</span></p>

  216. <pre><code>$ journalctl -u shellscript.service

  217. </code></pre>

  218. </li>

  219. </ul><h2 id="Questions-to-answer" data-id="Questions-to-answer"><a class="anchor hidden-xs" href="#Questions-to-answer" title="Questions-to-answer"><span class="octicon octicon-link"></span></a><span>Questions to answer</span></h2><blockquote>

  220. <p><span>Upload the scripts you create to moodle.</span><br>

  221. <span>Show test results of your implementation in your report.</span></p>

  222. </blockquote><ol>

  223. <li>

  224. <p><span>Show the following boot-up performance statistics on your system:</span></p>

  225. <ul>

  226. <li><span>Time spent in the kernel space before the user space was reached.</span></li>

  227. <li><span>Show an SVG image that contains services that have been started, and how long it took for them to initialize.</span></li>

  228. </ul>

  229. </li>

  230. <li>

  231. <p><span>Take the systemd unit </span><code>graphical.target</code><span> as your starting point, start tracing backwards using only the </span><code>Requires</code><span> variable. At what systemd unit do you reach a dead end where there is no more </span><code>Requires</code><span> variable?</span></p>

  232. <ul>

  233. <li><span>Provide brief explanation for each of the systemd units you encounter while performing this trace.</span></li>

  234. <li><span>The unit at this dead end </span><code>Wants</code><span> some systemd units. Why does it want these units?</span></li>

  235. </ul>

  236. <blockquote>

  237. <p><span>Show screenshots of every step as you trace.</span></p>

  238. </blockquote>

  239. </li>

  240. <li>

  241. <p><span>Create a simple web server in bash that shows the following: system uptime, inode usage, current memory, disk space usage statistics, and the last 15 lines of </span><code>/var/log/syslog</code><span>.</span></p>

  242. <ul>

  243. <li><span>The required information should be queried from the server everytime a user opens or refreshes the page.</span></li>

  244. <li><span>You do not need to save the results anywhere. Users only need live updates when the server is visited.</span></li>

  245. <li><span>The results should be displayed on a single page in an orderly manner that is easy to read.</span></li>

  246. <li><span>Create a systemd service on your system to run this script (web server). Show how you can start your new service, and configure it to run after system reboot.</span></li>

  247. <li><span>Your systemd service should restart the web server if the web server crashes or is killed.</span></li>

  248. <li><span>This service is allowed to use a maximum of 15% of the CPU and 256MB memory.</span></li>

  249. </ul>

  250. <blockquote>

  251. <p><span>Show all steps taken, and all unit files created in your report.</span><br>

  252. <span>At the end of this task, you must have at least one bash script, one service file, and one slice file all working together to achieve the objectives.</span></p>

  253. </blockquote>

  254. </li>

  255. <li>

  256. <p><span>Create a systemd service that will update your package sources list from the repository.</span></p>

  257. <ul>

  258. <li><span>The service should update the package source list five minutes after booting, and then every day after that.</span></li>

  259. <li><span>The schedule of the execution should be done with only systemd.</span></li>

  260. </ul>

  261. </li>

  262. </ol><h3 id="Bonus" data-id="Bonus"><a class="anchor hidden-xs" href="#Bonus" title="Bonus"><span class="octicon octicon-link"></span></a><span>Bonus</span></h3><ol start="5">

  263. <li><span>Create a custom target in </span><code>/etc/systemd/system/&lt;your_target&gt;.target</code><span>.</span>

  264. <ul>

  265. <li><span>Add a description of the target file.</span></li>

  266. <li><span>Create a directory </span><code>/etc/systemd/system/&lt;your_target&gt;.wants/</code></li>

  267. <li><span>Create sylinks to additional services you wish to enable in this new directory. It should be a symlink to services from </span><code>/usr/lib/systemd/system/</code><span> that you wish to enable.</span></li>

  268. </ul>

  269. </li>

  270. </ol></div>

  271.     <div class="ui-toc dropup unselectable hidden-print" style="display:none;">

  272.         <div class="pull-right dropdown">

  273.             <a id="tocLabel" class="ui-toc-label btn btn-default" data-toggle="dropdown" href="#" role="button" aria-haspopup="true" aria-expanded="false" title="Table of content">

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  276.             <ul id="ui-toc" class="ui-toc-dropdown dropdown-menu" aria-labelledby="tocLabel">

  277.                 <div class="toc"><ul class="nav">

  278. <li class=""><a href="#Lab-8-Systemd" title="Lab 8: Systemd">Lab 8: Systemd</a><ul class="nav">

  279. <li class=""><a href="#Task-1-Create-a-shell-script" title="Task 1: Create a shell script">Task 1: Create a shell script</a></li>

  280. <li class=""><a href="#Task-2-Create-a-systemd-file" title="Task 2: Create a systemd file">Task 2: Create a systemd file</a></li>

  281. <li class=""><a href="#Task-3-Enable-the-service" title="Task 3: Enable the service">Task 3: Enable the service</a></li>

  282. <li><a href="#Task-4-Systemd-control-groups-cgroups" title="Task 4: Systemd control groups (cgroups)">Task 4: Systemd control groups (cgroups)</a></li>

  283. <li><a href="#Questions-to-answer" title="Questions to answer">Questions to answer</a><ul class="nav">

  284. <li><a href="#Bonus" title="Bonus">Bonus</a></li>

  285. </ul>

  286. </li>

  287. </ul>

  288. </li>

  289. </ul>

  290. </div><div class="toc-menu"><a class="expand-toggle" href="#">Expand all</a><a class="back-to-top" href="#">Back to top</a><a class="go-to-bottom" href="#">Go to bottom</a></div>

  291.             </ul>

  292.         </div>

  293.     </div>

  294.     <div id="ui-toc-affix" class="ui-affix-toc ui-toc-dropdown unselectable hidden-print" data-spy="affix" style="top:17px;display:none;" null null>

  295.         <div class="toc"><ul class="nav">

  296. <li class=""><a href="#Lab-8-Systemd" title="Lab 8: Systemd">Lab 8: Systemd</a><ul class="nav">

  297. <li class=""><a href="#Task-1-Create-a-shell-script" title="Task 1: Create a shell script">Task 1: Create a shell script</a></li>

  298. <li class=""><a href="#Task-2-Create-a-systemd-file" title="Task 2: Create a systemd file">Task 2: Create a systemd file</a></li>

  299. <li class=""><a href="#Task-3-Enable-the-service" title="Task 3: Enable the service">Task 3: Enable the service</a></li>

  300. <li><a href="#Task-4-Systemd-control-groups-cgroups" title="Task 4: Systemd control groups (cgroups)">Task 4: Systemd control groups (cgroups)</a></li>

  301. <li><a href="#Questions-to-answer" title="Questions to answer">Questions to answer</a><ul class="nav">

  302. <li><a href="#Bonus" title="Bonus">Bonus</a></li>

  303. </ul>

  304. </li>

  305. </ul>

  306. </li>

  307. </ul>

  308. </div><div class="toc-menu"><a class="expand-toggle" href="#">Expand all</a><a class="back-to-top" href="#">Back to top</a><a class="go-to-bottom" href="#">Go to bottom</a></div>

  309.     </div>

  310.     <script src="https://cdnjs.cloudflare.com/ajax/libs/jquery/3.1.1/jquery.min.js" integrity="sha256-hVVnYaiADRTO2PzUGmuLJr8BLUSjGIZsDYGmIJLv2b8=" crossorigin="anonymous"></script>

  311.     <script src="https://cdnjs.cloudflare.com/ajax/libs/twitter-bootstrap/3.3.7/js/bootstrap.min.js" integrity="sha256-U5ZEeKfGNOja007MMD3YBI0A3OSZOQbeG6z2f2Y0hu8=" crossorigin="anonymous" defer></script>

  312.     <script src="https://cdnjs.cloudflare.com/ajax/libs/gist-embed/2.6.0/gist-embed.min.js" integrity="sha256-KyF2D6xPIJUW5sUDSs93vWyZm+1RzIpKCexxElmxl8g=" crossorigin="anonymous" defer></script>

  313.     <script>

  314.         var markdown = $(".markdown-body");

  315.         //smooth all hash trigger scrolling

  316.         function smoothHashScroll() {

  317.             var hashElements = $("a[href^='#']").toArray();

  318.             for (var i = 0; i < hashElements.length; i++) {

  319.                 var element = hashElements[i];

  320.                 var $element = $(element);

  321.                 var hash = element.hash;

  322.                 if (hash) {

  323.                     $element.on('click', function (e) {

  324.                         // store hash

  325.                         var hash = this.hash;

  326.                         if ($(hash).length <= 0) return;

  327.                         // prevent default anchor click behavior

  328.                         e.preventDefault();

  329.                         // animate

  330.                         $('body, html').stop(true, true).animate({

  331.                             scrollTop: $(hash).offset().top

  332.                         }, 100, "linear", function () {

  333.                             // when done, add hash to url

  334.                             // (default click behaviour)

  335.                             window.location.hash = hash;

  336.                         });

  337.                     });

  338.                 }

  339.             }

  340.         }

  341. 

  342.         smoothHashScroll();

  343.         var toc = $('.ui-toc');

  344.         var tocAffix = $('.ui-affix-toc');

  345.         var tocDropdown = $('.ui-toc-dropdown');

  346.         //toc

  347.         tocDropdown.click(function (e) {

  348.             e.stopPropagation();

  349.         });

  350. 

  351.         var enoughForAffixToc = true;

  352. 

  353.         function generateScrollspy() {

  354.             $(document.body).scrollspy({

  355.                 target: ''

  356.             });

  357.             $(document.body).scrollspy('refresh');

  358.             if (enoughForAffixToc) {

  359.                 toc.hide();

  360.                 tocAffix.show();

  361.             } else {

  362.                 tocAffix.hide();

  363.                 toc.show();

  364.             }

  365.             $(document.body).scroll();

  366.         }

  367. 

  368.         function windowResize() {

  369.             //toc right

  370.             var paddingRight = parseFloat(markdown.css('padding-right'));

  371.             var right = ($(window).width() - (markdown.offset().left + markdown.outerWidth() - paddingRight));

  372.             toc.css('right', right + 'px');

  373.             //affix toc left

  374.             var newbool;

  375.             var rightMargin = (markdown.parent().outerWidth() - markdown.outerWidth()) / 2;

  376.             //for ipad or wider device

  377.             if (rightMargin >= 133) {

  378.                 newbool = true;

  379.                 var affixLeftMargin = (tocAffix.outerWidth() - tocAffix.width()) / 2;

  380.                 var left = markdown.offset().left + markdown.outerWidth() - affixLeftMargin;

  381.                 tocAffix.css('left', left + 'px');

  382.             } else {

  383.                 newbool = false;

  384.             }

  385.             if (newbool != enoughForAffixToc) {

  386.                 enoughForAffixToc = newbool;

  387.                 generateScrollspy();

  388.             }

  389.         }

  390.         $(window).resize(function () {

  391.             windowResize();

  392.         });

  393.         $(document).ready(function () {

  394.             windowResize();

  395.             generateScrollspy();

  396.         });

  397. 

  398.         //remove hash

  399.         function removeHash() {

  400.             window.location.hash = '';

  401.         }

  402. 

  403.         var backtotop = $('.back-to-top');

  404.         var gotobottom = $('.go-to-bottom');

  405. 

  406.         backtotop.click(function (e) {

  407.             e.preventDefault();

  408.             e.stopPropagation();

  409.             if (scrollToTop)

  410.                 scrollToTop();

  411.             removeHash();

  412.         });

  413.         gotobottom.click(function (e) {

  414.             e.preventDefault();

  415.             e.stopPropagation();

  416.             if (scrollToBottom)

  417.                 scrollToBottom();

  418.             removeHash();

  419.         });

  420. 

  421.         var toggle = $('.expand-toggle');

  422.         var tocExpand = false;

  423. 

  424.         checkExpandToggle();

  425.         toggle.click(function (e) {

  426.             e.preventDefault();

  427.             e.stopPropagation();

  428.             tocExpand = !tocExpand;

  429.             checkExpandToggle();

  430.         })

  431. 

  432.         function checkExpandToggle () {

  433.             var toc = $('.ui-toc-dropdown .toc');

  434.             var toggle = $('.expand-toggle');

  435.             if (!tocExpand) {

  436.                 toc.removeClass('expand');

  437.                 toggle.text('Expand all');

  438.             } else {

  439.                 toc.addClass('expand');

  440.                 toggle.text('Collapse all');

  441.             }

  442.         }

  443. 

  444.         function scrollToTop() {

  445.             $('body, html').stop(true, true).animate({

  446.                 scrollTop: 0

  447.             }, 100, "linear");

  448.         }

  449. 

  450.         function scrollToBottom() {

  451.             $('body, html').stop(true, true).animate({

  452.                 scrollTop: $(document.body)[0].scrollHeight

  453.             }, 100, "linear");

  454.         }

  455.     </script>

  456. </body>

  457. 

  458. </html>