systemd systemd 1 systemd init systemd system and service manager /usr/lib/systemd/systemd OPTIONS init OPTIONS COMMAND 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. Separate instances are started for logged-in users to start their services. systemd is usually not invoked directly by the user, but is installed as the /sbin/init symlink and started during early boot. The user manager instances are started automatically through the user@.service5 service. For compatibility with SysV, if the binary is called as init and is not the first process on the machine (PID 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 telinit8 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.conf5 for more information. 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/, /proc/, and /dev/. systemd will also reset the system clock during early boot if it appears to be set incorrectly. See "System clock epoch" section below. Note that some but not all interfaces provided by systemd are covered by the Interface Portability and Stability Promise. The D-Bus API of systemd is described in org.freedesktop.systemd15 and org.freedesktop.LogControl15. Systems which invoke systemd in a container or initrd environment should implement the Container Interface or initrd Interface specifications, respectively. 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.unit5, however some are created automatically from other configuration files, dynamically from system state or programmatically at runtime. Units may be in a number of states, described in the following table. Note that the various unit types may have a number of additional substates, which are mapped to the generalized unit states described here. The following unit types are available: Service units, which start and control daemons and the processes they consist of. For details, see systemd.service5. Socket units, which encapsulate local IPC or network sockets in the system, useful for socket-based activation. For details about socket units, see systemd.socket5, for details on socket-based activation and other forms of activation, see daemon7. Target units are useful to group units, or provide well-known synchronization points during boot-up, see systemd.target5. Device units expose kernel devices in systemd and may be used to implement device-based activation. For details, see systemd.device5. Mount units control mount points in the file system, for details see systemd.mount5. Automount units provide automount capabilities, for on-demand mounting of file systems as well as parallelized boot-up. See systemd.automount5. Timer units are useful for triggering activation of other units based on timers. You may find details in systemd.timer5. Swap units are very similar to mount units and encapsulate memory swap partitions or files of the operating system. They are described in systemd.swap5. Path units may be used to activate other services when file system objects change or are modified. See systemd.path5. 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.slice5. Scope units are similar to service units, but manage foreign processes instead of starting them as well. See systemd.scope5. Units are named as their configuration files. Some units have special semantics. A detailed list is available in systemd.special7. 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.special7 for details about these target units. On first boot, systemd will enable or disable units according to preset policy. See systemd.preset5 and "First Boot Semantics" in machine-id5. systemd only keeps a minimal set of units loaded into memory. Specifically, the only units that are kept loaded into memory are those for which at least one of the following conditions is true: It is in an active, activating, deactivating or failed state (i.e. in any unit state except for inactive) It has a job queued for it It is a dependency of at least one other unit that is loaded into memory It has some form of resource still allocated (e.g. a service unit that is inactive but for which a process is still lingering that ignored the request to be terminated) It has been pinned into memory programmatically by a D-Bus call systemd will automatically and implicitly load units from disk — if they are not loaded yet — as soon as operations are requested for them. Thus, in many respects, the fact whether a unit is loaded or not is invisible to clients. Use systemctl list-units --all to comprehensively list all units currently loaded. Any unit for which none of the conditions above applies is promptly unloaded. Note that when a unit is unloaded from memory its accounting data is flushed out too. However, this data is generally not lost, as a journal log record is generated declaring the consumed resources whenever a unit shuts down. Processes systemd spawns are placed in individual Linux control groups named after the unit which they belong to in the private systemd hierarchy. (see Control Groups v2 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/), or in tools such as systemd-cgls1 or ps1 (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. Note that transactions are generated independently of a unit's state at runtime, hence, for example, if a start job is requested on an already started unit, it will still generate a transaction and wake up any inactive dependencies (and cause propagation of other jobs as per the defined relationships). This is because the enqueued job is at the time of execution compared to the target unit's state and is marked successful and complete when both satisfy. However, this job also pulls in other dependencies due to the defined relationships and thus leads to, in our example, start jobs for any of those inactive units getting queued as well. 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.generator7. 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 /usr/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 systemctl1 tool. Full list of directories is provided in systemd.unit5. User unit directories Similar rules apply for the user 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=systemduserunitdir. Global configuration is done in the directory reported by pkg-config systemd --variable=systemduserconfdir. The enable and disable commands of the systemctl1 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.unit5. 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. The service listens to various UNIX process signals that can be used to request various actions asynchronously. The signal handling is enabled very early during boot, before any further processes are invoked. However, a supervising container manager or similar that intends to request these operations via this mechanism must take into consideration that this functionality is not available during the earliest initialization phase. An sd_notify() notification message carrying the X_SYSTEMD_SIGNALS_LEVEL=2 field is emitted once the signal handlers are enabled, see below. This may be used to schedule submission of these signals correctly. 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-irreversibly. 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-irreversibly. If this signal is received more than 7 times per 2s, an immediate reboot is triggered. Note that pressing CtrlAltDel on the console will trigger this signal. Hence, if a reboot is hanging, pressing CtrlAltDel more than 7 times in 2 seconds 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-irreversibly. SIGRTMIN+4 Powers off the machine, starts the poweroff.target unit. This is mostly equivalent to systemctl start poweroff.target --job-mode=replace-irreversibly. SIGRTMIN+5 Reboots the machine, starts the reboot.target unit. This is mostly equivalent to systemctl start reboot.target --job-mode=replace-irreversibly. SIGRTMIN+6 Reboots the machine via kexec, starts the kexec.target unit. This is mostly equivalent to systemctl start kexec.target --job-mode=replace-irreversibly. SIGRTMIN+7 Reboots userspace, starts the soft-reboot.target unit. This is mostly equivalent to systemctl start soft-reboot.target --job-mode=replace-irreversibly. 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+17 Immediately reboots the userspace. SIGRTMIN+20 Enables display of status messages on the console, as controlled via systemd.show_status=1 on the kernel command line. You may want to use SetShowStatus() instead of SIGRTMIN+20 in order to prevent race conditions. See org.freedesktop.systemd15. SIGRTMIN+21 Disables display of status messages on the console, as controlled via systemd.show_status=0 on the kernel command line. You may want to use SetShowStatus() instead of SIGRTMIN+21 in order to prevent race conditions. See org.freedesktop.systemd15. SIGRTMIN+22 Sets the service manager's log level to debug, in a fashion equivalent to systemd.log_level=debug on the kernel command line. SIGRTMIN+23 Restores the log level to its configured value. The configured value is derived from – in order of priority – the value specified with systemd.log-level= on the kernel command line, or the value specified with LogLevel= in the configuration file, or the built-in default of info. SIGRTMIN+24 Immediately exits the manager (only available for --user instances). SIGRTMIN+25 Upon receiving this signal the systemd manager will reexecute itself. This is mostly equivalent to systemctl daemon-reexec except that it will be done asynchronously. The systemd system manager treats this signal the same way as SIGTERM. SIGRTMIN+26 Restores the log target to its configured value. The configured value is derived from – in order of priority – the value specified with systemd.log-target= on the kernel command line, or the value specified with LogTarget= in the configuration file, or the built-in default. SIGRTMIN+27 SIGRTMIN+28 Sets the log target to console on SIGRTMIN+27 (or kmsg on SIGRTMIN+28), in a fashion equivalent to systemd.log_target=console (or systemd.log_target=kmsg on SIGRTMIN+28) on the kernel command line. The environment block for the system manager is initially set by the kernel. (In particular, key=value assignments on the kernel command line are turned into environment variables for PID 1). For the user manager, the system manager sets the environment as described in the "Environment Variables in Spawned Processes" section of systemd.exec5. The DefaultEnvironment= setting in the system manager applies to all services including user@.service. Additional entries may be configured (as for any other service) through the Environment= and EnvironmentFile= settings for user@.service (see systemd.exec5). Also, additional environment variables may be set through the ManagerEnvironment= setting in systemd-system.conf5 and systemd-user.conf5. Some of the variables understood by systemd: $SYSTEMD_LOG_LEVEL This can be overridden with --log-level=. $SYSTEMD_LOG_COLOR This can be overridden with --log-color=. $SYSTEMD_LOG_TIME This can be overridden with --log-time=. $SYSTEMD_LOG_LOCATION This can be overridden with --log-location=. $SYSTEMD_LOG_TID $SYSTEMD_LOG_TARGET This can be overridden with --log-target=. $SYSTEMD_LOG_RATELIMIT_KMSG $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 to find its configuration. $SYSTEMD_UNIT_PATH $SYSTEMD_GENERATOR_PATH $SYSTEMD_ENVIRONMENT_GENERATOR_PATH Controls where systemd looks for unit files and generators. These variables may contain a list of paths, separated by colons (:). When set, if the list ends with an empty component (...:), this list is prepended to the usual set of paths. Otherwise, the specified list replaces the usual set of paths. $LISTEN_PID $LISTEN_FDS $LISTEN_FDNAMES Set by systemd for supervised processes during socket-based activation. See sd_listen_fds3 for more information. $NOTIFY_SOCKET Set by service manager for its services for status and readiness notifications. Also consumed by service manager for notifying supervising container managers or service managers up the stack about its own progress. See sd_notify3 and the relevant section below for more information. For further environment variables understood by systemd and its various components, see Known Environment Variables. When run as the system instance, systemd parses a number of options listed below. They can be specified as kernel command line arguments which are parsed from a number of sources depending on the environment in which systemd is executed. If run inside a Linux container, these options are parsed from the command line arguments passed 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 and from the SystemdOptions EFI variable (on EFI systems) instead. Options from /proc/cmdline have higher priority. Note: use of SystemdOptions is deprecated. The following variables are understood: 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.special7 for details about these units. The option prefixed with rd. is honored only in the 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 when it crashes. Defaults to disabled, meaning that no such switch is attempted. If set to enabled, the virtual terminal the kernel messages are written to is used instead. 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. Otherwise, no shell is spawned. Defaults to disabled, for security reasons, as the shell is not protected by password authentication. systemd.crash_action= Takes one of freeze, reboot or poweroff. Defaults to freeze. If set to freeze, the system will hang indefinitely when the system manager (PID 1) crashes. If set to reboot, the system manager (PID 1) will reboot the machine automatically when it crashes, after a 10s delay. If set to poweroff, the system manager (PID 1) will power off the machine immediately when it crashes. If combined with systemd.crash_shell, the configured crash action is executed 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.service5. 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 constants error and 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. With error, only messages about failures are shown, but boot is otherwise quiet. auto behaves like false until 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 error. If specified overrides the system manager configuration file option ShowStatus=, see systemd-system.conf5. systemd.status_unit_format= Takes name, description or combined as the value. If name, the system manager will use unit names in status messages. If combined, the system manager will use unit names and description in status messages. When specified, overrides the system manager configuration file option StatusUnitFormat=, see systemd-system.conf5. systemd.log_color systemd.log_level= systemd.log_location systemd.log_target= systemd.log_time systemd.log_tid systemd.log_ratelimit_kmsg Controls log output, with the same effect as the $SYSTEMD_LOG_COLOR, $SYSTEMD_LOG_LEVEL, $SYSTEMD_LOG_LOCATION, $SYSTEMD_LOG_TARGET, $SYSTEMD_LOG_TIME, $SYSTEMD_LOG_TID and $SYSTEMD_LOG_RATELIMIT_KMSG environment variables described above. systemd.log_color, systemd.log_location, systemd.log_time, systemd.log_tid and systemd.log_ratelimit_kmsg 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 and sockets. That is, controls the default for StandardOutput= and StandardError= (see systemd.exec5 for details). Takes one of inherit, null, tty, journal, journal+console, kmsg, kmsg+console. If the argument is omitted systemd.default-standard-output= defaults to journal and systemd.default-standard-error= to inherit. 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.set_credential= systemd.set_credential_binary= Sets a system credential, which can then be propagated to system services using the ImportCredential= or LoadCredential= setting, see systemd.exec5 for details. Takes a pair of credential name and value, separated by a colon. The systemd.set_credential= parameter expects the credential value in literal text form, the systemd.set_credential_binary= parameter takes binary data encoded in Base64. Note that the kernel command line is typically accessible by unprivileged programs in /proc/cmdline. Thus, this mechanism is not suitable for transferring sensitive data. Use it only for data that is not sensitive (e.g. public keys/certificates, rather than private keys), or in testing/debugging environments. For further information see System and Service Credentials documentation. systemd.import_credentials= Takes a boolean argument. If false disables importing credentials from the kernel command line, the DMI/SMBIOS OEM string table, the qemu_fw_cfg subsystem or the EFI kernel stub. quiet Turn off status output at boot, much like systemd.show_status=no 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.conf5 and locale7. For other kernel command line parameters understood by components of the core OS, please refer to kernel-command-line7. During initialization the service manager will import credentials from various sources into the system's set of credentials, which can then be propagated into services and consumed by generators: When the service manager first initializes it will read system credentials from SMBIOS Type 11 vendor strings io.systemd.credential:name=value, and io.systemd.credential.binary:name=value. At the same time it will import credentials from QEMU fw_cfg. (Note that the SMBIOS mechanism is generally preferred, because it is faster and generic.) Credentials may be passed via the kernel command line, using the systemd.set-credential= parameter, see above. Credentials may be passed from the UEFI environment via systemd-stub7. When the service manager is invoked during the initrd → host transition it will import all files in /run/credentials/@initrd/ as system credentials. Invoke systemd-creds1 as follows to see the list of credentials passed into the system: # systemd-creds --system list For further information see System and Service Credentials documentation. The service manager when run as PID 1 consumes the following system credentials: vmm.notify_socket Contains a AF_VSOCK or AF_UNIX address where to send a READY=1 notification message when the service manager has completed booting. See sd_notify3 and the next section for more information. Note that in case the hypervisor does not support SOCK_DGRAM over AF_VSOCK, SOCK_SEQPACKET will be tried instead. The credential payload for AF_VSOCK should be a string in the form vsock:CID:PORT. vsock-stream, vsock-dgram and vsock-seqpacket can be used instead of vsock to force usage of the corresponding socket type. This feature is useful for machine managers or other processes on the host to receive a notification via VSOCK when a virtual machine has finished booting. system.machine_id Takes a 128bit hexadecimal ID to initialize /etc/machine-id from, if the file is not set up yet. See machine-id5 for details. For a list of system credentials various other components of systemd consume, see systemd.system-credentials7. The service manager implements a readiness notification protocol both between the manager and its services (i.e. down the stack), and between the manager and a potential supervisor further up the stack (the latter could be a machine or container manager, or in case of a per-user service manager the system service manager instance). The basic protocol (and the suggested API for it) is described in sd_notify3. The notification socket the service manager (including PID 1) uses for reporting readiness to its own supervisor is set via the usual $NOTIFY_SOCKET environment variable (see above). Since this is directly settable only for container managers and for the per-user instance of the service manager, an additional mechanism to configure this is available, in particular intended for use in VM environments: the vmm.notify_socket system credential (see above) may be set to a suitable socket (typically an AF_VSOCK one) via SMBIOS Type 11 vendor strings. For details see above. The notification protocol from the service manager up the stack towards a supervisor supports a number of extension fields that allow a supervisor to learn about specific properties of the system and track its boot progress. Specifically the following fields are sent: An X_SYSTEMD_HOSTNAME=… message will be sent out once the initial hostname for the system has been determined. Note that during later runtime the hostname might be changed again programmatically, and (currently) no further notifications are sent out in that case. An X_SYSTEMD_MACHINE_ID=… message will be sent out once the machine ID of the system has been determined. See machine-id5 for details. An X_SYSTEMD_SIGNALS_LEVEL=… message will be sent out once the service manager installed the various UNIX process signal handlers described above. The field's value is an unsigned integer formatted as decimal string, and indicates the supported UNIX process signal feature level of the service manager. Currently, only a single feature level is defined: X_SYSTEMD_SIGNALS_LEVEL=2 covers the various UNIX process signals documented above – which are a superset of those supported by the historical SysV init system. Signals sent to PID 1 before this message is sent might not be handled correctly yet. A consumer of these messages should parse the value as an unsigned integer indication the level of support. For now only the mentioned level 2 is defined, but later on additional levels might be defined with higher integers, that will implement a superset of the currently defined behaviour. X_SYSTEMD_UNIT_ACTIVE=… and X_SYSTEMD_UNIT_INACTIVE=… messages will be sent out for each target unit as it becomes active or stops being active. This is useful to track boot progress and functionality. For example, once the ssh-access.target unit is reported started SSH access is typically available, see systemd.special7 for details. An X_SYSTEMD_SHUTDOWN=… message will be sent out very shortly before the system shuts down. The value is one of the strings reboot, halt, poweroff, kexec and indicates which kind of shutdown is being executed. An X_SYSTEMD_REBOOT_PARAMETER=… message will also be sent out very shortly before the system shuts down. Its value is the reboot argument as configured with systemctl --reboot-argument=…. Note that these extension fields are sent in addition to the regular READY=1 and RELOADING=1 notifications. systemd is only very rarely invoked directly, since it is started early and is already running by the time users may interact with it. Normally, tools like systemctl1 are used to give commands to the manager. Since systemd is usually not invoked directly, the options listed below are mostly useful for debugging and special purposes. Those options are used for testing and introspection, and systemd may be invoked with them at any time: --dump-configuration-items Dump understood unit configuration items. This outputs a terse but complete list of configuration items understood in unit definition files. --dump-bus-properties Dump exposed bus properties. This outputs a terse but complete list of properties exposed on D-Bus. --test Determine the initial start-up transaction (i.e. the list of jobs enqueued at start-up), dump it and exit — without actually executing any of the determined jobs. This option is useful for debugging only. Note that during regular service manager start-up additional units not shown by this operation may be started, because hardware, socket, bus or other kinds of activation might add additional jobs as the transaction is executed. Use --system to request the initial transaction of the system service manager (this is also the implied default), combine with --user to request the initial transaction of the per-user service manager instead. --system --user When used in conjunction with --test, selects whether to calculate the initial transaction for the system instance or for a per-user instance. These options have no effect when invoked without --test, as during regular (i.e. non---test) invocations the service manager will automatically detect whether it shall operate in system or per-user mode, by checking whether the PID it is run as is 1 or not. Note that it is not supported booting and maintaining a system with the service manager running in --system mode but with a PID other than 1. Those options correspond directly to options listed above in "Kernel Command Line". Both forms may be used equivalently for the system manager, but it is recommended to use the forms listed above in this context, because they are properly namespaced. When an option is specified both on the kernel command line and as a normal command line argument, the latter has higher precedence. When systemd is used as a user manager, the kernel command line is ignored and only the options described below are understood. Nevertheless, systemd is usually started in this mode through the user@.service5 service, which is shared between all users. It may be more convenient to use configuration files to modify settings (see systemd-user.conf5), or environment variables. See the "Environment" section above for a discussion of how the environment block is set. --unit= Set default unit to activate on startup. If not specified, defaults to default.target. See systemd.unit= above. --dump-core Enable core dumping on crash. This switch has no effect when running as user instance. Same as systemd.dump_core= above. --crash-vt=VT Switch to a specific virtual console (VT) on crash. This switch has no effect when running as user instance. Same as systemd.crash_chvt= above (but not the different spelling!). --crash-shell Run a shell on crash. This switch has no effect when running as user instance. See systemd.crash_shell= above. --crash-action= Specify what to do when the system manager (PID 1) crashes. This switch has no effect when systemd is running as user instance. See systemd.crash_action= above. --confirm-spawn Ask for confirmation when spawning processes. This switch has no effect when run as user instance. See systemd.confirm_spawn above. --show-status Show terse unit status information on the console during boot-up and shutdown. See systemd.show_status above. --log-color Highlight important log messages. See systemd.log_color above. --log-level= Set log level. See systemd.log_level above. --log-location Include code location in log messages. See systemd.log_location above. --log-target= Set log target. See systemd.log_target above. --log-time= Prefix console messages with timestamp. See systemd.log_time above. --machine-id= Override the machine-id set on the hard drive. See systemd.machine_id= above. --service-watchdogs Globally enable/disable all service watchdog timeouts and emergency actions. See systemd.service_watchdogs above. --default-standard-output= --default-standard-error= Sets the default output or error output for all services and sockets, respectively. See systemd.default_standard_output= and systemd.default_standard_error= above. When systemd is started or restarted, it may set the system clock to the "epoch". This mechanism is used to ensure that the system clock remains somewhat reasonably initialized and roughly monotonic across reboots, in case no battery-backed local RTC is available or it does not work correctly. The epoch is the lowest date above which the system clock time is assumed to be set correctly. When initializing, the local clock is advanced to the epoch if it was set to a lower value. As a special case, if the local clock is sufficiently far in the future (by default 15 years, but this can be configured at build time), the hardware clock is assumed to be broken, and the system clock is rewound to the epoch. The epoch is set to the highest of: the build time of systemd, the modification time ("mtime") of /usr/lib/clock-epoch, and the modification time of /var/lib/systemd/timesync/clock. /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_notify3. /run/systemd/private Used internally as communication channel between systemctl1 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. /usr/lib/clock-epoch The modification time ("mtime") of this file is used for the time epoch, see previous section. /var/lib/systemd/timesync/clock The modification time ("mtime") of this file is updated by systemd-timesyncd.service8. If present, the modification time of file is used for the epoch, see previous section. systemd 252 Kernel command-line arguments systemd.unified_cgroup_hierarchy and systemd.legacy_systemd_cgroup_controller were deprecated. Please switch to the unified cgroup hierarchy. The systemd Homepage systemd-system.conf5 locale.conf5 systemctl1 journalctl1 systemd-notify1 daemon7 sd-daemon3 org.freedesktop.systemd15 systemd.unit5 systemd.special7 pkg-config1 kernel-command-line7 bootup7 systemd.directives7 org.freedesktop.systemd15 For more information about the concepts and ideas behind systemd, please refer to the Original Design Document.