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Resume Wikipedia de Systemd

systemd est un système d’initialisation et un daemon qui a été spécifiquement conçu pour le noyau Linux comme alternative à System V. Il a pour but d’offrir un meilleur cadre pour la gestion des dépendances entre services, de permettre le chargement en parallèle des services au démarrage, et de réduire les appels aux scripts shell. Le projet a été lancé par Lennart Poettering en 2010 et publié sous licence GNU LGPL version 2.1. Le nom de ce programme vient de « system daemon » : le daemon du système.
Ce projet a suscité de vives controverses chez les développeurs de GNU/Linux.
En 2015, la plupart des distributions GNU/Linux ont adopté ce système d’initialisation.

Resume Wikipedia de Systemd

systemd est un système d’initialisation et un daemon qui a été spécifiquement conçu pour le noyau Linux comme alternative à System V. Il a pour but d’offrir un meilleur cadre pour la gestion des dépendances entre services, de permettre le chargement en parallèle des services au démarrage, et de réduire les appels aux scripts shell. Le projet a été lancé par Lennart Poettering en 2010 et publié sous licence GNU LGPL version 2.1. Le nom de ce programme vient de « system daemon » : le daemon du système.
Ce projet a suscité de vives controverses chez les développeurs de GNU/Linux.
En 2015, la plupart des distributions GNU/Linux ont adopté ce système d’initialisation.

SYSTEMD(1) systemd SYSTEMD(1)

NAME

systemd, init – systemd system and service manager

SYNOPSIS

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=
Show terse service status information while booting. This switch
has no effect when run as user instance. Takes a boolean argument
which may be omitted which is interpreted as true.

–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.

-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 12 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, or an operation timed out). 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 or ps
(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 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 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.

SIGINT
Upon receiving this signal the systemd system manager will start
the ctrl-alt-del.target unit. This is mostly equivalent to
systemctl start ctl-alt-del.target. 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 start 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.

SIGRTMIN+4
Powers off the machine, starts the poweroff.target unit. This is
mostly equivalent to systemctl start poweroff.target.

SIGRTMIN+5
Reboots the machine, starts the reboot.target unit. This is mostly
equivalent to systemctl start reboot.target.

SIGRTMIN+6
Reboots the machine via kexec, starts the kexec.target unit. This
is mostly equivalent to systemctl start kexec.target.

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 level 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
Controls whether colorized output should be generated.

$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. If yes, the systemd manager (PID 1) dumps
core when it crashes. Otherwise, no core dump is created. Defaults
to yes.

systemd.crash_chvt=
Takes a positive integer, or a boolean argument. 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 no, meaning that no such switch is attempted.
If set to yes, the VT the kernel messages are written to is
selected.

systemd.crash_shell=
Takes a boolean argument. If yes, the system manager (PID 1) spawns
a shell when it crashes, after a 10s delay. Otherwise, no shell is
spawned. Defaults to no, for security reasons, as the shell is not
protected by password authentication.

systemd.crash_reboot=
Takes a boolean argument. If yes, 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
no, 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. If yes, the system manager (PID 1) asks
for confirmation when spawning processes. Defaults to no.

systemd.show_status=
Takes a boolean argument or the constant auto. If yes, the systemd
manager (PID 1) shows terse service status updates on the console
during bootup. auto behaves like false until a service fails or
there is a significant delay in boot. Defaults to yes, unless quiet
is passed as kernel command line option, in which case it defaults
to auto.

systemd.log_target=, systemd.log_level=, systemd.log_color=,
systemd.log_location=
Controls log output, with the same effect as the
$SYSTEMD_LOG_TARGET, $SYSTEMD_LOG_LEVEL, $SYSTEMD_LOG_COLOR,
$SYSTEMD_LOG_LOCATION environment variables described above.

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.

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

rescue, single, s, S, 1
Boot into rescue mode. This is equivalent to
systemd.unit=rescue.target 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 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.

SEE ALSO

The systemd Homepage[8], systemd-system.conf(5), locale.conf(5),
systemctl, journalctl, systemd-notify, daemon(7), sd-
daemon(3), systemd.unit(5), systemd.special(5), pkg-config, kernel-
command-line(7), bootup(7), systemd.directives(7)

NOTES
1. cgroups.txt
https://www.kernel.org/doc/Documentation/cgroups/cgroups.txt

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

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

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

5. initrd Interface
http://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. systemd Homepage
http://www.freedesktop.org/wiki/Software/systemd/

systemd 229 SYSTEMD(1)