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Introduction

systemd is a system and service manager for Linux operating systems. It is used to manage services, processes, and system states. This guide covers creating, managing, and troubleshooting custom systemd services.

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Updating Systemd After Service Creation

After creating or modifying a systemd service unit file, always reload the systemd daemon to apply changes: 
sudo systemctl daemon-reload

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Creating a Systemd Service

  1. Service Unit File Path Service unit files are typically located in /etc/systemd/system/ for custom services and /lib/systemd/system/ for official services. To create a new service, use the following path: 
    vim /etc/systemd/system/"cmd-name".service
  2. Script Path Place your service script in a standard executable location: 
    vim /usr/sbin/script-name.sh

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Example Service Unit Files

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Custom Apache2 Service

To create a custom systemd service for Apache2: 
sudo nano /etc/systemd/system/customname.service
Service unit file content: 
[Unit]
Description=Apache2 Web Server
After=network.target

[Service]
Type=simple
ExecStart=/usr/sbin/apachectl start
ExecStop=/usr/sbin/apachectl stop
ExecReload=/usr/sbin/apachectl graceful
KillMode=process
Restart=always

[Install]
WantedBy=multi-user.target

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Official Apache2 Service

To view the official Apache2 service unit file: 
sudo vim /lib/systemd/system/apache2.service
Official service unit file content: 
[Unit]
Description=The Apache HTTP Server
After=network.target remote-fs.target nss-lookup.target
Documentation=https://httpd.apache.org/docs/2.4/

[Service]
Type=forking
Environment=APACHE_STARTED_BY_SYSTEMD=true
ExecStart=/usr/sbin/apachectl start
ExecStop=/usr/sbin/apachectl graceful-stop
ExecReload=/usr/sbin/apachectl graceful
KillMode=mixed
PrivateTmp=true
Restart=on-abort

[Install]
WantedBy=multi-user.target

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Running a Script

To create a service that executes a script: 
sudo vim /etc/systemd/system/example-service.service
Service unit file content: 
[Unit]
Description=Example systemd service.

[Service]
Type=simple
ExecStart=/bin/bash /usr/bin/test_service.sh

[Install]
WantedBy=multi-user.target

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Running a Script Every 30 Seconds

To create a service that runs a script every 30 seconds: 
sudo vim /etc/systemd/system/nmap-service.service
Service unit file content: 
[Unit]
Description=Demonstration of custom nmap service.
After=network.target

[Service]
Type=simple
User=root
ExecStart=/usr/bin/nmap -sS -O -oN /home/<user>/results.txt localhost
Restart=always
RestartSec=30

[Install]
WantedBy=multi-user.target

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Gunicorn Service for a Test Application

To create a service for running Gunicorn: 
sudo vim /etc/systemd/system/gunicorn-test-app.service
Service unit file content: 
[Unit]
Description=Gunicorn daemon for serving test-app
After=network.target

[Service]
User=root
Group=root
WorkingDirectory=/apps/test-app/
Environment="PATH=/apps/test-app/bin"
ExecStart=/apps/test-app/bin/gunicorn --workers 9 -t 0 --bind 127.0.0.1:5001 -m 007 wsgi:app --log-level debug --access-logfile /var/log/gunicorn/test_app_access.log --error-logfile /var/log/gunicorn/test_app_error.log
ExecReload=/bin/kill -s HUP $MAINPID
RestartSec=5

[Install]
WantedBy=multi-user.target

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Conclusion

Systemd is a powerful tool for managing system services and processes. By creating and configuring service unit files, you can control how services start, stop, and behave. Always remember to reload the systemd daemon after making changes to service files to ensure your configurations take effect.
This guide covers essential aspects of creating and managing systemd services, including paths for service unit files, sample configurations, and instructions for updating systemd after changes.