NAME

systemd, init - systemd system and service manager

SYNOPSIS

DESCRIPTION

systemd is a system and service manager for Linux operating systems. When run as first process on boot (as PID 1), it acts as init system that brings up and maintains userspace services. For compatibility with SysV, if systemd is called as init and a PID that is not 1, it will execute telinit and pass all command line arguments unmodified. That means init and telinit are mostly equivalent when invoked from normal login sessions. See telinit(8) for more information. When run as a system instance, systemd interprets the configuration file system.conf and the files in system.conf.d directories; when run as a user instance, systemd interprets the configuration file user.conf and the files in user.conf.d directories. See systemd-system.conf(5) for more information.

OPTIONS

The following options are understood: --test --dump-configuration-items --unit= --system, --user --dump-core --crash-vt=VT --crash-shell --crash-reboot --confirm-spawn --show-status= --log-target= --log-level= --log-color= --log-location= --default-standard-output=, --default-standard-error= --machine-id= --service-watchdogs= -h, --help --version

CONCEPTS

systemd provides a dependency system between various entities called "units" of 11 different types. Units encapsulate various objects that are relevant for system boot-up and maintenance. The majority of units are configured in unit configuration files, whose syntax and basic set of options is described in systemd.unit(5), however some are created automatically from other configuration, dynamically from system state or programmatically at runtime. Units may be "active" (meaning started, bound, plugged in, ..., depending on the unit type, see below), or "inactive" (meaning stopped, unbound, unplugged, ...), as well as in the process of being activated or deactivated, i.e. between the two states (these states are called "activating", "deactivating"). A special "failed" state is available as well, which is very similar to "inactive" and is entered when the service failed in some way (process returned error code on exit, or crashed, an operation timed out, or after too many restarts). If this state is entered, the cause will be logged, for later reference. Note that the various unit types may have a number of additional substates, which are mapped to the five generalized unit states described here. The following unit types are available: Units are named as their configuration files. Some units have special semantics. A detailed list is available in systemd.special(7). systemd knows various kinds of dependencies, including positive and negative requirement dependencies (i.e. Requires= and Conflicts=) as well as ordering dependencies ( After= and Before=). NB: ordering and requirement dependencies are orthogonal. If only a requirement dependency exists between two units (e.g. foo.service requires bar.service), but no ordering dependency (e.g. foo.service after bar.service) and both are requested to start, they will be started in parallel. It is a common pattern that both requirement and ordering dependencies are placed between two units. Also note that the majority of dependencies are implicitly created and maintained by systemd. In most cases, it should be unnecessary to declare additional dependencies manually, however it is possible to do this. Application programs and units (via dependencies) may request state changes of units. In systemd, these requests are encapsulated as 'jobs' and maintained in a job queue. Jobs may succeed or can fail, their execution is ordered based on the ordering dependencies of the units they have been scheduled for. On boot systemd activates the target unit default.target whose job is to activate on-boot services and other on-boot units by pulling them in via dependencies. Usually, the unit name is just an alias (symlink) for either graphical.target (for fully-featured boots into the UI) or multi-user.target (for limited console-only boots for use in embedded or server environments, or similar; a subset of graphical.target). However, it is at the discretion of the administrator to configure it as an alias to any other target unit. See systemd.special(7) for details about these target units. Processes systemd spawns are placed in individual Linux control groups named after the unit which they belong to in the private systemd hierarchy. (see cgroups.txt[1] for more information about control groups, or short "cgroups"). systemd uses this to effectively keep track of processes. Control group information is maintained in the kernel, and is accessible via the file system hierarchy (beneath /sys/fs/cgroup/systemd/), or in tools such as systemd-cgls(1) or ps(1) ( ps xawf -eo pid,user,cgroup,args is particularly useful to list all processes and the systemd units they belong to.). systemd is compatible with the SysV init system to a large degree: SysV init scripts are supported and simply read as an alternative (though limited) configuration file format. The SysV /dev/initctl interface is provided, and compatibility implementations of the various SysV client tools are available. In addition to that, various established Unix functionality such as /etc/fstab or the utmp database are supported. systemd has a minimal transaction system: if a unit is requested to start up or shut down it will add it and all its dependencies to a temporary transaction. Then, it will verify if the transaction is consistent (i.e. whether the ordering of all units is cycle-free). If it is not, systemd will try to fix it up, and removes non-essential jobs from the transaction that might remove the loop. Also, systemd tries to suppress non-essential jobs in the transaction that would stop a running service. Finally it is checked whether the jobs of the transaction contradict jobs that have already been queued, and optionally the transaction is aborted then. If all worked out and the transaction is consistent and minimized in its impact it is merged with all already outstanding jobs and added to the run queue. Effectively this means that before executing a requested operation, systemd will verify that it makes sense, fixing it if possible, and only failing if it really cannot work. systemd contains native implementations of various tasks that need to be executed as part of the boot process. For example, it sets the hostname or configures the loopback network device. It also sets up and mounts various API file systems, such as /sys or /proc. For more information about the concepts and ideas behind systemd, please refer to the Original Design Document[2]. Note that some but not all interfaces provided by systemd are covered by the Interface Stability Promise[3]. Units may be generated dynamically at boot and system manager reload time, for example based on other configuration files or parameters passed on the kernel command line. For details, see systemd.generator(7). Systems which invoke systemd in a container or initrd environment should implement the Container Interface[4] or initrd Interface[5] specifications, respectively.

DIRECTORIES

System unit directories User unit directories SysV init scripts directory SysV runlevel link farm directory

SIGNALS

SIGTERM SIGINT SIGWINCH SIGPWR SIGUSR1 SIGUSR2 SIGHUP SIGRTMIN+0 SIGRTMIN+1 SIGRTMIN+2 SIGRTMIN+3 SIGRTMIN+4 SIGRTMIN+5 SIGRTMIN+6 SIGRTMIN+13 SIGRTMIN+14 SIGRTMIN+15 SIGRTMIN+16 SIGRTMIN+20 SIGRTMIN+21 SIGRTMIN+22, SIGRTMIN+23 SIGRTMIN+24 SIGRTMIN+26, SIGRTMIN+27, SIGRTMIN+28

ENVIRONMENT

$SYSTEMD_LOG_LEVEL $SYSTEMD_LOG_TARGET $SYSTEMD_LOG_COLOR $SYSTEMD_LOG_LOCATION $XDG_CONFIG_HOME, $XDG_CONFIG_DIRS, $XDG_DATA_HOME, $XDG_DATA_DIRS $SYSTEMD_UNIT_PATH $SYSTEMD_SYSVINIT_PATH $SYSTEMD_SYSVRCND_PATH $SYSTEMD_COLORS $LISTEN_PID, $LISTEN_FDS, $LISTEN_FDNAMES $NOTIFY_SOCKET

KERNEL COMMAND LINE

When run as system instance systemd parses a number of kernel command line arguments[7]: systemd.unit=, rd.systemd.unit= systemd.dump_core systemd.crash_chvt systemd.crash_shell systemd.crash_reboot systemd.confirm_spawn systemd.service_watchdogs= systemd.show_status systemd.log_target=, systemd.log_level=, systemd.log_location=, systemd.log_color systemd.default_standard_output=, systemd.default_standard_error= systemd.setenv= systemd.machine_id= systemd.unified_cgroup_hierarchy systemd.legacy_systemd_cgroup_controller quiet debug emergency, rd.emergency, -b rescue, rd.rescue, single, s, S, 1 2, 3, 4, 5 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= For other kernel command line parameters understood by components of the core OS, please refer to kernel-command-line(7).

SOCKETS AND FIFOS

/run/systemd/notify /run/systemd/private /dev/initctl

SEE ALSO

The systemd Homepage[10], systemd-system.conf(5), locale.conf(5), systemctl(1), journalctl(1), systemd-notify(1), daemon(7), sd-daemon(3), systemd.unit(5), systemd.special(5), pkg-config(1), kernel-command-line(7), bootup(7), systemd.directives(7)

NOTES

1.

cgroups.txt

2.

Original Design Document

3.

Interface Stability Promise

4.

Container Interface

5.

initrd Interface

6.

XDG Base Directory specification

7.

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

8.

unified cgroup hierarchy

9.

legacy cgroup hierarchy

10.

systemd Homepage