Systemd


systemd is a software suite for system and service management on Linux built to unify service configuration and behavior across Linux distributions. Its main component is an init system used to bootstrap user space and manage user processes. It also provides replacements for various daemons and utilities, including device management, login management, network connection management, and event logging. The name systemd adheres to the Unix convention of naming daemons by appending the letter d, and also plays on the French phrase Système D.
Since 2015, nearly all Linux distributions have adopted systemd. It has been praised by developers and users of distributions that adopted it for providing a stable, fast out-of-the-box solution for issues that had existed in the Linux space for years. At the time of its adoption, it was the only parallel boot and init system offering centralized management of processes, daemons, services, and mount points.
Critics of systemd contend it suffers from feature creep and has damaged interoperability across Unix-like operating systems. In addition, they contend systemd's large feature set creates a larger attack surface. This has led to the development of several minor Linux distributions replacing systemd with other init systems like SysVinit or OpenRC.

History

and Kay Sievers, the software engineers then working for Red Hat who initially developed systemd, started a project to replace Linux's conventional System V init in 2010. An April 2010 blog post from Poettering, titled "Rethinking PID 1", introduced an experimental version of what would later become systemd. They sought to surpass the efficiency of the init daemon in several ways. They wanted to improve the software framework for expressing dependencies, to allow more processes to run concurrently or in parallel during system booting, and to reduce the computational overhead of the shell.
In May 2011, Fedora Linux became the first major Linux distribution to enable systemd by default, replacing Upstart. The reasoning at the time was that systemd provided extensive parallelization during startup, better management of processes and overall a saner, dependency-based approach to control of the system.
In October 2012, Arch Linux made systemd the default, switching from SysVinit. Developers had debated since August 2012 and concluded it was faster and had more features than SysVinit and that maintaining SysVinit was not worth the effort. Some thought the criticism of systemd was not based on actual shortcomings of the software but rather personal dislike of Poettering and a general opposition to change. Several complaints about systemd—including its use of D-bus, C instead of bash, and an optional on-disk journal format—were instead described as advantages by the Arch maintainers.
Between 2013 and 2014, the Debian Technical Committee engaged in a widely publicized debate on the mailing list about which init system to use as the default in Debian 8 before settling on systemd. Soon after, Debian developer Joey Hess, Technical Committee members Russ Allbery and Ian Jackson, and systemd package maintainer Tollef Fog Heen resigned from their positions, citing the extraordinary levels of stress caused by disputes on systemd integration within the Debian and FOSS community that rendered regular maintenance virtually impossible. Mark Shuttleworth announced soon afterwards that the Debian-based Ubuntu would use systemd to replace its old Upstart init system.
In August 2015, systemd started providing a login shell, callable via .
In September 2016, a security bug was discovered that allowed any unprivileged user to perform a denial-of-service attack against systemd. Rich Felker, developer of musl, stated that this bug reveals a major "system development design flaw". In 2017 another security bug was discovered in systemd-resolved,, which "allows disruption of service" by a "malicious DNS server". Later in 2017, the Pwnie Awards gave author Lennart Poettering a "lamest vendor response" award due to his handling of the vulnerabilities.

Design

Poettering describes systemd development as "never finished, never complete, but tracking progress of technology". In May 2014, Poettering further described systemd as unifying "pointless differences between distributions", by providing the following three general functions:
includes features like on-demand starting of daemons, snapshot support, process tracking and Inhibitor Locks. It is not just the name of the init daemon but also refers to the entire software bundle around it, which, in addition to the init daemon, includes the daemons, and, and many other low-level components. In January 2013, Poettering described systemd not as one program, but rather a large software suite that includes 69 individual binaries. As an integrated software suite, systemd replaces the startup sequences and runlevels controlled by the traditional init daemon, along with the shell scripts executed under its control. systemd also integrates many other services that are common on Linux systems by handling user logins, the system console, device hotplugging, scheduled execution, logging, hostnames and locales.
Like the init daemon, is a daemon that manages other daemons, which, including itself, are background processes. is the first daemon to start during booting and the last daemon to terminate during shutdown. The daemon serves as the root of the user space's process tree; the first process has a special role on Unix systems, as it replaces the parent of a process when the original parent terminates. Therefore, the first process is particularly well suited for the purpose of monitoring daemons.
executes elements of its startup sequence in parallel, which is theoretically faster than the traditional startup sequence approach. For inter-process communication, makes Unix domain sockets and D-Bus available to the running daemons. The state of itself can also be preserved in a snapshot for future recall.

Core components and libraries

Following its integrated approach, systemd also provides replacements for various daemons and utilities, including the startup shell scripts, pm-utils, inetd,, syslog, watchdog, cron and. systemd's core components include:
  • is a system and service manager for Linux operating systems.
  • is a command to introspect and control the state of the systemd system and service manager. Not to be confused with sysctl.
  • may be used to determine system boot-up performance statistics and retrieve other state and tracing information from the system and service manager.
tracks processes using the Linux kernel's cgroups subsystem instead of using process identifiers ; thus, daemons cannot "escape", not even by double-forking. not only uses cgroups, but also augments them with and, two utility programs that facilitate the creation and management of Linux containers. Since version 205, systemd also offers ControlGroupInterface, which is an API to the Linux kernel cgroups. The Linux kernel cgroups are adapted to support kernfs, and are being modified to support a unified hierarchy.

Ancillary components

Beside its primary purpose of providing a Linux init system, the systemd suite can provide additional functionality, including the following components:
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; : provides network name resolution to local applications
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; : is a client NTP daemon for synchronizing the system clock across the network.
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Configuration of systemd

is configured exclusively via plain-text files although GUI tools such as systemd-manager are also available.
records initialization instructions for each daemon in a configuration file that uses a declarative language, replacing the traditionally used per-daemon startup shell scripts. The syntax of the language is inspired by files.
Unit-file types include:

  • Adoption

Linux distributionDate added to software repositoryEnabled by default?Date released as defaultRuns without?
Alpine Linux
Android
Arch Linux
antiX Linux
Artix Linux
CentOS
CoreOS
Debian . In , a number of alternative init systems are supported
Devuan
Fedora Linux
Gentoo Linux
GNU Guix System
Knoppix
Linux Mint
Mageia
Manjaro Linux
openSUSE
Parabola GNU/Linux-libre
Red Hat Enterprise Linux
Slackware
Solus
Source Mage
SUSE Linux Enterprise Server
Ubuntu in Yaketty
Void Linux, removed

While many distributions boot systemd by default, some allow other init systems to be used; in this case switching the init system is possible by installing the appropriate packages. A fork of Debian called Devuan was developed to avoid systemd and has reached version 5.0 for stable usage. In December 2019, the Debian project voted in favour of retaining systemd as the default init system for the distribution, but with support for "exploring alternatives".