🌿💻 The Packets Garden

Recent Notes

My Linux Upskill Challenge: Day 7

🌱 🪴 ⏱️ 8 min read

Introduction

This is my journal note following the Linux Upskill Challenge: Day 7.

Before we Begin

In this lesson, we explore how services work in Linux, how they’re configured, and where to find their logs. The example used is installing and experimenting with the Apache2 web server.

Installing Apache2 on Ubuntu

sudo apt update
sudo apt install apache2

How to verify Apache is running

  • Open your browser and go to http://<server-ip> or use curl in the server terminal:

    curl http://localhost

Note

If you’r using an cloud hosted virtual machine, you will probably need to do extra steps in order to allow incoming request to you server on the port 80, perhaps this guides will help you:

Understanding Services in Linux

What is a service in Linux?

  • A service is a background process (also called a daemon) that is managed by the system to start, stop, or restart as needed.
  • In our case apache2 applications is running as a “service” inside Linux.

How are services managed?

  • On modern systems like Ubuntu, services are managed using systemd via the systemctl command.

Process, Daemons, Services, and Units

Process

  • A process is a running instance of a program.
  • For example, if you have a Bash script in your home folder, that’s just a file — it does nothing until you execute it.
  • When you run the script, a process is created (with a Process ID or PID) and it starts using system resources. So a process is the live, running version of a program.
  • You can even run the same program (or script) multiple times — each execution creates a new, separate process.

Every process has its own Process ID (PID).

Daemon

  • A daemon is a process that runs in the background and typically does not interact directly with the user.
  • Daemons usually start when the system boots and continue running “silently” in the background. They often handle essential tasks like logging, printing, networking, or system services like SSH.
  • There’s a naming convention where many daemons end with the letter d, such as sshd, systemd, or httpd. However, this is not a strict rule — some daemons like cron, cups, or rsyslog don’t follow this pattern.

Service

  • When a daemon is managed by the system’s init system (like systemd), it’s typically referred to as a service. This means it has an associated unit file that defines how to start, stop, and monitor it.
  • Examples of daemons that run as managed services include: httpd (Apache), nginx (Nginx), and docker (Docker).
  • To understand the difference, think of it this way: not every daemon is a service — meaning there can be background daemons that are not managed by the init system (like systemd). These may be started manually or by other programs.

Unit

  • systemd manages units — a unit is a representation of a resource that needs to be controlled.
  • Simply put, a unit is something that systemd organizes and keeps track of.
  • There are different types of units depending on the resource they describe or represent.
  • One of the most common unit types is the service unit (.service), which defines how to manage a service or application. It includes instructions on how to start or stop the service, when it should be started automatically, and any dependencies or ordering with other software.
  • Other unit types include: .socket, .mount, .automount, .swap, .target, .path, .timer, .snapshot

For example, a .service unit describes how to manage a system service like apache2, while a .mount unit tells systemd how to handle mounting a filesystem.

  • Units are defined using configuration files called unit files.

What is  systemd ?

  • systemd it’s the core init system used in most modern Linux distributions (like Ubuntu, Debian, Fedora, RHEL, etc.).
  • It’s the background process that manages all the services and other processes that are running on your system.
  • It’s like the 🧠 brain, or🗼 Control Tower of everything that is running on Linux
  • It is the first process (PID 1) that runs after the Linux kernel finishes booting.
  • Its job is to initialize the system, start all necessary services, and manage them during runtime.

Other Linux distribution (specialy oldest) could have a different init systems, like OpenRC, Upstart, etc..

The command systemctl

  • systemctlis the command line tool to interact with systemd
  • You use systemctlto start/stop/enable/disable services, among other things.
  • So systemctlis the user-facing CLI tool to manage systemd.

Start and Stop services

  • You can use this commands to start and stop the Apache2 service.
sudo systemctl start apache2
sudo systemctl stop apache2

Restart and Reload services

  • To restart a running service, you can use the restart command:
sudo systemctl restart apache2
  • You can also reload a service with the reload command, this means that the application will be able to reload its configuration files (without restarting):
sudo systemctl reload apache2

Check status

  • In case you want to check the status you should use:
systemctl status apache2
  • You should observe sometime like this if the service is running OK
● apache2.service - The Apache HTTP Server
     Loaded: loaded (/usr/lib/systemd/system/apache2.service; enabled; preset: enabled)
     Active: active (running) since Tue 2025-05-27 21:55:51 CST; 12h ago
       Docs: https://httpd.apache.org/docs/2.4/
    Process: 6291 ExecStart=/usr/sbin/apachectl start (code=exited, status=0/SUCCESS)
   Main PID: 6294 (apache2)
      Tasks: 55 (limit: 4549)
     Memory: 5.6M (peak: 6.3M)
        CPU: 2.565s
     CGroup: /system.slice/apache2.service
             ├─6294 /usr/sbin/apache2 -k start
             ├─6296 /usr/sbin/apache2 -k start
             └─6297 /usr/sbin/apache2 -k start
 
May 27 21:55:51 ubuntu systemd[1]: Starting apache2.service - The Apache HTTP Server...
May 27 21:55:51 ubuntu apachectl[6293]: AH00558: apache2: Could not reliably determine the server's fully qualified domain name, >

Enabling and Disabling Services

  • To enable a service to start automatically on boot, use the enable command:
sudo systemctl enable apache2
  • To disable a service for automatic start, you can use:
sudo systemctl disable apache2
  • To see a list of all of the active units that systemd knows about, we can use the list-units command:
systemctl list-units

Service configuration Files

  • As must of the services in Linux Apache2 configurations files are located in the usual place: /etc/.

  • Specifically, the main configuration file for Apache is here:

    /etc/apache2/apache2.conf

  • You can open it using less, but you could also use vim to explore or edit it.

    less /etc/apache2/apache2.conf

  • Inside that file, you will found this line:

    IncludeOptional conf-enabled/*.conf

  • That tells Apache to also load all .conf files inside the conf-enabled/ folder when it starts.

  • So it’s like modular config — instead of editing a single big file, you can drop in specific .conf files.

  • This pattern (many small config files + include directives) seems pretty common in Linux tools.

  • Another important configuration files in Apache2 includes

/etc/apache2/sites-available/
/etc/apache2/sites-enabled/
  • /etc/apache2/sites-available/: this directory contains configuration files for all available virtual hosts, that are available but not enables
  • /etc/apache2/sites-enabled/: - This contains symbolic links (symlinks) to the .conf files in sites-available/. If a site is linked here, it is active.

This lesson isn’t about setting up Apache2 web server or deploying a web page, so I didn’t dive into the details, only a very basic guide here

Service logs

Services saves their logs in var/logs, as expected, Apache keeps its logs under /var/log/apache2/

cd /var/log/apache2
ls -l
  • I checked the access.log file — I could actually see the request I made when I visited the test page in the browser.
less access.log
192.168.64.1 - - [27/May/2025:21:55:55 -0600] "GET / HTTP/1.1" 200 3459 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36"
192.168.64.1 - - [27/May/2025:22:17:44 -0600] "GET /sample HTTP/1.1" 404 491 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36"
192.168.64.1 - - [28/May/2025:08:27:29 -0600] "GET / HTTP/1.1" 200 689 "-" "Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/136.0.0.0 Safari/537.36"
  • I also looked at error.log — luckily, it was empty (which is a good sign 😅).

Appendix: Apache2 very basic setup

  • In Apache2, the default webpage files are located in /var/www/html
  • This is defined in the configuration file /etc/apache2/sites-enabled/000-default.conf, specifically in the parameter _DocumentRoot_.
  • To modify the default web page to custom simple page, you need to edit the var/www/html/index.html, using vimof course!
  • Delete everything on the current file and subtitute with this:
<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Welcome to My Apache Server</title>
  <style>
    body {
      font-family: Arial, sans-serif;
      background-color: #f3f3f3;
      text-align: center;
      padding-top: 100px;
    }
    h1 {
      color: #2c3e50;
    }
    p {
      color: #555;
    }
  </style>
</head>
<body>
  <h1>🚀 It works!</h1>
  <p>This is the new default page served by Apache.</p>
  <p>You can edit this page at <code>/var/www/html/index.html</code>.</p>
</body>
</html>
  • You will see this simple web page on your browser in http://<server-ip>

Key takeaways

  • systemd is the program responsible for managing and coordinating other programs that run in the background on a Linux system — known as services or daemons. But it goes beyond just services: it also manages devices, mount points, sockets, timers, and other essential components.
  • Think of it as the modern orchestrator of the system’s startup process and overall state.
  • Every sevice has their own configuration files and service logs that are stores in /etc and /var/logs respectively

Additional Resources

Nota diaria: 2025-05-27

Recent Notes

Graph View

Backlinks