|Original author(s)||Lennart Poettering, Kay Sievers|
|Developer(s)||Lennart Poettering, Kay Sievers and others|
|Initial release||30 March 2010|
|Stable release||215 (July 3, 2014[±])|
|License||first GPLv2+, currently GNU LGPL 2.1+ |
systemd is a system management daemon designed for Linux and programmed exclusively for the Linux API. For systems using it, it is the first process which is executed in user space during the Linux startup process. Therefore, systemd serves as the root of the user space's process tree. The name systemd adheres to the Unix convention of making daemons easier to distinguish by having letter d as the last one in their actual filenames.
systemd is also the name of a software bundle. This includes systemd daemon, logind and a couple of other low-level components of an operating system.
Systemd was developed for Linux to replace the init system inherited from UNIX System V and Berkeley Software Distribution (BSD) operating systems. Like init, systemd is a daemon that manages other daemons. All daemons, including systemd, are background processes. Systemd is the first daemon to start (during booting) and the last daemon to terminate (during shutdown).
Lennart Poettering and Kay Sievers, the software engineers who initially developed systemd, 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 processing to be done concurrently or in parallel during system booting; and to reduce the computational overhead of the shell.
Systemd's initialization instructions for each daemon are recorded in a declarative configuration file rather than a shell script. For inter-process communication, systemd makes Unix domain sockets and D-Bus available to the running daemons. Because systemd tracks processes using Linux cgroups instead of process identifiers (PIDs), daemons cannot "escape" systemd; not even by double-forking. Systemd is also capable of aggressive parallelization.
Among systemd's auxiliary features are a cron-like job scheduler called systemd Calendar Timers, and an event logging subsystem called journal. The system administrator may choose whether to log system events with systemd or syslog. Systemd's logfile is a binary file. The state of systemd itself can be preserved in a snapshot for future recall.
In April 2012, the source tree for udev (a device manager for the Linux kernel, which handles the /dev directory and all user space actions when adding/removing devices, including firmware loading) was merged into systemd.
Systemd also provides replacements for the following daemons and utilities:
Since version 205, systemd offers ControlGroupInterface, 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.
In version 209, networkd was integrated, which provides abilities for systemd to perform various network configurations; as of this version, support is limited to statically assigned addresses and basic support for bridging configuration.
In the interest of enhancing the interoperability between systemd and the GNOME desktop environment, systemd coauthor Lennart Poettering asked the GNOME Project to consider making systemd an external dependency of GNOME 3.2.
In November 2012, the GNOME Project concluded that basic GNOME functionality should not rely on systemd. However, in contradiction of this statement, GNOME 3.8 introduced a de facto dependency on systemd by introducing session management behaviors which depend on how systemd operates. While the developers of Gentoo attempted to adapt these changes in OpenRC, the implementation contained too many bugs, causing the distribution to mark systemd as a dependency of GNOME.
Adoption of systemd has been very controversial. Linus Torvalds and Theodore Ts'o have expressed reservations about the systemd philosophy and especially the attitudes of the key developers toward users and bug reports. Articles run in Linux Advocates have characterized systemd as "the new PulseAudio," and as "an accident waiting to happen." One Fuduntu contributor is quoted as stating that systemd has limited software choice:
Systemd, whether by design, or circumstance, is largely becoming non-optional. Inclusion of core technologies such as dbus and udev are reducing choice for linux users and developers, rather than expanding them—which is the very antithesis of the idea of Free/Open Source software.
Concerning systemd, I do like the idea of a faster boot time (obviously), but I also like controlling the startup of the system with shell scripts that are readable, and I'm guessing that's what most Slackware users prefer too. I don't spend all day rebooting my machine, and having looked at systemd config files it seems to me a very foreign way of controlling a system to me, and attempting to control services, sockets, devices, mounts, etc., all within one daemon flies in the face of the UNIX concept of doing one thing and doing it well.
In January 2013, Lennart Poettering attempted to address concerns about systemd in a blog post called The Biggest Myths.
Eric S. Raymond declined to comment on systemd at first, but stated, "I'm aware there’s a controversy." Then in a March 2014 interview on Slashdot, he expressed some concerns about the goals and architecture of systemd:
I want to study it carefully because I'm a bit troubled by what I hear about the feature set and the goals. From that, I fear it may be one of those projects that is teetering right at the edge of manageable complexity – OK as long as an architect with a strong sense of design discipline is running things, but very prone to mission creep and bloat and likely to turn into a nasty hairball over the longer term.
In May 2011, Fedora became the first major Linux distribution to enable systemd by default. As of October 2013[update], Slackware does not support or use systemd, but Slackware's lead Patrick Volkerding has not ruled out the possibility for switching to it.
|Linux distribution||Date added to software repository[a]||Enabled by default?|
|Arch Linux||October 2012||Yes|
|Debian GNU/Linux||April 2012||Chosen for next release[b]|
|Fedora||May 2011 (v15)||Yes|
|Frugalware Linux||August 2011 (v1.5)||Yes|
|Gentoo Linux [c]||2011||No|
|Mageia||May 2012 (v2.0)||Yes|
|openSUSE||March 2011 (v11.4)||Yes|
|Red Hat Enterprise Linux||June 2014 (v7.0)||Yes|
|Sabayon Linux||August 2013 (v13.08)||Yes|
|Ubuntu[d]||April 2013 (v13.04)||Planned|
- Dates are for the general availability release.
- The Debian Technical Committee voted to make systemd the default system management daemon for Linux in the "jessie" release. As a result, Ubuntu founder Mark Shuttleworth announced that Ubuntu would migrate towards it from its self-developed competitor Upstart for a future release, in order to maintain consistency with the distribution that Ubuntu is based upon.
- systemd is supported in Gentoo as an alternative to OpenRC, the default init system for those who "want to use systemd instead, or are planning to use Gnome 3.8 and later (which requires systemd)"
- Ubuntu's development documentation offers instructions on how to use systemd as an experimental option.
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- Red Hat Unveils Red Hat Enterprise Linux 7, 2014-06-10
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