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 systemd startup onFedora 17 | |
| Original author(s) | Lennart Poettering[1] |
|---|---|
| Developer(s) | Red Hat (Lennart Poettering,Kay Sievers,Harald Hoyer, Daniel Mack, Tom Gundersen, David Herrmann);[2] 345 different authors in 2018[3] and 2,032 different authors in total[4] |
| Initial release | 30 March 2010; 14 years ago (2010-03-30) |
| Stable release | 257.4[5]  / 6 March 2025 |
| Repository | |
| Written in | C |
| Operating system | Linux |
| Type | |
| License | LGPLv2.1+[6] |
| Website | systemd.io |
systemd is asoftware suite that provides an array of system components forLinux[7] operating systems. The main aim is to unify service configuration and behavior acrossLinux distributions.[8] Its primary component is a "system and service manager" — aninit system used tobootstrapuser space and manageuser processes. It also provides replacements for variousdaemons and utilities, including device management, login management, network connection management, and event logging. The namesystemd adheres to theUnix convention of naming daemons by appending the letterd.[9] It also plays on the term "System D", which refers to a person's ability to adapt quickly and improvise to solve problems.[10]
Since 2015, the majority of Linux distributions have adopted systemd, having replaced other init systems such asSysV init. 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 adoption of systemd on most Linux distributions, it was the only software suite that offered reliableparallelism during boot as well as centralized management ofprocesses, daemons, services andmount points.
Critics of systemd contend that it suffers frommission creep andbloat, the latter affecting other software (such as theGNOME desktop), adding dependencies on systemd, reducing its compatibility with otherUnix-like operating systems and making it difficult for sysadmins to integrate alternative solutions. In addition, they contend that the complexity of systemd results in a largerattack surface, reducing the overall security of the platform.[14] Concerns have also been raised aboutRed Hat and its parent companyIBM controlling the scene of init systems on Linux.[15][1] A number of new forked Linux distributions have since been created that use alternative init systems like SysV orOpenRC.[16] Systemd has not been adopted by Unix-like systems outside the Linux space, likeFreeBSD,OpenBSD andSolaris.
Lennart Poettering andKay Sievers, thesoftware engineers then working forRed Hat who initially developed systemd,[2] started a project to replace Linux's conventional System V init in 2010.[17] An April 2010 blog post from Poettering, titled "Rethinking PID 1", introduced an experimental version of what would later become systemd.[18] They sought to surpass the efficiency of theinit daemon in several ways. They wanted to improve thesoftware framework for expressing dependencies, to allow more processes to runconcurrently or inparallel during systembooting, and to reduce thecomputational overhead of theshell.
In May 2011,Fedora Linux became the first major Linux distribution to enable systemd by default, replacingUpstart. 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.[19]
In October 2012,Arch Linux made systemd the default, switching fromSysVinit.[20] Developers had debated since August 2012[13] and came to the conclusion that it was faster and had more features than SysVinit, and that maintaining the latter was not worth the effort in patches.[21] Some of them thought that the criticism towards the implementation of systemd was not based on actual shortcomings of the software, rather the disliking of Lennart from a part of the Linux community and the general hesitation for change. Specifically, some of the complaints regarding systemd not being programmed in bash, it being bigger and more extensive than SysVinit, the use ofD-bus, and the optional on-disk format of the journal were regarded as advantages by programmers.[22]
Between October 2013 and February 2014, a long debate among the Debian Technical Committee occurred on theDebian mailing list,[23] discussing which init system to use as the default in Debian 8 "jessie", and culminating in a decision in favor of systemd. The debate was widely publicized[24][25] and in the wake of the decision the debate continues on the Debian mailing list. In February 2014, after Debian's decision was made,Mark Shuttleworth announced in his blog thatUbuntu would follow in implementing systemd, discarding its ownUpstart.[26][27]
In November 2014, Debian Developer Joey Hess,[28] Debian Technical Committee members Russ Allbery[29] andIan Jackson,[30] and systemd package-maintainer Tollef Fog Heen[31] resigned from their positions. All four justified their decision on the public Debian mailing list and in personal blogs with their exposure to extraordinary stress-levels related to ongoing disputes on systemd integration within the Debian andFOSS community that rendered regular maintenance virtually impossible.
In August 2015, systemd started providing a login shell, callable viamachinectl shell.[32]
In September 2016, asecurity bug was discovered that allowed any unprivileged user to perform a denial-of-service attack against systemd.[33] Rich Felker, developer ofmusl, stated that this bug reveals a major "system development design flaw".[34] In 2017 another security bug was discovered in systemd,CVE-2017-9445, which "allows disruption of service" by a "malicious DNS server".[35][36] Later in 2017, thePwnie Awards gave author Lennart Poettering a "lamest vendor response" award due to his handling of the vulnerabilities.[37]
telephony,bootmode,dlog, andtizen service are from Tizen and are not components of systemd.[38]
systemd-nspawn[39]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:[40]
systemd includes features like on-demand starting of daemons, snapshot support, process tracking[41] and Inhibitor Locks.[42] It is not just the name of the init daemon but also refers to the entire software bundle around it, which, in addition to thesystemd init daemon, includes the daemonsjournald,logind andnetworkd, 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.[43] As an integratedsoftware suite, systemd replaces thestartup sequences andrunlevels controlled by the traditional init daemon, along with theshell scripts executed under its control. systemd also integrates many other services that are common on Linux systems by handling user logins, thesystem console, device hotplugging (seeudev), scheduled execution (replacingcron), logging, hostnames and locales.
Like the init daemon,systemd is a daemon that manages other daemons, which, includingsystemd itself, arebackground processes.systemd is the first daemon to start duringbooting and the last daemon to terminate duringshutdown. Thesystemd daemon serves as the root of the user space'sprocess 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.[44] Forinter-process communication (IPC),systemd makesUnix domain sockets andD-Bus available to the running daemons. The state ofsystemd itself can also be preserved in asnapshot for future recall.
Following its integrated approach, systemd also provides replacements for various daemons and utilities, including the startup shell scripts, pm-utils,inetd,acpid,syslog, watchdog,cron andatd. systemd's core components include:
systemd tracks processes using the Linux kernel'scgroups subsystem instead of usingprocess identifiers (PIDs); thus, daemons cannot "escape"systemd, not even by double-forking.systemd not only uses cgroups, but also augments them withsystemd-nspawn andmachinectl, two utility programs that facilitate the creation and management ofLinux containers.[45] Since version 205, systemd also offers ControlGroupInterface, which is an API to the Linux kernel cgroups.[46] The Linux kernel cgroups are adapted to supportkernfs,[47] and are being modified to support a unified hierarchy.[48]
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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~/.identity.homed manages the user's home directory in various ways such as a plain directory, abtrfs subvolume, aLinux Unified Key Setup volume, an fscrypt directory, or mounted from anSMB server.networkctl may be used to review the state of the network links as seen by systemd-networkd.[60] Configuration of new interfaces has to be added under the /lib/systemd/network/ as a new file ending with .network extension.
systemd is configured exclusively viaplain-text files although GUI tools such as systemd-manager are also available.
systemd records initialization instructions for each daemon in aconfiguration file (referred to as a "unit file") that uses adeclarative language, replacing the traditionally used per-daemon startupshell scripts. The syntax of the language is inspired by.ini files.[66]
Unit-file types[67] include:
| Linux distribution | Date added tosoftware repository[a] | Enabled by default? | Date released as default | Runs without? |
|---|---|---|---|---|
| Alpine Linux | N/A (not in repository) | No | — | Yes |
| Android | N/A (not in repository) | No | — | Yes |
| Arch Linux | January 2012[72] | Yes | October 2012[73] | No |
| antiX Linux | N/A (not in repository) | No | — | Yes |
| Artix Linux | N/A (not in repository) | No | — | Yes |
| CentOS | July 2014 | Yes | July 2014 (v7.0) | No |
| CoreOS | July 2013 | Yes | October 2013 (v94.0.0)[74][75] | No |
| Debian | April 2012[76] | Yes | April 2015 (v8.0)[77] | Jessie is the last release supporting installing without systemd.[78] Inbullseye, a number of alternative init systems are supported |
| Devuan | N/A (not in repository) | No | — | Yes |
| Fedora Linux | November 2010 (v14)[79] | Yes | May 2011 (v15) | No |
| Gentoo Linux[b] | July 2011[80][82][83] | Optional[84] | — | Yes |
| GNU Guix System | N/A (not in repository) | No | — | Yes |
| Knoppix | — | No[85][86] | — | Yes |
| Linux Mint | June 2016 (v18.0) | Yes | August 2018 (LMDE 3) | No[87] |
| Mageia | January 2011 (v1.0)[88] | Yes | May 2012 (v2.0)[89] | No[90] |
| Manjaro Linux | November 2013 | Yes | November 2013 | No |
| openSUSE | March 2011 (v11.4)[91] | Yes | September 2012 (v12.2)[92] | No |
| Parabola GNU/Linux-libre | January 2012[72] | Optional[93] | — | Yes |
| Red Hat Enterprise Linux | June 2014 (v7.0)[94] | Yes | June 2014 (v7.0) | No |
| Slackware | N/A (not in repository) | No | — | Yes |
| Solus | — | Yes | — | No |
| Source Mage | June 2011[95] | No | — | Yes |
| SUSE Linux Enterprise Server | October 2014 (v12) | Yes | October 2014 (v12) | No |
| Ubuntu | April 2013 (v13.04) | Yes | April 2015 (v15.04) | Upstart option removed in Yaketty (16.10)[96][97][c] |
| Void Linux | June 2011, removedJune 2015[98] | No | — | Yes |
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. Afork of Debian calledDevuan 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]
In the interest of enhancing the interoperability between systemd and theGNOME desktop environment, systemd coauthor Lennart Poettering askedthe GNOME Project to consider making systemd an externaldependency of GNOME 3.2.[102]
In November 2012, the GNOME Project concluded that basic GNOME functionality should not rely on systemd.[103] However, GNOME 3.8 introduced a compile-time choice between thelogind and ConsoleKit API, the former being provided at the time only by systemd. Ubuntu provided a separatelogind binary, but systemd became ade facto dependency of GNOME for mostLinux distributions, in particular since ConsoleKit is no longer actively maintained and upstream recommends the use ofsystemd-logind instead.[104] The developers ofGentoo Linux also attempted to adapt these changes inOpenRC, but the implementation contained too many bugs, causing the distribution to mark systemd as a dependency of GNOME.[105][106]
GNOME has further integratedlogind.[107] As ofMutter version 3.13.2,logind is a dependency forWayland sessions.[108]
The design of systemd has ignited controversy within thefree-software community. Critics regard systemd as overly complex and suffering from continuedfeature creep, arguing that its architecture violates theUnix 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, which is similar to the problems created byPulseAudio, another of Lennart Poettering's projects.[109][110]
In a 2012 interview,Slackware's leadPatrick 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.[111] As of August 2018[update], Slackware does not support or use systemd, but Volkerding has not ruled out the possibility of switching to it.[112]
In January 2013, Lennart Poettering attempted to address concerns about systemd in a blog post calledThe Biggest Myths.[43]
In February 2014,musl's Rich Felker opined thatPID 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 reapingzombie processes, and that the additional functionality added by systemd can be provided elsewhere and unnecessarily increases the complexity and attack surface of PID 1.[113]
In March 2014,Eric S. Raymond commented that systemd's design goals were prone tomission creep andsoftware bloat.[114] In April 2014,Linus Torvalds expressed reservations about the attitude ofKay Sievers, a key systemd developer, toward users and bug reports in regard to modifications to the Linux kernel submitted by Sievers.[115] In late April 2014, a campaign to boycott systemd was launched, with a website listing various reasons against its adoption.[116][117]
In an August 2014 article published inInfoWorld, 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".[118] The article also characterizes the architecture of systemd as similar to that ofsvchost.exe, a critical system component inMicrosoft Windows with a broad functional scope.[118]
In a September 2014ZDNet interview, prominent Linux kernel developerTheodore 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.[119] On social media, Ts'o also later compared the attitudes of Sievers and his co-developer, Lennart Poettering, to that of GNOME's developers.[120]
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.[121]
In 2012, theGentoo Linux project created afork ofudev in order to avoid dependency on the systemd architecture. The resulting fork is calledeudev and it makes udev functionality available without systemd.[122] A stated goal of the project is to keep eudev independent of any Linux distribution or init system.[123] In 2021, Gentoo announced that support of eudev would cease at the beginning of 2022. An independent group of maintainers have since taken up eudev.[124]
Elogind is the systemd project's "logind", extracted to be a standalone daemon. It integrates with PAM to know the set of users that are logged into a system and whether they are logged in graphically, on the console, or remotely. Elogind exposes this information via the standard org.freedesktop.login1D-Bus interface, as well as through the file system using systemd's standard/run/systemd layout. Elogind also provides "libelogind", which is a subset of the facilities offered by "libsystemd". There is a "libelogind.pc"pkg-config file as well.[125]
ConsoleKit was forked in October 2014 byXfce developers wanting its features to still be maintained and available on operating systems other than Linux. While not ruling out the possibility of reviving the original repository in the long term, the main developer considers ConsoleKit2 a temporary necessity until systembsd matures.[126]
LoginKit was an attempt to implement a logind (systemd-logind)shim, which would allow packages that depend on systemd-logind to work without dependency on a specific init system.[127] The project has been defunct since February 2015.[128]
In 2014, aGoogle Summer of Code project named "systembsd" was started in order to provide alternative implementations of these APIs forOpenBSD. The original project developer began it in order to ease his transition from Linux to OpenBSD.[129] Project development finished in July 2016.[130]
The systembsd project did not provide an init replacement, but aimed to provide OpenBSD with compatible daemons forhostnamed,timedated,localed, andlogind. The project did not create new systemd-like functionality, and was only meant to act as a wrapper over the native OpenBSD system. The developer aimed for systembsd to be installable as part of theports collection, not as part of a base system, stating that "systemd and *BSD differ fundamentally in terms of philosophy and development practices."[129]
Notsystemd intends to implement all systemd's features working on any init system.[131] It was forked by theParabola GNU/Linux-libre developers to build packages with their development tools without the necessity of having systemd installed to run systemd-nspawn. Development ceased in July 2018.[132]
In 2014,uselessd was created as a lightweight fork of systemd. The project sought to remove features and programs deemed unnecessary for an init system, as well as address other perceived faults.[133] Project development halted in January 2015.[134]
uselessd supported themusl andμClibc libraries, so it may have been used onembedded systems, whereas systemd only supportsglibc. The uselessd project had planned further improvements on cross-platform compatibility, as well as architectural overhauls and refactoring for the Linux build in the future.[135]
InitWare is a modular refactor of systemd, porting the system to BSD platforms without glibc or Linux-specific system calls. It is known to work on DragonFly BSD, FreeBSD, NetBSD, and GNU/Linux. Components considered unnecessary are dropped.[136]
systemd uses many Linux-specific features, and does not limit itself to POSIX. That unlocks a lot of functionality a system that is designed for portability to other operating systems cannot provide.
Yes, it is written systemd, not system D or System D, or even SystemD. And it isn't system d either. Why? Because it's a system daemon, and under Unix/Linux those are in lower case, and get suffixed with a lower case d.
systemd defines itself as a system and service manager. The project was initiated in 2010 by Lennart Poettering and Kay Sievers to create an integrated set of tools for managing a Linux system including an init daemon.
It certainly is not something that comes with systemd from upstream.
networkctl(1) – Linux UserManual – User Commands... a slice ... is a concept for hierarchically managing resources of a group of processes.
...script-based KNOPPIX system start with sysvinit
...Knoppix 'boot process continues to run via Sys-V init with few bash scripts that start the system services efficiently sequentially or in parallel. (The original German text: Knoppix' Startvorgang läuft nach wie vor per Sys-V-Init mit wenigen Bash-Skripten, welche die Systemdienste effizient sequenziell oder parallel starten.)
The recently released openSUSE 12.2 does migrate from SysVinit to systemd
ConsoleKit is currently not actively maintained. The focus has shifted to the built-in seat/user/session management of Software/systemd called systemd-logind!
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