Systemd

A daemon is a background process that runs continuously on a computer system, often without direct interaction with users. Daemons are typically started during the system boot process and perform various tasks, such as handling system events, providing services, or performing periodic maintenance

Init System:

systemd replaces the traditional init system as the first process (PID 1) that gets executed on Linux systems during the boot process. It initializes the system and manages the execution of user and system processes.

Service Management:

systemd provides a sophisticated service management framework. It can start, stop, restart, enable, and disable services. Services are defined using unit files with a .service extension.

Dependency Management:

systemd handles dependencies between services, ensuring that services start in the correct order based on their dependencies.

Logging:

systemd includes its own logging system called journalctl, which collects and manages system logs. It allows for centralized logging and querying of log data.

Device Management:

systemd manages device events, automounting and hot-plugging devices as needed. This includes udev integration for dynamic device management.

Timers:

systemd includes a timer unit for scheduling tasks. Timer units can trigger services or other actions at specified intervals.

Security Features:

systemd includes various security features, such as cgroups (control groups) for process isolation, and the ability to run services in their own user and group namespaces.

User Sessions:

systemd supports user sessions, allowing users to start and manage their own services within their login sessions.

Socket Activation:

systemd supports socket activation, where services are only started when a connection is received on a specified socket. This can improve resource utilization and reduce startup times.

Few important commands

To check the status of a running service
sudo systemctl status

sudo systemctl status nginx

To start a service
sudo systemctl start

sudo systemctl start nginx

To restart a running service
sudo systemctl restart

sudo systemctl restart nginx

To reload a service running:
sudo systemctl reload

sudo systemctl reload nginx

To stop a service:
sudo systemctl stop

sudo systemctl stop nginx

To auto-start a service on boot:
sudo systemctl enable

sudo systemctl enable nginx

To not auto-start a service on boot:
sudo systemctl disable

sudo systemctl disable nginx

Where is systemd's unit files?
1- /etc/systemd/system
2- /run/systemd/system
3- /lib/systemd/system

To edit a systemd configuration file
sudo systemctl edit

sudo systemctl edit nginx

This creates a override.conf file, which takes priority over your root service file.

To edit a systemd configuration file fully:
sudo systemctl edit --full

sudo systemd edit --full nginx

This opens the actual service file and you can edit it, and it creates a file in /etc/systemd/system, which is the highest priority directory. And you can override the current configurations.