Linux Lab 16: Systemd Services

This lab is a deep dive into managing services with systemd, the init system used by most modern Linux distributions. You will learn how to troubleshoot failed services, edit unit files, reload the systemd daemon, and configure automatic restarts.

[Image of systemd service lifecycle diagram]

Table of Contents

• Linux Lab 16: Systemd Services
• Table of Contents
  • Key Concepts
  • Step-by-Step Walkthrough
  • 1. Checking Service Status
  • 2. Attempting to Start a Broken Service
  • 3. Diagnosing the Failure (journalctl)
  • 4. Fixing the Service Unit File
  • 5. Verifying the Fix
  • 6. Enabling the Service
  • 7. Configuring Auto-Restart
  • 8. Reloading the Daemon
  • Command Reference

---

Key Concepts

• Systemd: The system and service manager for Linux. It is the first process started by the kernel (PID 1).
• Unit File: A configuration file (usually ending in .service) that tells systemd how to manage a specific service. It defines what command to run, when to restart, and dependencies.
• systemctl: The main command-line tool for controlling systemd.
• journalctl: The command for viewing logs collected by systemd.
• Daemon Reload: When you change a unit file on disk, systemd doesn't know about it automatically. You must run systemctl daemon-reload to tell it to re-read the configuration.

---

Step-by-Step Walkthrough

1. Checking Service Status

Question: What is the status of sample.service? Command: sudo systemctl status sample.service Output: Loaded: error (Reason: Invalid argument), Active: inactive (dead) Answer: inactive

Explanation: The service is not running. The "Loaded: error" indicates a problem with the configuration file itself.

2. Attempting to Start a Broken Service

Task: Try starting the service. Command: sudo systemctl start sample.service Result: Failed.

Explanation: Systemd cannot start a service with a broken or incomplete configuration file.

3. Diagnosing the Failure (journalctl)

Question: Why did the service start fail? Command: sudo journalctl -u sample.service Output: Service lacks both ExecStart= and ExecStop= Answer: Service Section Not Defined (specifically, the ExecStart directive is missing).

Explanation: * journalctl -u [unit]: Shows logs specifically for that unit. * The error message is explicit: a service must know what command to run to start (ExecStart).

4. Fixing the Service Unit File

Task: Update the [Service] section to run /bin/bash /root/sample_script.sh. Command: sudo vi /etc/systemd/system/sample.service

Edit: Add the ExecStart line under [Service].

[Service]
ExecStart=/bin/bash /root/sample_script.sh

Save and exit (:wq).

5. Verifying the Fix

Task: Start the service and check the status. Commands: 1. sudo systemctl start sample.service 2. sudo systemctl status sample.service

Output: Active: active (running) Answer: running

6. Enabling the Service

Task: Configure the service to start automatically on boot. Command: sudo systemctl enable sample.service

Explanation: * Start: Runs the service now. * Enable: Configures the service to start at boot time. It usually creates a symbolic link in /etc/systemd/system/multi-user.target.wants/.

7. Configuring Auto-Restart

Task: Ensure the service restarts automatically if it stops. Command: sudo vi /etc/systemd/system/sample.service

Edit: Add Restart=always to the [Service] section.

[Service]
ExecStart=/bin/bash /root/sample_script.sh
Restart=always

Save and exit.

Why? This makes the service robust. If the script crashes or is killed, systemd will automatically try to start it again.

8. Reloading the Daemon

Problem: After editing the file, systemctl restart gives a warning: Warning: The unit file... changed on disk. Run 'systemctl daemon-reload'. Task: Fix the warning. Command: sudo systemctl daemon-reload

Explanation: Whenever you modify a unit file in /etc/systemd/system/, systemd needs to reload its internal cache to see the changes. Until you run daemon-reload, it is still using the old version of the file.

---

Command Reference

Command	Purpose	Example
systemctl status [unit]	Check status of a service	systemctl status sshd
systemctl start [unit]	Start a service immediately	sudo systemctl start nginx
systemctl stop [unit]	Stop a service immediately	sudo systemctl stop nginx
systemctl restart [unit]	Restart a service	sudo systemctl restart nginx
systemctl enable [unit]	Enable start on boot	sudo systemctl enable nginx
systemctl daemon-reload	Reload unit file changes	sudo systemctl daemon-reload
journalctl -u [unit]	View logs for a specific service	journalctl -u nginx