User:Johannbg/QA/Systemd/Systemd

Name
systemd, init — systemd System and Session Manager

Synopsis
systemd [OPTIONS...] init [OPTIONS...] {COMMAND}

Description
systemd is a system and session 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 system instance, systemd interprets the configuration file system.conf, otherwise session.conf. See systemd.conf(5) for more information.

Options
The following options are understood:

-h, --help Prints a short help text and exits.

--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.

--introspect= Extract D-Bus interface introspection data. This is mostly useful at install time to generate data suitable for the D-Bus interfaces repository. Optionally the interface name for the introspection data may be specified. If omitted, the introspection data for all interfaces is dumped.

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

--system, --session Tell systemd to run a system instance (resp. session instance), even if the process ID is not 1 (resp. is 1), i.e. system is not (resp. is) run as init process. 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.

--dump-core Dump core on crash. This switch has no effect when run as session instance.

--crash-shell Run shell on crash. This switch has no effect when run as session instance.

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

--show-status Show terse service status information while booting. This switch has no effect when run as session instance.

--log-target= Set log target. Argument must be one of console, syslog, kmsg, syslog-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.

Concepts
systemd provides a dependency system between various entities called "units". Units encapsulate various objects that are relevant for system boot-up and maintainance. 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 or dynamically from system state. Units may be active (meaning started, bound, plugged in, ... depending on the unit type), or inactive (meaning stopped, unbound, unplugged, ...), as well as in the process of being activated or deactivated, i.e. between the two states. The following unit types are available:

Service units
Control daemons and the processes they consist of. For details see systemd.service(5).

Socket units
Capsulate 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).

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

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

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

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

Snapshot units
Can be used to temporarily save the state of the set of systemd units, which later may be restored by activating the saved snapshot unit. For more information see systemd.automount(5).

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

Swap units
Very similar to mount units and encapsulated memory swap partitions or files of the operating systemd. They are described in systemd.swap(5).

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

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

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 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 /cgroup/systemd/), or in tools such as 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 host name 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.

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/share/systemd/system and /usr/share/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 systemd-install(1) tool.

Session unit directories
Similar rules apply for the session unit directories. However, here the XDG Base Directory specification is followed to find units. Applications should place their unit files in the directory returned by pkg-config systemd --variable=systemdsessionunitdir. Global configuration is done in the directory reported by pkg-config systemd --variable=systemdsessionconfdir. The systemd-install(1) tool can handle both global (i.e. for all users) and private (for one user) enabling/disabling of units.

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 session managers will start the exit.target unit when this signal is received. This is mostly equivalent to systemctl --session 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.

systemd session 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 session 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 systemctl 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.

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 session manager uses these variables in accordance to the XDG Base Directory specification 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.

$LISTEN_PID, $LISTEN_FDS 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 few kernel command line arguments:

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.

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.dump_core= Takes a boolean argument. If true systemd dumps core when it crashes. Otherwise no core dump is created. Defaults to true.

systemd.crash_shell= Takes a boolean argument. If true systemd spawns a shell when it crashes. Otherwise no core dump is created. Defaults to false, for security reasons, as the shell is not protected by any password authentication.

systemd.crash_chvt= Takes an integer argument. If positive systemd activates the specified virtual terminal when it crashes. Defaults to -1.

systemd.show_status= Takes a boolean argument. If true shows terse service status updates on the console during bootup. Defaults to true.

Sockets and FIFOs
@/org/freedesktop/systemd1/notify Daemon status notification socket. This is an AF_UNIX datagram socket in the Linux abstract namespace, and is used to implement the daemon notification logic as implemented by sd_notify(3).

@/org/freedesktop/systemd1/logger Used internally by the systemd-logger.service unit to connect STDOUT and/or STDERR of spawned processes to syslog(3) or the kernel log buffer. This is an AF_UNIX stream socket in the Linux abstract namespace.

@/org/freedesktop/systemd1/private Used internally as communication channel between systemctl(1) and the systemd process. This is an AF_UNIX stream socket in the Linux abstract namespace. 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.