Systemd

Systemd is the init system used by most mainstream Linux distributions — Debian, Fedora, Arch, Ubuntu — since roughly the early 2010s. It replaced sysvinit's numbered shell scripts with declarative unit files that describe what a service needs rather than the exact sequence to start it, letting the system start independent services in parallel and cutting boot times substantially.

A unit is a small text file — .service for a program, .mount for a filesystem, .timer for a scheduled trigger, .socket for something that starts on first connection — with an explicit After=/Requires= graph instead of a filename number. systemctl status nginx reads that graph and its live state; journalctl -u nginx reads its structured, binary-logged output rather than a scattered pile of text files under /var/log.

Systemd's scope is the recurring objection: it also absorbed device management (udev), network configuration, DNS resolution, login sessions, and time sync, well past what "init" traditionally meant. Proponents call this consistency — one dependency graph, one logging format, one tool — critics call it a monolith that violates the Unix preference for small composable tools. Both are describing the same design honestly.

Its most consequential feature for daily debugging is the systemd unit's automatic restart and resource limits: a service can be told to restart on failure, sandboxed with a private /tmp, or capped on memory, all as unit-file settings rather than code the service itself has to implement.