systemd
also: init, PID 1
systemd is a modern system and service manager for Linux that replaces traditional init systems, handling startup processes, service management, and system initialization.
systemd is the default init system on most modern Linux distributions. It manages the entire lifecycle of the system from boot to shutdown, starting services, managing processes, and handling dependencies between them.
Unlike older init systems (like SysVinit), systemd uses unit files to define services, sockets, timers, and other system resources. Unit files are typically stored in /etc/systemd/system/ or /usr/lib/systemd/system/ and use a simple key-value format. For example, a web server service unit might specify how to start the service, what it depends on, and how to restart it if it fails.
You interact with systemd using the systemctl command. Common operations include systemctl start nginx, systemctl enable apache2 (to auto-start at boot), and systemctl status mysql to check service status. systemd also manages system logging through journald and provides tools like journalctl for viewing logs.