SYSTEMD(1)

📦

version 262.1 · 16 versions

💖 systemd(1) — systemd system and service manager 💖

✨ NAME ✨

systemd, init — systemd system and service manager. That's the whole vibe, bestie. 👑

✨ SYNOPSIS ✨

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

init [OPTIONS...]

okay so that's how you invoke it. No cap. 💅

✨ DESCRIPTION ✨

okay so systemd is literally the system and service manager for Linux operating systems. When it runs as the first process on boot — as PID 1, no cap — it acts as the init system that brings up and maintains userspace services. And for logged-in users? Separate instances get started to start their services too. 💅

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@.service(5) service. 🦋

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. 📖

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. ⏰

Heads up: some but not all interfaces provided by systemd are covered by the Interface Portability and Stability Promise[1]. 🙈

The D-Bus API of systemd is described in org.freedesktop.systemd1(5) and org.freedesktop.LogControl1(5). 🔥

Systems which invoke systemd in a container or initrd environment should implement the Container Interface[2] or initrd Interface[3] specifications, respectively. Periodt. 💅

✨ UNITS ✨

okay so 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 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. 👑

Table 1. Unit ACTIVE states

active — Started, bound, plugged in, ..., depending on the unit type.
inactive — Stopped, unbound, unplugged, ..., depending on the unit type.
failed — Similar to inactive, but the unit failed in some way (process returned error code on exit, crashed, an operation timed out, or after too many restarts).
activating — Changing from inactive to active.
deactivating — Changing from active to inactive.
maintenance — Unit is inactive and a maintenance operation is in progress.
reloading — Unit is active and it is reloading its configuration.
refreshing — Unit is active and a new mount is being activated in its namespace.

The following unit types are available:

Service units, which start and control daemons and the processes they consist of. For details, see systemd.service(5).
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).
Target units are 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 provide automount capabilities, for on-demand mounting of file systems as well as parallelized boot-up. See systemd.automount(5).
Timer units are useful for triggering activation of other units based on timers. You may find details in systemd.timer(5).
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).
Path units may be used to activate other services when file system objects change or are modified. See systemd.path(5).
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).
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. 👑

On first boot, systemd will enable or disable units according to preset policy. See systemd.preset(5) and "First Boot Semantics" in machine-id(5). ✨

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[4] 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-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.). 🦋

okay so bestie, before we get into the wild stuff, lemme just say: systemd is fully compatible with a bunch of the classic Unix stuff you already know and love — think /etc/fstab, think the utmp database. it plays nice. it's not trying to reinvent your whole life. 🫶

THE TRANSACTION SYSTEM (aka systemd does a vibe check before it commits)

systemd runs a minimal transaction system, and honestly it's giving careful bestie energy. here's the tea ☕:

whenever a unit is asked to start up or shut down, systemd adds that unit and all its dependencies to a temporary transaction. then it checks whether the transaction is consistent — which basically means: is the ordering of all the units cycle-free? no loops, no circular drama. 🔄🚫

if it's not consistent, systemd doesn't just give up, it tries to fix it up — it removes non-essential jobs from the transaction that might be causing the loop. it also tries to suppress non-essential jobs that would stop a running service (rude, why would we do that to a working service 💀). finally it checks whether the jobs in the transaction contradict jobs that have already been queued, and if so, it optionally aborts the whole transaction.

and if everything worked out — transaction consistent, impact minimized — it gets merged with all the already-outstanding jobs and added to the run queue. ✨

so effectively: before executing anything you asked for, systemd verifies that it actually makes sense, fixes it if it can, and only fails if it really cannot work. main character energy, honestly. 👑

⚠️ heads up though: transactions are generated independently of a unit's state at runtime. so for example, if you request a start job on a unit that's already started, it will still generate a transaction and wake up any inactive dependencies (and cause propagation of other jobs as per the defined relationships). why?? because the enqueued job gets compared to the target unit's state at the time of execution, and it's marked successful and complete when both satisfy. but — this job also pulls in other dependencies due to the defined relationships, which means, in our example, start jobs for any of those inactive units get queued too. it's giving "i'm just gonna do a little extra, as a treat" 🙈

also! units can be generated dynamically at boot and at system manager reload time — for example, based on other configuration files or parameters passed on the kernel command line. for the full deets on that, go read systemd.generator(7). 🧚

✨ DIRECTORIES ✨

💖 System unit directories

the systemd system manager reads unit configuration from a whole bunch of different directories, so let's map them out:

if you're a package and you wanna install unit files, you put them in the directory returned by pkg-config systemd --variable=systemdsystemunitdir
other directories that get checked are /usr/local/lib/systemd/system and /usr/lib/systemd/system
user configuration always takes precedence 👑 (yes, always)
pkg-config systemd --variable=systemdsystemconfdir hands you the path of the system configuration directory
and packages should only mess with the content of these directories using the enable and disable commands of the systemctl(1) tool. like, don't just go in there with a shovel, bestie 🪓
the full list of directories lives in systemd.unit(5)

🌸 User unit directories

similar rules apply for the user unit directories — however! here, the XDG Base Directory specification[5] is followed to find units. so it's a slightly different hunt. 🔍

applications should drop their unit files in the directory returned by pkg-config systemd --variable=systemduserunitdir
global configuration happens in the directory reported by pkg-config systemd --variable=systemduserconfdir
the enable and disable commands of systemctl(1) can handle both global (i.e. for all users) and private (for one user) enabling/disabling of units. versatile icon 💅
and again, the full list of directories is in systemd.unit(5)

💀 SIGNALS 💀

the service listens to a bunch of UNIX process signals that you can use to request various actions asynchronously. so if you're the type to yeet a signal instead of typing a command, this is your section. 😌

here's the important bit though: signal handling is enabled very early during boot — before any further processes are invoked. BUT if you're a supervising container manager or similar and you're planning to request these operations via this mechanism, you gotta take into consideration that this functionality is not available during the earliest initialization phase. 🙅

so how do you know when it's safe? an sd_notify() notification message carrying the X_SYSTEMD_SIGNALS_LEVEL=2 field is emitted once the signal handlers are enabled — see below. use that to schedule your signal submissions correctly. schedule your slay. 🎀

`SIGTERM`

when the systemd system manager receives this signal, it 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 they get this signal. mostly equivalent to:

systemctl --user start exit.target --job-mode=replace-irreversibly

`SIGINT`

when the systemd system manager receives this, it starts the ctrl-alt-del.target unit. mostly equivalent to:

systemctl start ctrl-alt-del.target --job-mode=replace-irreversibly

now here's the spicy part 🌶️ — if this signal is received more than 7 times per 2s, an immediate reboot is triggered. and note that pressing Ctrl+Alt+Del on the console will trigger this signal. so hence: if a reboot is hanging, pressing Ctrl+Alt+Del more than 7 times in 2 seconds is a relatively safe way to trigger an immediate reboot. 🚨 (yeah, button mashing is a legitimate technique here, no cap)

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. mostly equivalent to:

systemctl start kbrequest.target

this signal is ignored by systemd user managers. they simply do not care 💅

`SIGPWR`

when this signal is received, the systemd manager will start the sigpwr.target unit. 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. so it's the "spill the whole diary" signal 📖

`SIGHUP`

reloads the complete daemon configuration. mostly equivalent to:

systemctl daemon-reload

`SIGRTMIN+0`

enters default mode, starts the default.target unit. mostly equivalent to:

systemctl isolate default.target

`SIGRTMIN+1`

enters rescue mode, starts the rescue.target unit. mostly equivalent to:

systemctl isolate rescue.target

`SIGRTMIN+2`

enters emergency mode, starts the emergency.service unit. mostly equivalent to:

systemctl isolate emergency.service

`SIGRTMIN+3`

halts the machine, starts the halt.target unit. mostly equivalent to:

systemctl start halt.target --job-mode=replace-irreversibly

`SIGRTMIN+4`

powers off the machine, starts the poweroff.target unit. mostly equivalent to:

systemctl start poweroff.target --job-mode=replace-irreversibly

`SIGRTMIN+5`

reboots the machine, starts the reboot.target unit. mostly equivalent to:

systemctl start reboot.target --job-mode=replace-irreversibly

`SIGRTMIN+6`

reboots the machine via kexec, starts the kexec.target unit. mostly equivalent to:

systemctl start kexec.target --job-mode=replace-irreversibly

`SIGRTMIN+7`

reboots userspace, starts the soft-reboot.target unit. mostly equivalent to:

systemctl start soft-reboot.target --job-mode=replace-irreversibly

added in version 254. 🆕

`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. ⚡

added in version 254. 🆕

`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.systemd1(5).

`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.systemd1(5).

`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".

added in version 239. 🆕

`SIGRTMIN+24`

immediately exits the manager (only available for --user instances).

added in version 195. 🆕

`SIGRTMIN+25`

when this signal is received, the systemd manager will reexecute itself. 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.

added in version 250. 🆕

`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.

added in version 239. 🆕

`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.

added in version 239. 🆕

🌸 ENVIRONMENT 🌸

okay bestie, quick vibe check on where systemd actually gets its environment from, because it's giving layers 💅

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.exec(5). 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.exec(5)). Also, additional environment variables may be set through the ManagerEnvironment= setting in systemd-system.conf(5) and systemd-user.conf(5).

Some of the variables understood by systemd:

$SYSTEMD_LOG_LEVEL — the max log level of messages it'll actually emit (anything with a higher log level, aka less important, gets yeeted 🗑️). Takes a comma-separated list of values. A value may be either one of (in order of decreasing importance) emerg, alert, crit, err, warning, notice, info, debug, or an integer in the range 0...7. See syslog(3) for more. Each value may optionally be prefixed with one of console, syslog, kmsg or journal followed by a colon to set the max log level for that specific log target (e.g. SYSTEMD_LOG_LEVEL=debug,console:info specifies to log at debug level except when logging to the console which should be at info level). Note that the global maximum log level takes priority over any per-target maximum log levels. This can be overridden with --log-level=.
$SYSTEMD_LOG_COLOR — a boolean. If true, messages written to the tty will be colored according to priority 🎨. This can be overridden with --log-color=.
$SYSTEMD_LOG_TIME — a boolean. If true, console log messages will be prefixed with a timestamp ⏰. This can be overridden with --log-time=. Added in version 246.
$SYSTEMD_LOG_LOCATION — a boolean. If true, messages will be prefixed with a filename and line number in the source code where the message originates 📍. This can be overridden with --log-location=.
$SYSTEMD_LOG_TID — a boolean. If true, messages will be prefixed with the current numerical thread ID (TID) 🧵. Added in version 247.
$SYSTEMD_LOG_TARGET — the destination for log messages. One of console (log to the attached tty), console-prefixed (log to the attached tty but with prefixes encoding the log level and "facility", see syslog(3)), kmsg (log to the kernel circular log buffer), journal (log to the journal), journal-or-kmsg (log to the journal if available, and to kmsg otherwise), auto (determine the appropriate log target automatically, the default), null (disable log output). This can be overridden with --log-target=.
$SYSTEMD_LOG_RATELIMIT_KMSG — whether to ratelimit kmsg or not. Takes a boolean. Defaults to "true". If disabled, systemd will not ratelimit messages written to kmsg. Added in version 254.
$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[5] 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. ⚠️
$SYSTEMD_PAGER, $PAGER — pager to use when --no-pager is not given. $SYSTEMD_PAGER is used if set; otherwise $PAGER is used. If neither $SYSTEMD_PAGER nor $PAGER are set, a set of well-known pager implementations is tried in turn, including less(1) and more(1), until one is found. If no pager implementation is discovered, no pager is invoked. Setting those environment variables to an empty string or the value "cat" is equivalent to passing --no-pager. 📄

Note: if $SYSTEMD_PAGERSECURE is not set, $SYSTEMD_PAGER and $PAGER can only be used to disable the pager (with "cat" or ""), and are otherwise ignored. 💀

$SYSTEMD_LESS — override the options passed to less (by default "FRSXMK"). Users might want to change two options in particular:
- K — this option instructs the pager to exit immediately when Ctrl+C is pressed. To allow less to handle Ctrl+C itself to switch back to the pager command prompt, unset this option. If the value of $SYSTEMD_LESS does not include "K", and the pager that is invoked is less, Ctrl+C will be ignored by the executable, and needs to be handled by the pager.
- X — this option instructs the pager to not send termcap initialization and deinitialization strings to the terminal. It is set by default to allow command output to remain visible in the terminal even after the pager exits. Nevertheless, this prevents some pager functionality from working, in particular paged output cannot be scrolled with the mouse 🖱️.

Note that setting the regular $LESS environment variable has no effect for less invocations by systemd tools. See less(1) for more discussion.

$SYSTEMD_LESSCHARSET — override the charset passed to less (by default "utf-8", if the invoking terminal is determined to be UTF-8 compatible). Note that setting the regular $LESSCHARSET environment variable has no effect for less invocations by systemd tools. 🔤
$SYSTEMD_PAGERSECURE — okay this one is important bestie, listen up. Common pager commands like less(1), in addition to "paging", i.e. scrolling through the output, support opening of or writing to other files and running arbitrary shell commands. When commands are invoked with elevated privileges, for example under sudo(8) or pkexec(1), the pager becomes a security boundary 🚨. Care must be taken that only programs with strictly limited functionality are used as pagers, and unintended interactive features like opening or creation of new files or starting of subprocesses are not allowed. "Secure mode" for the pager may be enabled as described below, if the pager supports that (most pagers are not written in a way that takes this into consideration). It is recommended to either explicitly enable "secure mode" or to completely disable the pager using --no-pager or PAGER=cat when allowing untrusted users to execute commands with elevated privileges.

This option takes a boolean argument. When set to true, the "secure mode" of the pager is enabled. In "secure mode", LESSSECURE=1 will be set when invoking the pager, which instructs the pager to disable commands that open or create new files or start new subprocesses. Currently only less(1) is known to understand this variable and implement "secure mode".

When set to false, no limitation is placed on the pager. Setting SYSTEMD_PAGERSECURE=0 or not removing it from the inherited environment may allow the user to invoke arbitrary commands. 🙈

When $SYSTEMD_PAGERSECURE is not set, systemd tools attempt to automatically figure out if "secure mode" should be enabled and whether the pager supports it. "Secure mode" is enabled if the effective UID is not the same as the owner of the login session, see geteuid(2) and sd_pid_get_owner_uid(3), or when running under sudo(8) or similar tools ($SUDO_UID is set [6]). In those cases, SYSTEMD_PAGERSECURE=1 will be set and pagers which are not known to implement "secure mode" will not be used at all. Note that this autodetection only covers the most common mechanisms to elevate privileges and is intended as convenience. It is recommended to explicitly set $SYSTEMD_PAGERSECURE or disable the pager.

Note that if the $SYSTEMD_PAGER or $PAGER variables are to be honoured, other than to disable the pager, $SYSTEMD_PAGERSECURE must be set too.

$SYSTEMD_COLORS — takes a boolean argument, or a special value. By default (unset), systemd and related utilities will use colors in their output if possible. If $COLORTERM is set to "truecolor" or "24bit", 24-bit colors will be enabled, 256 colors otherwise, unless $NO_COLOR or $TERM indicates colors are disabled. 🌈
- true — same as unset, except that $NO_COLOR is ignored.
- false — the output will be monochrome.
- "16", "256", "24bit" — always use the base 16 ANSI colors, 256 colors, or 24 bit color, respectively.
- "auto-16", "auto-256", "auto-24bit" — use the given quantity of colours, subject to $TERM, and what the console is connected to.
$SYSTEMD_URLIFY — the value must be a boolean. Controls whether clickable links should be generated in the output for terminal emulators supporting this. This can be specified to override the decision that systemd makes based on $TERM and other conditions. 🔗
$LISTEN_PID, $LISTEN_PIDFDID, $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 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_notify(3) and the relevant section below for more information. 📬

For further environment variables understood by systemd and its various components, see Known Environment Variables[7].

👑 KERNEL COMMAND LINE 👑

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 instead. 🐧

The following variables are understood:

systemd.unit=, rd.systemd.unit= — overrides which unit gets activated on boot, bestie. Defaults to default.target. Use this when you wanna temporarily boot into a different boot unit, like rescue.target or emergency.service — peep systemd.special(7) for the tea on those units. The one with the rd. prefix is only honored in the initrd, and the one without the prefix is only honored in the main system. ✨
systemd.dump_core — takes a boolean argument, or just write it bare and it's enabled. If it's on, the systemd manager (PID 1) dumps core when it crashes. If not, no core dump gets created. Defaults to enabled.

Added in version 233.

systemd.crash_chvt — takes a positive integer, or a boolean. Can also be written with no argument at all, which hits the same as a positive boolean. If you give it a positive integer (range 1–63), the system manager (PID 1) will activate that virtual terminal when it crashes. Defaults to disabled, meaning no such switch is attempted. If you set it to enabled, the virtual terminal that kernel messages are written to is used instead.

Added in version 233.

systemd.crash_shell — takes a boolean, or bare = enabled. If on, the system manager (PID 1) spawns a shell when it crashes. Otherwise, no shell. Defaults to disabled, for security reasons — because that shell is not protected by password authentication. 💀

Added in version 233.

systemd.crash_action= — takes one of "freeze", "reboot" or "poweroff". Defaults to "freeze". If it's "freeze", the system hangs indefinitely when the system manager (PID 1) crashes. If it's "reboot", the system manager (PID 1) reboots the machine automatically on crash, after a 10s delay. If it's "poweroff", the system manager (PID 1) powers the machine off immediately when it crashes. And if you combine it with systemd.crash_shell, the configured crash action runs after the shell exits.

Added in version 256.

systemd.confirm_spawn — takes a boolean, or a path to the virtual console where the confirmation messages should go. Can also be written bare, same effect as a positive boolean. If enabled, the system manager (PID 1) asks for confirmation when spawning processes using /dev/console. If you hand it a path or a console name (like "ttyS0"), that virtual console gets used instead. Defaults to disabled.

Added in version 233.

systemd.service_watchdogs= — takes a boolean. If disabled, all service runtime watchdogs (WatchdogSec=) and emergency actions (e.g. OnFailure= or StartLimitAction=) are ignored by the system manager (PID 1) — go read systemd.service(5). Defaults to enabled, i.e. watchdogs and failure actions get processed like normal. Heads up: the hardware watchdog is not affected by this option.

Added in version 237.

systemd.show_status — takes a boolean, or the constants error and auto. Can also be written bare, 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 show up and boot is otherwise quiet. auto behaves like false until there's a significant delay in boot. Defaults to enabled — unless quiet is passed as a kernel command line option, in which case it defaults to error. If you specify it, it overrides the system manager configuration file option ShowStatus=, see systemd-system.conf(5).

Added in version 233.

systemd.status_unit_format= — takes name, description or combined as the value. If name, the system manager uses unit names in status messages. If combined, it uses unit names and description in status messages. When specified, it overrides the system manager configuration file option StatusUnitFormat=, see systemd-system.conf(5).

Added in version 243.

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, 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. And systemd.log_color, systemd.log_location, systemd.log_time, systemd.log_tid and systemd.log_ratelimit_kmsg can all be specified without an argument, with the same effect as a positive boolean. 🎀
systemd.default_standard_output=, systemd.default_standard_error= — controls the default standard output and error output for services and sockets. I.e. it controls the default for StandardOutput= and StandardError= (see systemd.exec(5) for the 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. Use it to set default environment variables that get added to forked child processes. You can use it 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 you want the same machine-id every single boot.

Added in version 229.

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.exec(5) for the 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, while the systemd.set_credential_binary= parameter takes binary data encoded in Base64.

⚠️ Not me about to say this loudly: the kernel command line is typically accessible by unprivileged programs in /proc/cmdline. So 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. fr fr.

For further information see System and Service Credentials[8] documentation.

Added in version 251.

systemd.import_credentials= — takes a boolean argument. If false, it disables importing credentials from the kernel command line, the DMI/SMBIOS OEM string table, the qemu_fw_cfg subsystem or the EFI kernel stub.

Added in version 251.

systemd.credentials_boot_policy= — controls under which conditions a credential encrypted with a "null" key (i.e. a credential wrapped in a systemd-creds envelope but offering neither confidentiality nor authenticity, see systemd-creds(1)) is accepted at boot. This gives you a fallback for provisioning credentials before a machine has a key to encrypt them against — like during first boot, or on systems with no TPM2 device. Takes one of strict, tofu, relaxed or off:
- strict — never accepts a "null" key, i.e. always insists on proper encryption.
- tofu (trust on first use) — accepts a "null" key during first boot, or when no TPM2 device is available.
- relaxed — accepts a "null" key when SecureBoot is disabled, or when no TPM2 device is available.
- off — always accepts a "null" key.

Defaults to relaxed. Note that this policy only applies to the default acceptance decision; it has no effect on credentials encrypted against a host key or TPM2 device, which are always accepted. 💅

For further information see System and Service Credentials[8] documentation.

Added in version 262.

quiet — turns 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. So passing this option turns off the usual output from both the system manager and the kernel. 😌

Added in version 186.

debug — turns 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. So passing this option turns on the debug output from both the system manager and the kernel. 🐛✨

Added in version 205.

emergency, rd.emergency — boot into emergency mode. Those options are equivalent to systemd.unit=emergency.target or rd.systemd.unit=emergency.target, respectively. They were initially provided for compatibility with SysV, but they're kept around as convenient shortcuts.

Added in version 186.

rescue, rd.rescue, single, 1 — boot into rescue mode. Those options are equivalent to either systemd.unit=rescue.target or rd.systemd.unit=rescue.target. They were initially provided for compatibility with SysV, but they're kept around as convenient shortcuts.

Added in version 186.

3, 5 — shortcuts to boot into specific targets: 3 is equivalent to systemd.unit=multi-user.target and 5 is equivalent to systemd.unit=graphical.target. Those options were initially provided for compatibility with SysV, but they're kept around as convenient shortcuts.

Added in version 186.

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 info, see locale.conf(5) and locale(7). 🌍

Added in version 186.

systemd.minimum_uptime_sec= — takes a time in seconds. Specifies the minimum uptime of the system before the system shuts down. For more information see the MinimumUptimeSec= setting described in systemd-system.conf(5).

Added in version 261.

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

💖 SYSTEM CREDENTIALS 💖

ok so when the service manager boots up it's gonna import credentials from a whole bunch of places into the system's credential set, and then it can pass them along into services and let generators snack on them too 🍓. here's every single source, no skips:

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". (heads up tho: 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-stub(7).
When the service manager is invoked during the initrd → host transition it will import all files in /run/credentials/@initrd/ as system credentials.

want to see the list of credentials that got passed into the system? invoke systemd-creds(1) like this:

# systemd-creds --system list

for further information see System and Service Credentials[8] documentation.

and when the service manager is running as PID 1, it consumes these system credentials, bestie:

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_notify(3) 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.

Added in version 254.

system.machine_id — Takes a 128bit hexadecimal ID to initialize /etc/machine-id from, if the file is not set up yet. See machine-id(5) for details.

Added in version 254.

for a list of system credentials various other components of systemd consume, see systemd.system-credentials(7).

✨ READINESS PROTOCOL ✨

so the service manager implements a readiness notification protocol in both directions, iconic behavior honestly 👑

between the manager and its services (i.e. down the stack), and
between the manager and a potential supervisor further up the stack — that supervisor could be a machine or container manager, or, in the 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_notify(3).

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, there's an additional mechanism to configure this 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.

now the glow-up part ✨: the notification protocol going up the stack from the service manager towards a supervisor supports a bunch of extension fields, so a supervisor can learn about specific properties of the system and track its boot progress. Specifically, these fields get 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.

Added in version 256.

An X_SYSTEMD_MACHINE_ID=... message will be sent out once the machine ID of the system has been determined. See machine-id(5) for details.

Added in version 256.

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 that indicates 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.

Added in version 256.

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.special(7) for details.

Added in version 256.

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.

Added in version 256.

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=....

Added in version 256.

ngl the important bit: these extension fields are sent in addition to the regular "READY=1" and "RELOADING=1" notifications. both. always. periodt 💅

✨ OPTIONS ✨

okay so real talk: you are basically never going to run systemd yourself 💀. It boots up mad early and it's already chillin' in the background by the time you even get a shell. Usually you just poke it with tools like systemctl(1). So since calling systemd directly is like, a special occasion thing, the options below are mostly for debugging and other niche purposes. iykyk.

🧐 Introspection and debugging options

These ones are for testing and snooping on what's going on inside. You can call systemd with them literally whenever:

--dump-configuration-items — dumps all the unit config items it understands. it's giving terse but complete: a full list of every configuration item it knows about in unit definition files.
--dump-bus-properties — dumps the bus properties it exposes. again, terse but complete — every property it puts out on D-Bus.

Added in version 239.

--test — figures out the initial start-up transaction (aka the list of jobs enqueued at start-up), dumps it, and dips — without actually running any of those jobs. purely a debugging thing bestie. heads up: during a regular service manager start-up, extra units that this doesn't show might get started anyway, because hardware, socket, bus or other kinds of activation can pile on more jobs while the transaction runs. use --system to ask for the initial transaction of the system service manager (that's also the implied default), or pair it with --user to get the initial transaction of the per-user service manager instead.
--system, --user — when you use these with --test, they pick whether to calculate the initial transaction for the system instance or for a per-user instance. Without --test these do absolutely nothing, because during regular (i.e. non---test) invocations the service manager just auto-detects whether it's in system or per-user mode by checking if the PID it's running as is 1 or not. and lowkey important: it is not supported to boot and maintain a system with the service manager running in --system mode but with a PID other than 1. periodt.
-h, --help — prints a short help text and exits. 🎀
--version — prints a short version string and exits.

🔁 Options that duplicate kernel command line settings

These options map directly onto the options listed up in "Kernel Command Line". Both forms work the same for the system manager, but pls use the forms listed above in that context, because they're properly namespaced. And if an option shows up both on the kernel command line and as a normal command line argument, the command line argument wins — higher precedence, no debate 💅.

When systemd is used as a user manager, the kernel command line is ignored entirely and only the options below are understood. That said, systemd is usually started in this mode through the user@.service(5) service, which is shared between all users. It might be comfier to use configuration files to tweak settings (see systemd-user.conf(5)), or environment variables. Scroll up to the "Environment" section for a whole discussion of how the environment block gets set up.

--unit= — sets the default unit to activate on startup. If you don't specify it, it defaults to default.target. See systemd.unit= above.
--dump-core — enables core dumping on crash. This switch does nothing when running as a user instance. Same as systemd.dump_core= above.
--crash-vt=VT — switches to a specific virtual console (VT) on crash. No effect when running as a user instance. Same as systemd.crash_chvt= above (but not the different spelling!).

Added in version 227.

--crash-shell — runs a shell on crash. No effect when running as a user instance. See systemd.crash_shell= above.
--crash-action= — specifies what to do when the system manager (PID 1) crashes. No effect when systemd is running as a user instance. See systemd.crash_action= above.

Added in version 256.

--confirm-spawn — asks for confirmation when spawning processes. No effect when run as a user instance. See systemd.confirm_spawn above.
--show-status — shows terse unit status info on the console during boot-up and shutdown. See systemd.show_status above.

Added in version 244.

--log-color — highlights important log messages. See systemd.log_color above.

Added in version 244.

--log-level= — sets the log level. See systemd.log_level above.
--log-location — includes code location in log messages. See systemd.log_location above.

Added in version 244.

--log-target= — sets the log target. See systemd.log_target above.
--log-time= — prefixes console messages with a timestamp. See systemd.log_time above.

Added in version 246.

--machine-id= — overrides the machine-id set on the hard drive. See systemd.machine_id= above.

Added in version 229.

--service-watchdogs — globally enables/disables all service watchdog timeouts and emergency actions. See systemd.service_watchdogs above.

Added in version 237.

--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.

⏰ SYSTEM CLOCK EPOCH ⏰

so when systemd starts or restarts, it may set the system clock to the "epoch". this whole mechanism exists to keep the system clock somewhat reasonably initialized and roughly monotonic across reboots, in case there's no battery-backed local RTC available, or the one you've got is just not doing its job 😭.

The epoch is the lowest date above which the system clock time is assumed to be set correctly. When initializing, the local clock gets advanced to the epoch if it was set to a lower value. Special case bestie: 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 gets 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.

📁 FILES 📁

/run/systemd/notify — daemon status notification socket. This is an AF_UNIX datagram socket and it's used to implement the daemon notification logic as implemented by sd_notify(3).
/run/systemd/private — used internally as the 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. like, hands off 🫶.
/usr/lib/clock-epoch — the modification time ("mtime") of this file is used for the time epoch, see the previous section.

Added in version 247.

/var/lib/systemd/timesync/clock — the modification time ("mtime") of this file is updated by systemd-timesyncd.service(8). If present, the modification time of this file is used for the epoch, see the previous section.

Added in version 257.

📜 HISTORY 📜

systemd 252 — the kernel command-line arguments systemd.unified_cgroup_hierarchy and systemd.legacy_systemd_cgroup_controller were deprecated. Please switch to the unified cgroup hierarchy. it's giving "move on, babe" ✨

For more info about the concepts and ideas behind systemd, pls refer to the Original Design Document[10]. 🦋

📝 NOTES 📝

Interface Portability and Stability Promise https://systemd.io/PORTABILITY_AND_STABILITY/
Container Interface https://systemd.io/CONTAINER_INTERFACE
initrd Interface https://systemd.io/INITRD_INTERFACE/
Control Groups v2 https://docs.kernel.org/admin-guide/cgroup-v2.html
XDG Base Directory specification https://standards.freedesktop.org/basedir-spec/basedir-spec-latest.html
It is recommended for other tools to set and check $SUDO_UID as appropriate, treating it is a common interface.
Known Environment Variables https://systemd.io/ENVIRONMENT
System and Service Credentials https://systemd.io/CREDENTIALS
systemd Homepage https://systemd.io/
Original Design Document https://0pointer.de/blog/projects/systemd.html

and that's the whole page bestie, we made it to the end 🥹💖 this absolute icon of a manpage is systemd version 262.1, filed under SYSTEMD(1). go forth and slay your services, mwah 💅🎀✨

📦 where this page comes from

arch rolling: systemd 262.1-1 · LGPL-2.1-or-later AND CC0-1.0 AND GPL-2.0-or-later AND MIT-0
💖 girlified by deepseek-ai/DeepSeek-V4.1-Flash on 2026-10-19: a language model's rewrite of the original, under its terms

SYSTEMD(1)

💖 systemd(1) — systemd system and service manager 💖

✨ NAME ✨

✨ SYNOPSIS ✨

✨ DESCRIPTION ✨

✨ UNITS ✨

Table 1. Unit ACTIVE states

THE TRANSACTION SYSTEM (aka systemd does a vibe check before it commits)

✨ DIRECTORIES ✨

💖 System unit directories

🌸 User unit directories

💀 SIGNALS 💀

SIGTERM

SIGINT

SIGWINCH

SIGPWR

SIGUSR1

SIGUSR2

SIGHUP

SIGRTMIN+0

SIGRTMIN+1

SIGRTMIN+2

SIGRTMIN+3

SIGRTMIN+4

SIGRTMIN+5

SIGRTMIN+6

SIGRTMIN+7

SIGRTMIN+13

SIGRTMIN+14

SIGRTMIN+15

SIGRTMIN+16

SIGRTMIN+17

SIGRTMIN+20

SIGRTMIN+21

SIGRTMIN+22

SIGRTMIN+23

SIGRTMIN+24

SIGRTMIN+25

SIGRTMIN+26

SIGRTMIN+27, SIGRTMIN+28

🌸 ENVIRONMENT 🌸

👑 KERNEL COMMAND LINE 👑

💖 SYSTEM CREDENTIALS 💖

✨ READINESS PROTOCOL ✨

✨ OPTIONS ✨

🧐 Introspection and debugging options

🔁 Options that duplicate kernel command line settings

⏰ SYSTEM CLOCK EPOCH ⏰

📁 FILES 📁

📜 HISTORY 📜

👀 SEE ALSO 👀

📝 NOTES 📝

`SIGTERM`

`SIGINT`

`SIGWINCH`

`SIGPWR`

`SIGUSR1`

`SIGUSR2`

`SIGHUP`

`SIGRTMIN+0`

`SIGRTMIN+1`

`SIGRTMIN+2`

`SIGRTMIN+3`

`SIGRTMIN+4`

`SIGRTMIN+5`

`SIGRTMIN+6`

`SIGRTMIN+7`

`SIGRTMIN+13`

`SIGRTMIN+14`

`SIGRTMIN+15`

`SIGRTMIN+16`

`SIGRTMIN+17`

`SIGRTMIN+20`

`SIGRTMIN+21`

`SIGRTMIN+22`

`SIGRTMIN+23`

`SIGRTMIN+24`

`SIGRTMIN+25`

`SIGRTMIN+26`

`SIGRTMIN+27`, `SIGRTMIN+28`