Ubuntu Manpage: systemd, init - systemd system and service manager

Provided by: systemd_237-3ubuntu10.57_amd64

NAME

       systemd, init - systemd system and service manager

SYNOPSIS

       /lib/systemd/systemd [OPTIONS...]

       init [OPTIONS...] {COMMAND}

DESCRIPTION

       systemd is a system and service manager for Linux operating systems. When run as first
       process on boot (as PID 1), it acts as init system that brings up and maintains userspace
       services.

       For compatibility with SysV, if systemd is called as init and a PID that is not 1, it will
       execute telinit and pass all command line arguments unmodified. That means init and
       telinit are mostly equivalent when invoked from normal login sessions. See telinit(8) for
       more information.

       When run as a system instance, systemd interprets the configuration file system.conf and
       the files in system.conf.d directories; when run as a user instance, systemd interprets
       the configuration file user.conf and the files in user.conf.d directories. See systemd-
       system.conf(5) for more information.

OPTIONS

The following options are understood:

--test
Determine startup sequence, dump it and exit. This is an option useful for debugging
only.

--dump-configuration-items
Dump understood unit configuration items. This outputs a terse but complete list of
configuration items understood in unit definition files.

--unit=
Set default unit to activate on startup. If not specified, defaults to default.target.

--system, --user
For --system, tell systemd to run a system instance, even if the process ID is not 1,
i.e. systemd is not run as init process. --user does the opposite, running a user
instance even if the process ID is 1. Normally, it should not be necessary to pass
these options, as systemd automatically detects the mode it is started in. These
options are hence of little use except for debugging. Note that it is not supported
booting and maintaining a full system with systemd running in --system mode, but PID
not 1. In practice, passing --system explicitly is only useful in conjunction with
--test.

--dump-core
Enable core dumping on crash. This switch has no effect when running as user instance.
This setting may also be enabled during boot on the kernel command line via the
systemd.dump_core= option, see below.

--crash-vt=VT
Switch to a specific virtual console (VT) on crash. Takes a positive integer in the
range 1–63, or a boolean argument. If an integer is passed, selects which VT to switch
to. If yes, the VT kernel messages are written to is selected. If no, no VT switch is
attempted. This switch has no effect when running as user instance. This setting may
also be enabled during boot, on the kernel command line via the systemd.crash_vt=
option, see below.

--crash-shell
Run a shell on crash. This switch has no effect when running as user instance. This
setting may also be enabled during boot, on the kernel command line via the
systemd.crash_shell= option, see below.

--crash-reboot
Automatically reboot the system on crash. This switch has no effect when running as
user instance. This setting may also be enabled during boot, on the kernel command
line via the systemd.crash_reboot= option, see below.

--confirm-spawn
Ask for confirmation when spawning processes. This switch has no effect when run as
user instance.

--show-status=
Takes a boolean argument or the special value auto. If on, terse unit status
information is shown on the console during boot-up and shutdown. If off, no such
status information is shown. If set to auto behavior is similar to off, except that it
is automatically switched to on, as soon as the first unit failure or significant boot
delay is encountered. This switch has no effect when invoked as user instance. If
specified, overrides both the kernel command line setting systemd.show_status= (see
below) and the configuration file option ShowStatus=, see systemd-system.conf(5).

--log-target=
Set log target. Argument must be one of console, journal, kmsg, journal-or-kmsg, null.

--log-level=
Set log level. As argument this accepts a numerical log level or the well-known
syslog(3) symbolic names (lowercase): emerg, alert, crit, err, warning, notice, info,
debug.

--log-color=
Highlight important log messages. Argument is a boolean value. If the argument is
omitted, it defaults to true.

--log-location=
Include code location in log messages. This is mostly relevant for debugging purposes.
Argument is a boolean value. If the argument is omitted it defaults to true.

--default-standard-output=, --default-standard-error=
Sets the default output or error output for all services and sockets, respectively.
That is, controls the default for StandardOutput= and StandardError= (see
systemd.exec(5) for details). Takes one of inherit, null, tty, journal,
journal+console, syslog, syslog+console, kmsg, kmsg+console. If the argument is
omitted --default-standard-output= defaults to journal and --default-standard-error=
to inherit.

--machine-id=
Override the machine-id set on the hard drive, useful for network booting or for
containers. May not be set to all zeros.

--service-watchdogs=
Globally enable/disable all service watchdog timeouts and emergency actions. This
setting may also be specified during boot, on the kernel command line via the
systemd.service_watchdogs= option, see below. Defaults to enabled.

-h, --help
Print a short help text and exit.

--version
Print a short version string and exit.

CONCEPTS

systemd provides a dependency system between various entities called "units" of 11
different types. Units encapsulate various objects that are relevant for system boot-up
and maintenance. The majority of units are configured in unit configuration files, whose
syntax and basic set of options is described in systemd.unit(5), however some are created
automatically from other configuration, dynamically from system state or programmatically
at runtime. Units may be "active" (meaning started, bound, plugged in, ..., depending on
the unit type, see below), or "inactive" (meaning stopped, unbound, unplugged, ...), as
well as in the process of being activated or deactivated, i.e. between the two states
(these states are called "activating", "deactivating"). A special "failed" state is
available as well, which is very similar to "inactive" and is entered when the service
failed in some way (process returned error code on exit, or crashed, an operation timed
out, or after too many restarts). If this state is entered, the cause will be logged, for
later reference. Note that the various unit types may have a number of additional
substates, which are mapped to the five generalized unit states described here.

The following unit types are available:

1. Service units, which start and control daemons and the processes they consist of. For
details, see systemd.service(5).

2. Socket units, which encapsulate local IPC or network sockets in the system, useful for
socket-based activation. For details about socket units, see systemd.socket(5), for
details on socket-based activation and other forms of activation, see daemon(7).

3. Target units are useful to group units, or provide well-known synchronization points
during boot-up, see systemd.target(5).

4. Device units expose kernel devices in systemd and may be used to implement
device-based activation. For details, see systemd.device(5).

5. Mount units control mount points in the file system, for details see systemd.mount(5).

6. Automount units provide automount capabilities, for on-demand mounting of file systems
as well as parallelized boot-up. See systemd.automount(5).

7. Timer units are useful for triggering activation of other units based on timers. You
may find details in systemd.timer(5).

8. Swap units are very similar to mount units and encapsulate memory swap partitions or
files of the operating system. They are described in systemd.swap(5).

9. Path units may be used to activate other services when file system objects change or
are modified. See systemd.path(5).

10. Slice units may be used to group units which manage system processes (such as service
and scope units) in a hierarchical tree for resource management purposes. See
systemd.slice(5).

11. Scope units are similar to service units, but manage foreign processes instead of
starting them as well. See systemd.scope(5).

Units are named as their configuration files. Some units have special semantics. A
detailed list is available in systemd.special(7).

systemd knows various kinds of dependencies, including positive and negative requirement
dependencies (i.e. Requires= and Conflicts=) as well as ordering dependencies (After= and
Before=). NB: ordering and requirement dependencies are orthogonal. If only a requirement
dependency exists between two units (e.g. foo.service requires bar.service), but no
ordering dependency (e.g. foo.service after bar.service) and both are requested to start,
they will be started in parallel. It is a common pattern that both requirement and
ordering dependencies are placed between two units. Also note that the majority of
dependencies are implicitly created and maintained by systemd. In most cases, it should be
unnecessary to declare additional dependencies manually, however it is possible to do
this.

Application programs and units (via dependencies) may request state changes of units. In
systemd, these requests are encapsulated as 'jobs' and maintained in a job queue. Jobs may
succeed or can fail, their execution is ordered based on the ordering dependencies of the
units they have been scheduled for.

On boot systemd activates the target unit default.target whose job is to activate on-boot
services and other on-boot units by pulling them in via dependencies. Usually, the unit
name is just an alias (symlink) for either graphical.target (for fully-featured boots into
the UI) or multi-user.target (for limited console-only boots for use in embedded or server
environments, or similar; a subset of graphical.target). However, it is at the discretion
of the administrator to configure it as an alias to any other target unit. See
systemd.special(7) for details about these target units.

Processes systemd spawns are placed in individual Linux control groups named after the
unit which they belong to in the private systemd hierarchy. (see cgroups.txt[1] for more
information about control groups, or short "cgroups"). systemd uses this to effectively
keep track of processes. Control group information is maintained in the kernel, and is
accessible via the file system hierarchy (beneath /sys/fs/cgroup/systemd/), or in tools
such as systemd-cgls(1) or ps(1) (ps xawf -eo pid,user,cgroup,args is particularly useful
to list all processes and the systemd units they belong to.).

systemd is compatible with the SysV init system to a large degree: SysV init scripts are
supported and simply read as an alternative (though limited) configuration file format.
The SysV /dev/initctl interface is provided, and compatibility implementations of the
various SysV client tools are available. In addition to that, various established Unix
functionality such as /etc/fstab or the utmp database are supported.

systemd has a minimal transaction system: if a unit is requested to start up or shut down
it will add it and all its dependencies to a temporary transaction. Then, it will verify
if the transaction is consistent (i.e. whether the ordering of all units is cycle-free).
If it is not, systemd will try to fix it up, and removes non-essential jobs from the
transaction that might remove the loop. Also, systemd tries to suppress non-essential jobs
in the transaction that would stop a running service. Finally it is checked whether the
jobs of the transaction contradict jobs that have already been queued, and optionally the
transaction is aborted then. If all worked out and the transaction is consistent and
minimized in its impact it is merged with all already outstanding jobs and added to the
run queue. Effectively this means that before executing a requested operation, systemd
will verify that it makes sense, fixing it if possible, and only failing if it really
cannot work.

systemd contains native implementations of various tasks that need to be executed as part
of the boot process. For example, it sets the hostname or configures the loopback network
device. It also sets up and mounts various API file systems, such as /sys or /proc.

For more information about the concepts and ideas behind systemd, please refer to the
Original Design Document[2].

Note that some but not all interfaces provided by systemd are covered by the Interface
Stability Promise[3].

Units may be generated dynamically at boot and system manager reload time, for example
based on other configuration files or parameters passed on the kernel command line. For
details, see systemd.generator(7).

Systems which invoke systemd in a container or initrd environment should implement the
Container Interface[4] or initrd Interface[5] specifications, respectively.

DIRECTORIES

System unit directories
The systemd system manager reads unit configuration from various directories. Packages
that want to install unit files shall place them in the directory returned by
pkg-config systemd --variable=systemdsystemunitdir. Other directories checked are
/usr/local/lib/systemd/system and /lib/systemd/system. User configuration always takes
precedence. pkg-config systemd --variable=systemdsystemconfdir returns the path of
the system configuration directory. Packages should alter the content of these
directories only with the enable and disable commands of the systemctl(1) tool. Full
list of directories is provided in systemd.unit(5).

User unit directories
Similar rules apply for the user unit directories. However, here the XDG Base
Directory specification[6] is followed to find units. Applications should place their
unit files in the directory returned by pkg-config systemd
--variable=systemduserunitdir. Global configuration is done in the directory reported
by pkg-config systemd --variable=systemduserconfdir. The enable and disable commands
of the systemctl(1) tool can handle both global (i.e. for all users) and private (for
one user) enabling/disabling of units. Full list of directories is provided in
systemd.unit(5).

SysV init scripts directory
The location of the SysV init script directory varies between distributions. If
systemd cannot find a native unit file for a requested service, it will look for a
SysV init script of the same name (with the .service suffix removed).

SysV runlevel link farm directory
The location of the SysV runlevel link farm directory varies between distributions.
systemd will take the link farm into account when figuring out whether a service shall
be enabled. Note that a service unit with a native unit configuration file cannot be
started by activating it in the SysV runlevel link farm.

SIGNALS

SIGTERM
Upon receiving this signal the systemd system manager serializes its state, reexecutes
itself and deserializes the saved state again. This is mostly equivalent to systemctl
daemon-reexec.

systemd user managers will start the exit.target unit when this signal is received.
This is mostly equivalent to systemctl --user start exit.target
--job-mode=replace-irreversible.

SIGINT
Upon receiving this signal the systemd system manager will start the
ctrl-alt-del.target unit. This is mostly equivalent to systemctl start
ctrl-alt-del.target --job-mode=replace-irreversible. If this signal is received more
than 7 times per 2s, an immediate reboot is triggered. Note that pressing Ctrl-Alt-Del
on the console will trigger this signal. Hence, if a reboot is hanging, pressing
Ctrl-Alt-Del more than 7 times in 2s is a relatively safe way to trigger an immediate
reboot.

systemd user managers treat this signal the same way as SIGTERM.

SIGWINCH
When this signal is received the systemd system manager will start the
kbrequest.target unit. This is mostly equivalent to systemctl start kbrequest.target.

This signal is ignored by systemd user managers.

SIGPWR
When this signal is received the systemd manager will start the sigpwr.target unit.
This is mostly equivalent to systemctl start sigpwr.target.

SIGUSR1
When this signal is received the systemd manager will try to reconnect to the D-Bus
bus.

SIGUSR2
When this signal is received the systemd manager will log its complete state in
human-readable form. The data logged is the same as printed by systemd-analyze dump.

SIGHUP
Reloads the complete daemon configuration. This is mostly equivalent to systemctl
daemon-reload.

SIGRTMIN+0
Enters default mode, starts the default.target unit. This is mostly equivalent to
systemctl isolate default.target.

SIGRTMIN+1
Enters rescue mode, starts the rescue.target unit. This is mostly equivalent to
systemctl isolate rescue.target.

SIGRTMIN+2
Enters emergency mode, starts the emergency.service unit. This is mostly equivalent to
systemctl isolate emergency.service.

SIGRTMIN+3
Halts the machine, starts the halt.target unit. This is mostly equivalent to systemctl
start halt.target --job-mode=replace-irreversible.

SIGRTMIN+4
Powers off the machine, starts the poweroff.target unit. This is mostly equivalent to
systemctl start poweroff.target --job-mode=replace-irreversible.

SIGRTMIN+5
Reboots the machine, starts the reboot.target unit. This is mostly equivalent to
systemctl start reboot.target --job-mode=replace-irreversible.

SIGRTMIN+6
Reboots the machine via kexec, starts the kexec.target unit. This is mostly equivalent
to systemctl start kexec.target --job-mode=replace-irreversible.

SIGRTMIN+13
Immediately halts the machine.

SIGRTMIN+14
Immediately powers off the machine.

SIGRTMIN+15
Immediately reboots the machine.

SIGRTMIN+16
Immediately reboots the machine with kexec.

SIGRTMIN+20
Enables display of status messages on the console, as controlled via
systemd.show_status=1 on the kernel command line.

SIGRTMIN+21
Disables display of status messages on the console, as controlled via
systemd.show_status=0 on the kernel command line.

SIGRTMIN+22, SIGRTMIN+23
Sets the log level to "debug" (or "info" on SIGRTMIN+23), as controlled via
systemd.log_level=debug (or systemd.log_level=info on SIGRTMIN+23) on the kernel
command line.

SIGRTMIN+24
Immediately exits the manager (only available for --user instances).

SIGRTMIN+26, SIGRTMIN+27, SIGRTMIN+28
Sets the log target to "journal-or-kmsg" (or "console" on SIGRTMIN+27, "kmsg" on
SIGRTMIN+28), as controlled via systemd.log_target=journal-or-kmsg (or
systemd.log_target=console on SIGRTMIN+27 or systemd.log_target=kmsg on SIGRTMIN+28)
on the kernel command line.

ENVIRONMENT

$SYSTEMD_LOG_LEVEL
systemd reads the log level from this environment variable. This can be overridden
with --log-level=.

$SYSTEMD_LOG_TARGET
systemd reads the log target from this environment variable. This can be overridden
with --log-target=.

$SYSTEMD_LOG_COLOR
Controls whether systemd highlights important log messages. This can be overridden
with --log-color=.

$SYSTEMD_LOG_LOCATION
Controls whether systemd prints the code location along with log messages. This can be
overridden with --log-location=.

$XDG_CONFIG_HOME, $XDG_CONFIG_DIRS, $XDG_DATA_HOME, $XDG_DATA_DIRS
The systemd user manager uses these variables in accordance to the XDG Base Directory
specification[6] to find its configuration.

$SYSTEMD_UNIT_PATH
Controls where systemd looks for unit files.

$SYSTEMD_SYSVINIT_PATH
Controls where systemd looks for SysV init scripts.

$SYSTEMD_SYSVRCND_PATH
Controls where systemd looks for SysV init script runlevel link farms.

$SYSTEMD_COLORS
The value must be a boolean. Controls whether colorized output should be generated.
This can be specified to override the decision that systemd makes based on $TERM and
what the console is connected to.

$LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES
Set by systemd for supervised processes during socket-based activation. See
sd_listen_fds(3) for more information.

$NOTIFY_SOCKET
Set by systemd for supervised processes for status and start-up completion
notification. See sd_notify(3) for more information.

KERNEL COMMAND LINE

When run as system instance systemd parses a number of kernel command line arguments[7]:

systemd.unit=, rd.systemd.unit=
Overrides the unit to activate on boot. Defaults to default.target. This may be used
to temporarily boot into a different boot unit, for example rescue.target or
emergency.service. See systemd.special(7) for details about these units. The option
prefixed with "rd." is honored only in the initial RAM disk (initrd), while the one
that is not prefixed only in the main system.

systemd.dump_core
Takes a boolean argument or enables the option if specified without an argument. If
enabled, the systemd manager (PID 1) dumps core when it crashes. Otherwise, no core
dump is created. Defaults to enabled.

systemd.crash_chvt
Takes a positive integer, or a boolean argument. Can be also specified without an
argument, with the same effect as a positive boolean. If a positive integer (in the
range 1–63) is specified, the system manager (PID 1) will activate the specified
virtual terminal (VT) when it crashes. Defaults to disabled, meaning that no such
switch is attempted. If set to enabled, the VT the kernel messages are written to is
selected.

systemd.crash_shell
Takes a boolean argument or enables the option if specified without an argument. If
enabled, the system manager (PID 1) spawns a shell when it crashes, after a 10s delay.
Otherwise, no shell is spawned. Defaults to disabled, for security reasons, as the
shell is not protected by password authentication.

systemd.crash_reboot
Takes a boolean argument or enables the option if specified without an argument. If
enabled, the system manager (PID 1) will reboot the machine automatically when it
crashes, after a 10s delay. Otherwise, the system will hang indefinitely. Defaults to
disabled, in order to avoid a reboot loop. If combined with systemd.crash_shell, the
system is rebooted after the shell exits.

systemd.confirm_spawn
Takes a boolean argument or a path to the virtual console where the confirmation
messages should be emitted. Can be also specified without an argument, with the same
effect as a positive boolean. If enabled, the system manager (PID 1) asks for
confirmation when spawning processes using /dev/console. If a path or a console name
(such as "ttyS0") is provided, the virtual console pointed to by this path or
described by the give name will be used instead. Defaults to disabled.

systemd.service_watchdogs=
Takes a boolean argument. If disabled, all service runtime watchdogs (WatchdogSec=)
and emergency actions (e.g. OnFailure= or StartLimitAction=) are ignored by the
system manager (PID 1); see systemd.service(5). Defaults to enabled, i.e. watchdogs
and failure actions are processed normally. The hardware watchdog is not affected by
this option.

systemd.show_status
Takes a boolean argument or the constant auto. Can be also specified without an
argument, with the same effect as a positive boolean. If enabled, the systemd manager
(PID 1) shows terse service status updates on the console during bootup. auto behaves
like false until a unit fails or there is a significant delay in boot. Defaults to
enabled, unless quiet is passed as kernel command line option, in which case it
defaults to auto. If specified overrides the system manager configuration file option
ShowStatus=, see systemd-system.conf(5). However, the process command line option
--show-status= takes precedence over both this kernel command line option and the
configuration file option.

systemd.log_target=, systemd.log_level=, systemd.log_location=, systemd.log_color
Controls log output, with the same effect as the $SYSTEMD_LOG_TARGET,
$SYSTEMD_LOG_LEVEL, $SYSTEMD_LOG_LOCATION, $SYSTEMD_LOG_COLOR environment variables
described above. systemd.log_color can be specified without an argument, with the
same effect as a positive boolean.

systemd.default_standard_output=, systemd.default_standard_error=
Controls default standard output and error output for services, with the same effect
as the --default-standard-output= and --default-standard-error= command line arguments
described above, respectively.

systemd.setenv=
Takes a string argument in the form VARIABLE=VALUE. May be used to set default
environment variables to add to forked child processes. May be used more than once to
set multiple variables.

systemd.machine_id=
Takes a 32 character hex value to be used for setting the machine-id. Intended mostly
for network booting where the same machine-id is desired for every boot.

systemd.unified_cgroup_hierarchy
When specified without an argument or with a true argument, enables the usage of
unified cgroup hierarchy[8] (a.k.a. cgroups-v2). When specified with a false argument,
fall back to hybrid or full legacy cgroup hierarchy.

If this option is not specified, the default behaviour is determined during
compilation (the --with-default-hierarchy= option). If the kernel does not support
unified cgroup hierarchy, the legacy hierarchy will be used even if this option is
specified.

systemd.legacy_systemd_cgroup_controller
Takes effect if the full unified cgroup hierarchy is not used (see previous option).
When specified without an argument or with a true argument, disables the use of
"hybrid" cgroup hierarchy (i.e. a cgroups-v2 tree used for systemd, and legacy cgroup
hierarchy[9], a.k.a. cgroups-v1, for other controllers), and forces a full "legacy"
mode. When specified with a false argument, enables the use of "hybrid" hierarchy.

quiet
Turn off status output at boot, much like systemd.show_status=false would. Note that
this option is also read by the kernel itself and disables kernel log output. Passing
this option hence turns off the usual output from both the system manager and the
kernel.

debug
Turn on debugging output. This is equivalent to systemd.log_level=debug. Note that
this option is also read by the kernel itself and enables kernel debug output. Passing
this option hence turns on the debug output from both the system manager and the
kernel.

emergency, rd.emergency, -b
Boot into emergency mode. This is equivalent to systemd.unit=emergency.target or
rd.systemd.unit=emergency.target, respectively, and provided for compatibility reasons
and to be easier to type.

rescue, rd.rescue, single, s, S, 1
Boot into rescue mode. This is equivalent to systemd.unit=rescue.target or
rd.systemd.unit=rescue.target, respectively, and provided for compatibility reasons
and to be easier to type.

2, 3, 4, 5
Boot into the specified legacy SysV runlevel. These are equivalent to
systemd.unit=runlevel2.target, systemd.unit=runlevel3.target,
systemd.unit=runlevel4.target, and systemd.unit=runlevel5.target, respectively, and
provided for compatibility reasons and to be easier to type.

locale.LANG=, locale.LANGUAGE=, locale.LC_CTYPE=, locale.LC_NUMERIC=, locale.LC_TIME=,
locale.LC_COLLATE=, locale.LC_MONETARY=, locale.LC_MESSAGES=, locale.LC_PAPER=,
locale.LC_NAME=, locale.LC_ADDRESS=, locale.LC_TELEPHONE=, locale.LC_MEASUREMENT=,
locale.LC_IDENTIFICATION=
Set the system locale to use. This overrides the settings in /etc/locale.conf. For
more information, see locale.conf(5) and locale(7).

For other kernel command line parameters understood by components of the core OS, please
refer to kernel-command-line(7).

SOCKETS AND FIFOS

       /run/systemd/notify
           Daemon status notification socket. This is an AF_UNIX datagram socket and is used to
           implement the daemon notification logic as implemented by sd_notify(3).

       /run/systemd/private
           Used internally as communication channel between systemctl(1) and the systemd process.
           This is an AF_UNIX stream socket. This interface is private to systemd and should not
           be used in external projects.

       /dev/initctl
           Limited compatibility support for the SysV client interface, as implemented by the
           systemd-initctl.service unit. This is a named pipe in the file system. This interface
           is obsolete and should not be used in new applications.

NOTES

        1. cgroups.txt
           https://www.kernel.org/doc/Documentation/cgroup-v1/cgroups.txt

        2. Original Design Document
           http://0pointer.de/blog/projects/systemd.html

        3. Interface Stability Promise
           https://www.freedesktop.org/wiki/Software/systemd/InterfaceStabilityPromise

        4. Container Interface
           https://www.freedesktop.org/wiki/Software/systemd/ContainerInterface

        5. initrd Interface
           https://www.freedesktop.org/wiki/Software/systemd/InitrdInterface

        6. XDG Base Directory specification
           http://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html

        7. If run inside a Linux container these arguments may be passed as command line
           arguments to systemd itself, next to any of the command line options listed in the
           Options section above. If run outside of Linux containers, these arguments are parsed
           from /proc/cmdline instead.

        8. unified cgroup hierarchy
           https://www.kernel.org/doc/Documentation/cgroup-v2.txt

        9. legacy cgroup hierarchy
           https://www.kernel.org/doc/Documentation/cgroup-v1/

       10. systemd Homepage
           https://www.freedesktop.org/wiki/Software/systemd/