systemd

systemd
systemd startup on Fedora 17
Original author	Lennart Poettering[1]
Developers	systemd team;[2] 345 different authors in 2018[3] and 2,032 different authors in total [4]
Release	30 March 2010; 16 years ago (2010-03-30)
Stable release	261.2[5]  / 23 July 2026
Written in	C
Operating system	Linux
Type	System software Process supervisor Service manager
License	LGPLv2.1+[6]
Website	systemd.io
Repository	github.com/systemd/systemd

systemd is a software suite for system and service management on Linux^[7] built to unify service configuration and behavior across Linux distributions.^[8] Its main component is an init system used to bootstrap user space and manage user processes. It also provides 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,^[9] and also plays on the French phrase Système D (a person's ability to quickly adapt and improvise in the face of problems).^[10]

In Linux distributions, systemd is used as a replacement for Sysvinit and other init systems. 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.^[11][12][13] 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.^{[citation needed]}

Critics of systemd contend it suffers from feature creep and has damaged interoperability across Unix-like operating systems (as it does not run on non-Linux Unix derivatives like BSD or Solaris). In addition, they contend systemd's large feature set creates a larger attack surface.^[14] This has led to the development of several minor Linux distributions replacing systemd with other init systems like SysVinit or OpenRC.^[15]

History

Lennart Poettering and Kay Sievers, the software engineers then working for Red Hat who initially developed systemd,^[2] started a project to replace Linux's conventional System V init in 2010.^[16] An April 2010 blog post from Poettering, titled "Rethinking PID 1", introduced an experimental version of what would later become systemd.^[17] 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.^[18]

In October 2012, Arch Linux made systemd the default, switching from SysVinit.^[19] Developers had debated since August 2012^[13] and concluded it was faster and had more features than SysVinit and that maintaining SysVinit was not worth the effort.^[20] 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.^[21]

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.^[22][23][24] Soon after, Debian developer Joey Hess,^[25] Technical Committee members Russ Allbery^[26] and Ian Jackson,^[27] and systemd package maintainer Tollef Fog Heen^[28] 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.^[29][30]

In August 2015, systemd started providing a login shell, callable via machinectl shell.^[31]

In September 2016, a security bug was discovered that allowed any unprivileged user to perform a denial-of-service attack against systemd.^[32] Rich Felker, developer of musl, stated that this bug reveals a major "system development design flaw".^[33] In 2017 another security bug was discovered in systemd-resolved, CVE-2017-9445, which "allows disruption of service" by a "malicious DNS server".^[34][35] Later in 2017, the Pwnie Awards gave author Lennart Poettering a "lamest vendor response" award due to his handling of the vulnerabilities.^[36]

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:^[39]

• A system and service manager (manages both the system, by applying various configurations, and its services)
• A software platform (serves as a basis for developing other software)
• The glue between applications and the kernel (provides various interfaces that expose functionalities provided by the kernel)

systemd includes features like on-demand starting of daemons, snapshot support, process tracking^[40] and Inhibitor Locks.^[41] 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 systemd init daemon, includes the daemons journald, logind and networkd, 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.^[42] 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 (see udev), scheduled execution (replacing cron), logging, hostnames and locales.

Like the init daemon, systemd is a daemon that manages other daemons, which, including systemd itself, are background processes. systemd is the first daemon to start during booting and the last daemon to terminate during shutdown. The systemd daemon serves as the root of the user space's process tree; the first process (PID 1) 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.

systemd executes elements of its startup sequence in parallel, which is theoretically faster than the traditional startup sequence approach.^[43] For inter-process communication (IPC), systemd makes Unix domain sockets and D-Bus available to the running daemons. The state of systemd itself can also be preserved in a snapshot for future recall.

Ancillary components

Beside its primary purpose of providing a Linux init system, the systemd suite can provide additional functionality, including the following components:

journald

systemd-journald is a daemon responsible for event logging, with append-only binary files serving as its logfiles. The system administrator may choose whether to log system events with systemd-journald, syslog-ng or rsyslog.

libudev

libudev is the standard library for utilizing udev, which allows third-party applications to query udev resources.

localed

localed manages the system locale and keyboard layout.

logind

systemd-logind is a daemon that manages user logins and seats in various ways. It is an integrated login manager that offers multiseat improvements^[48] and replaces ConsoleKit, which is no longer maintained.^[49] For X11 display managers the switch to logind requires a minimal amount of porting.^[50] It was integrated in systemd version 30.

hostnamed

hostnamed manages the system hostname.

homed

homed is a daemon that provides portable human-user accounts that are independent of current system configuration. homed moves various pieces of data such as UID/GID from various places across the filesystem into one file, ~/.identity. homed manages the user's home directory in various ways such as a plain directory, a btrfs subvolume, a Linux Unified Key Setup volume, an fscrypt directory, or mounted from an SMB server.

networkd

networkd is a daemon to handle the configuration of the network interfaces; in version 209, when it was first integrated, support was limited to statically assigned addresses and basic support for bridging configuration.^{[51][52][53][54][55]} In July 2014, systemd version 215 was released, adding new features such as a DHCP server for IPv4 hosts, and VXLAN support.^[56][57] networkctl may be used to review the state of the network links as seen by systemd-networkd.^[58] Configuration of new interfaces has to be added under the /lib/systemd/network/ as a new file ending with .network extension.

resolved

provides network name resolution to local applications

systemd-boot

systemd-boot is a boot manager, formerly known as gummiboot. Kay Sievers merged it into systemd with rev 220.

systemd-bsod

systemd-bsod is an error reporter used to generate Blue Screen of Death.

systemd-nspawn

systemd-nspawn may be used to run a command or OS in a namespace container.

timedated

systemd-timedated is a daemon that can be used to control time-related settings, such as the system time, system time zone, or selection between UTC and local time-zone system clock. It is accessible through D-Bus.^[59] It was integrated in systemd version 30.

timesyncd

timesyncd is a client NTP daemon for synchronizing the system clock across the network.

tmpfiles

systemd-tmpfiles is a utility that takes care of creation and clean-up of temporary files and directories. It is normally run once at startup and then in specified intervals.

udevd

udev is a device manager for the Linux kernel, which handles the /dev directory and all user space actions when adding/removing devices, including firmware loading. In April 2012, the source tree for udev was merged into the systemd source tree.^[60][61] In order to match the version number of udev, systemd maintainers bumped the version number directly from 44 to 183.^[62]

On 29 May 2014, support for firmware loading through udev was dropped from systemd, as it was decided that the kernel should be responsible for loading firmware.^[63]

Configuration of systemd

systemd is configured exclusively via plain-text files although GUI tools such as systemd-manager are also available.

systemd records initialization instructions for each daemon in a configuration file (referred to as a "unit file") that uses a declarative language, replacing the traditionally used per-daemon startup shell scripts. The syntax of the language is inspired by .ini files.^{[64][example needed]}

Unit-file types^[65] include:

• .service
• .socket
• .device (automatically initiated by systemd^[66])
• .mount
• .automount
• .swap
• .target
• .path
• .timer (which can be used as a cron-like job scheduler^[67])
• .snapshot
• .slice (used to group and manage processes and resources^[68])
• .scope (used to group worker processes, not intended to be configured via unit files^[69])

Adoption

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^[99][100] 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".^[101]

Reception

The design of systemd has ignited controversy within the free-software community. Critics regard systemd as overly complex and suffering from continued feature creep, arguing that its architecture violates the Unix philosophy. There is also concern that it forms a system of interlocked dependencies, thereby giving distribution maintainers little choice but to adopt systemd as more user-space software comes to depend on its components.^[109]

In a 2012 interview, Slackware's lead developer, Patrick Volkerding, expressed reservations about the systemd architecture, stating his belief that its design was contrary to the Unix philosophy of interconnected utilities with narrowly defined functionalities.^[110] As of August 2018^[update], Slackware does not support or use systemd, but Volkerding has not ruled out the possibility of switching to it.^[111]  Slackware 15, released in February 2022, continues to use SysV init with a BSD-style /etc/rc.d boot scripts tree.

In January 2013, Lennart Poettering attempted to address concerns about systemd in a blog post called The Biggest Myths.^[42]

In February 2014, musl's Rich Felker opined that PID 1 is too special to be saddled with additional responsibilities, believing that PID 1 should only be responsible for starting the rest of the init system and reaping zombie processes, and that the additional functionality added by systemd can be provided elsewhere and unnecessarily increases the complexity and attack surface of PID 1.^[112]

In March 2014, Eric S. Raymond commented that systemd's design goals were prone to mission creep and software bloat.^[113] In April 2014, Linus Torvalds expressed reservations about the attitude of Kay Sievers, a key systemd developer, toward users and bug reports in regard to modifications to the Linux kernel submitted by Sievers.^[114] In late April 2014, a campaign to boycott systemd was launched, with a website listing various reasons against its adoption.^[115][116]

In an August 2014 article published in InfoWorld, Paul Venezia wrote about the systemd controversy and attributed the controversy to violation of the Unix philosophy, and to "enormous egos who firmly believe they can do no wrong".^[117] The article also characterizes the architecture of systemd as similar to that of svchost.exe, a critical system component in Microsoft Windows with a broad functional scope.^[117]

In a September 2014 ZDNet interview, prominent Linux kernel developer Theodore Ts'o expressed his opinion that the dispute over systemd's centralized design philosophy, more than technical concerns, indicates a dangerous general trend toward uniformizing the Linux ecosystem, alienating and marginalizing parts of the open-source community, and leaving little room for alternative projects. He cited similarities with the attitude he found in the GNOME project toward non-standard configurations.^[118] On social media, Ts'o also later compared the attitudes of Sievers and his co-developer, Lennart Poettering, to that of GNOME's developers.^[119]

Forks and alternative implementations

Forks of systemd are closely tied to critiques of it outlined in the above section. Forks generally try to improve on at least one of portability (to other libcs and Unix-like systems), modularity, or size. A few forks have collaborated under the FreeInit banner.^[120]

All Systems Go!

All Systems Go! is the annual systemd conference, which is held in Berlin.^[136] The main topic is systemd but other Linux areas are covered as well, like TPM, DBus, desktop environment, containers, eBPF etc.

See also

• BusyBox
• launchd
• Linux distributions without systemd
• Operating system service management
• readahead
• runit
• Service Management Facility
• GNU Daemon Shepherd
• Upstart
• svchost.exe

Notes

References

External links

• Official website
• Systemd on GitHub
• Rethinking PID 1