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- Lab 8: Systemd - HackMD
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- <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="">
- <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
- </code></pre>
- </li>
- <li><span>Add the following to the file.</span><pre><code class="bash hljs"><div class="wrapper"><div class="gutter linenumber"><span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span></div><div class="code"><span class="hljs-meta">#!/bin/bash</span>
-
- <span class="hljs-keyword">while</span> <span class="hljs-literal">true</span>;
- <span class="hljs-keyword">do</span> <span class="hljs-built_in">dd</span> <span class="hljs-keyword">if</span>=/dev/zero of=/dev/null
- <span class="hljs-keyword">done</span> &
-
- <span class="hljs-keyword">while</span> <span class="hljs-literal">true</span>;
- <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">$(top -bn1)</span>"</span> \
- | nc -l -k -p 8080 -q 1;
- <span class="hljs-keyword">done</span>
- </div></div></code></pre>
- <blockquote>
- <p><code>dd</code><span> has been added for a purpose which you will see in further sections.</span><br>
- <span>Consider the web server as a means of remotely viewing the resource usage of </span><code>dd</code><span>.</span></p>
- </blockquote>
- </li>
- <li><span>Save the script and set execute permission.</span><pre><code>$ sudo chmod +x /usr/bin/script.sh
- </code></pre>
- </li>
- </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
- </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>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span></div><div class="code">[Unit]
- Description=My custom web service to show system processes
-
- [Service]
- ExecStart=/usr/bin/script.sh
-
- [Install]
- WantedBy=multi-user.target
- </div></div></code></pre><blockquote>
- <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>
- </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>
- <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
- </code></pre>
- </li>
- <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
- $ sudo systemctl start shellscript.service
- </code></pre>
- <blockquote>
- <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>
- </blockquote>
- </li>
- <li><span>Finally, verify that the script is up and running as a systemd service.</span><pre><code>$ sudo systemctl status shellscript.service
- </code></pre>
- </li>
- <li><span>Example output when we access the web service via port 8080 is shown below.</span><br>
- <img src="https://i.imgur.com/GfB4DAB.jpg" alt="" loading="lazy"></li>
- <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>
- </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>
- <li>
- <p><span>You can view the cgroup of a service with the </span><code>systemctl status <service_name></code><span> command.</span><br>
- <span>Let’s view the status of our custom system service </span><code>shellscript.service</code><span> again.</span></p>
- <pre><code>$ systemctl status shellscript.service
- </code></pre>
- <p><span>You get an output similar to the following</span></p>
- <pre><code>● shellscript.service - My Shell Script
- Loaded: loaded (/lib/systemd/system/shellscript.service; enabled; vendor preset: enabled)
- Active: active (running) since Sun 2022-10-23 19:39:02 +04; 1s ago
- Main PID: 6821 (script.sh)
- Tasks: 4 (limit: 9415)
- Memory: 1.0M
- CPU: 1.945s
- CGroup: /system.slice/shellscript.service
- ├─6821 /bin/bash /usr/bin/script.sh
- ├─6822 /bin/bash /usr/bin/script.sh
- ├─6824 nc -l -k -p 8080 -q 1
- └─6825 dd if=/dev/zero of=/dev/null
- </code></pre>
- <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>
- <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>
- <span>Subsequent lines under the </span><code>CGroup</code><span> are the other commands executed in the service.</span></p>
- </li>
- <li>
- <p><span>View your system’s cgroup hierarchy</span></p>
- <pre><code>$ systemctl status
- </code></pre>
- </li>
- <li>
- <p><span>You can view the system resource usage by each cgroup</span></p>
- <pre><code>$ systemd-cgtop
- </code></pre>
- </li>
- <li>
- <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>
- <span>View a list of all systemd slices</span></p>
- <pre><code>$ systemctl -t slice --all
- </code></pre>
- </li>
- <li>
- <p><span>Create a systemd slice at </span><code>/etc/systemd/system/testslice.slice</code><span>. Add the following to the file.</span></p>
- <pre><code class="hljs"><div class="wrapper"><div class="gutter linenumber"><span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span></div><div class="code">[Unit]
- Description=Custom systemd slice for SNA lab on systemd.
- Before=slices.target
-
- [Slice]
- MemoryAccounting=true
- CPUAccounting=true
- MemoryMax=10%
- CPUQuota=10%
- </div></div></code></pre>
- <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>
- </li>
- <li>
- <p><span>Let’s add our custom service </span><code>shellscript.service</code><span>to this new control group.</span><br>
- <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>
- <pre><code class="hljs"><div class="wrapper"><div class="gutter linenumber"><span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span>
- <span></span></div><div class="code">[Unit]
- Description=My custom web service to show system processes
-
- [Service]
- ExecStart=/usr/bin/script.sh
- Slice=testslice.slice
-
- [Install]
- WantedBy=multi-user.target
- </div></div></code></pre>
- </li>
- <li>
- <p><span>Reload the daemon and restart the service to apply the changes</span></p>
- <pre><code>$ sudo systemctl daemon-reload
- $ sudo systemctl restart shellscript.service
- </code></pre>
- </li>
- <li>
- <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>
- <img src="https://i.imgur.com/SQ5xb0b.jpg" alt="" loading="lazy"></p>
- </li>
- <li>
- <p><span>Run </span><code>$ systemd-cgtop</code><span> to view resource usage by cgroups.</span></p>
- <pre><code>Control Group Tasks %CPU Memory Input/s Output/s
- / 774 13,9 1.7G - -
- testslice.slice 2 9,9 956.0K - -
- testslice.slice/shellscript.service 2 9,9 576.0K - -
- user.slice 489 3,1 1.5G - -
- user.slice/user-1000.slice 489 3,4 1.3G - -
- </code></pre>
- </li>
- <li>
- <p><span>View the hierarchy and other artifacts about the cgroup.</span></p>
- <pre><code>$ systemctl -t slice --all
- $ systemctl status
- </code></pre>
- </li>
- <li>
- <p><span>View systemd log for your service</span></p>
- <pre><code>$ journalctl -u shellscript.service
- </code></pre>
- </li>
- </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>
- <p><span>Upload the scripts you create to moodle.</span><br>
- <span>Show test results of your implementation in your report.</span></p>
- </blockquote><ol>
- <li>
- <p><span>Show the following boot-up performance statistics on your system:</span></p>
- <ul>
- <li><span>Time spent in the kernel space before the user space was reached.</span></li>
- <li><span>Show an SVG image that contains services that have been started, and how long it took for them to initialize.</span></li>
- </ul>
- </li>
- <li>
- <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>
- <ul>
- <li><span>Provide brief explanation for each of the systemd units you encounter while performing this trace.</span></li>
- <li><span>The unit at this dead end </span><code>Wants</code><span> some systemd units. Why does it want these units?</span></li>
- </ul>
- <blockquote>
- <p><span>Show screenshots of every step as you trace.</span></p>
- </blockquote>
- </li>
- <li>
- <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>
- <ul>
- <li><span>The required information should be queried from the server everytime a user opens or refreshes the page.</span></li>
- <li><span>You do not need to save the results anywhere. Users only need live updates when the server is visited.</span></li>
- <li><span>The results should be displayed on a single page in an orderly manner that is easy to read.</span></li>
- <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>
- <li><span>Your systemd service should restart the web server if the web server crashes or is killed.</span></li>
- <li><span>This service is allowed to use a maximum of 15% of the CPU and 256MB memory.</span></li>
- </ul>
- <blockquote>
- <p><span>Show all steps taken, and all unit files created in your report.</span><br>
- <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>
- </blockquote>
- </li>
- <li>
- <p><span>Create a systemd service that will update your package sources list from the repository.</span></p>
- <ul>
- <li><span>The service should update the package source list five minutes after booting, and then every day after that.</span></li>
- <li><span>The schedule of the execution should be done with only systemd.</span></li>
- </ul>
- </li>
- </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">
- <li><span>Create a custom target in </span><code>/etc/systemd/system/<your_target>.target</code><span>.</span>
- <ul>
- <li><span>Add a description of the target file.</span></li>
- <li><span>Create a directory </span><code>/etc/systemd/system/<your_target>.wants/</code></li>
- <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>
- </ul>
- </li>
- </ol></div>
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- <li class=""><a href="#Lab-8-Systemd" title="Lab 8: Systemd">Lab 8: Systemd</a><ul class="nav">
- <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>
- <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>
- <li class=""><a href="#Task-3-Enable-the-service" title="Task 3: Enable the service">Task 3: Enable the service</a></li>
- <li><a href="#Task-4-Systemd-control-groups-cgroups" title="Task 4: Systemd control groups (cgroups)">Task 4: Systemd control groups (cgroups)</a></li>
- <li><a href="#Questions-to-answer" title="Questions to answer">Questions to answer</a><ul class="nav">
- <li><a href="#Bonus" title="Bonus">Bonus</a></li>
- </ul>
- </li>
- </ul>
- </li>
- </ul>
- </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>
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- <div class="toc"><ul class="nav">
- <li class=""><a href="#Lab-8-Systemd" title="Lab 8: Systemd">Lab 8: Systemd</a><ul class="nav">
- <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>
- <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>
- <li class=""><a href="#Task-3-Enable-the-service" title="Task 3: Enable the service">Task 3: Enable the service</a></li>
- <li><a href="#Task-4-Systemd-control-groups-cgroups" title="Task 4: Systemd control groups (cgroups)">Task 4: Systemd control groups (cgroups)</a></li>
- <li><a href="#Questions-to-answer" title="Questions to answer">Questions to answer</a><ul class="nav">
- <li><a href="#Bonus" title="Bonus">Bonus</a></li>
- </ul>
- </li>
- </ul>
- </li>
- </ul>
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