systemd is a set of building blocks for Linux systems comprising an init system designed for aggressive parallelisation and capable of bus- and socket-based activation, per-unit control groups, journalling, network device management, volume mounts, time synchronisation and name resolution. It aims to replace SysV-style init.


  • Units describe a resource under systemd's management. They're usually defined in INI files with one of the below extensions, though some are managed at runtime:
    • service for one-shot or long-running processes.
    • socket for socket-based activation of a service.
    • device for a udev or sysfs device, allowing declaration of dependencies.
    • mount for a mountpoint.
    • automount for a mountpoint managed with automount.
    • swap for swap devices or files.
    • target for synchronisation points during boot (e.g. network available) or the equivalent of runlevels (e.g. multi-user).
    • path for path-based activation.
    • timer for timer-based activation.
    • slice for hierarchical management of process groups, though the root slice (-.slice) is not backed by a file.
    • scope units allow grouping processes at runtime, e.g. to set per-group resource limits.
  • Drop-ins (short for "drop-in replacements") are additional configuration files used to augment an existing configuration with either an override or some additional values. This is commonly represented as a ${unit}.d directory containing an arbitrary number of configuration files.
  • The journal serves as a replacement for syslog; handling early boot, kernel and unit log messages. Logs are recorded in a structured and indexed binary file, and retained up to the limit set by SystemMaxUse via a ring buffer.

SysV init compatibility

systemd-initctl (managed by the systemd-initctl.service unit) provides a /dev/initctl FIFO compatible with SysV init.


  • /usr/lib/systemd contains system-installed units, usually installed by the system package manager.
  • /etc/systemd is for site units.
  • ~/.config/systemd contains user units.