How to Create and Run New Service Units in Systemd

Several days ago, I encountered a 32-bit CentOS 8 distribution and decided to test it on an older 32-bit system. Post-boot, I discovered a network connectivity issue; the connection would drop, requiring manual restoration after each reboot. This prompted me to explore automated solutions. This article details how to achieve this using systemd service units.

Before diving into the specifics, let's briefly examine systemd service units and their functionality. We'll cover the fundamentals of systemd service units, their interaction with "targets," and the process of configuring a service unit to execute a script on boot. The focus will be on practical steps.

What are Systemd Service Units?

A systemd service unit is a configuration file defining a service's system behavior. This could encompass network services, applications, or scripts designed to run during boot or at specific boot stages.

These units are organized into targets, representing milestones in the boot process. For instance, upon reaching the multi-user target (runlevel 3), specific services initiate. Consider targets as service "groups" collaborating at various boot phases.

To view services active within a target (e.g., graphical.target), utilize the systemctl command:

systemctl --type=service

This displays all active services in the current target. Some services run continuously; others execute once and terminate.

Monitoring Service Status

To check a service's status (active or inactive), use systemctl status:

systemctl status firewalld.service

This command checks firewalld's status. Observe its active (running) and enabled (auto-starts on reboot) states.

Temporarily stop a service (until the next boot) with:

systemctl stop firewalld.service
systemctl status firewalld.service

This halts firewalld for the current session but doesn't prevent future restarts.

Enabling and Disabling Services

To ensure a service auto-starts on boot, enable it (creating a symbolic link in the target's "wants" folder):

systemctl enable firewalld.service

Disabling is achieved via:

systemctl disable firewalld.service

Creating a Custom Service Unit

To create a boot-time script service, create a new service unit in /etc/systemd/system. This directory houses existing service unit files and target folders.

cd /etc/systemd/system
ls -l

Create connection.service using Vim or a similar editor:

vim connection.service

Add the following:

[Unit]
Description=Bring up network connection
After=network.target

[Service]
ExecStart=/root/scripts/conup.sh

[Install]
WantedBy=multi-user.target

Explanation:

• [Unit]: Metadata; describes the unit and specifies execution after network.target (ensuring network initialization).
• [Service]: Defines the command (conup.sh script execution).
• [Install]: Specifies loading at the multi-user.target.

Enable the service for auto-start on reboot:

systemctl enable connection.service

Verify enabling by checking multi-user.target.wants:

ls -l multi-user.target.wants/

The connection.service symbolic link should be present. Now, create the script.

Creating the Script

Create the conup.sh script to restore the network connection:

cd /root
mkdir scripts
cd scripts
vi conup.sh

Add this line (assuming the interface is enp0s3):

#!/bin/bash
nmcli connection up enp0s3

Make the script executable:

chmod  x conup.sh

The service is now ready.

SELinux Considerations (RHEL/CentOS)

On RHEL-based systems (CentOS, Rocky Linux), SELinux might block script execution unless the correct security context is applied.

Temporarily set the context:

chcon -t bin_t /root/scripts/conup.sh

For a permanent solution:

semanage fcontext -a -t bin_t "/root/scripts/conup.sh"
restorecon -v /root/scripts/conup.sh

This ensures continued script execution after reboots or SELinux policy updates.

Testing the Service

Test without rebooting by manually starting the service:

systemctl start connection.service

If successful, the network connection should restore. For simpler scripts (e.g., touch /tmp/testbootfile), check for /tmp/testbootfile creation to confirm service execution.

Conclusion

This guide provides a comprehensive understanding of systemd service units, their creation, management, and application in automating tasks such as network connection restoration on boot. This enhances system automation and efficiency.

The above is the detailed content of How to Create and Run New Service Units in Systemd. For more information, please follow other related articles on the PHP Chinese website!