systemd System and Service Manager

systemd is a system and service manager for Linux, functioning as PID 1 (init system) and providing a comprehensive framework for managing system services, devices, mounts, sockets, timers, and other system resources. Originally developed by Lennart Poettering and Kay Sievers at Red Hat in 2010, systemd has become the default init system for virtually all major Linux distributions including Red Hat Enterprise Linux (RHEL 7+), CentOS, Fedora, Ubuntu (15.04+), Debian (8+), Arch Linux, and openSUSE. Key features: service management via unit files (.service) defining how daemons are started, stopped, and monitored, with dependency ordering (After=, Before=, Requires=, Wants=), socket activation (services start on demand when a connection arrives), and D-Bus activation (services start when a D-Bus client requests them). Unit types: service (.service), socket (.socket), device (.device), mount (.mount), automount (.automount), swap (.swap), timer (.timer), path (.path), slice (.slice), scope (.scope), target (.target). Parallel startup: systemd starts services in parallel by analyzing dependency trees, dramatically reducing boot time compared to SysV init's serial startup. Socket activation: services are started on demand when their first request arrives. Timer units: replacement for cron with calendar-based (OnCalendar=) and monotonic scheduling. Journal (systemd-journald): structured, indexed logging system replacing traditional syslog, with binary format, rich metadata, and journalctl query interface. Network configuration (systemd-networkd): native network interface, DHCP, and routing configuration. Name resolution (systemd-resolved): DNS and mDNS resolver with caching and DNS-over-TLS. Logind (systemd-logind): session and seat management. Cgroups v2 hierarchy management for resource control (CPU, memory, IO). nspawn: lightweight container runtime. Bootloader (systemd-boot): UEFI boot manager. Homed: portable home directory management. GPL-2.0.