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documentation.suse.com / SUSE Linux Enterprise Server Documentation / Administration Guide / Booting a Linux system / The systemd daemon
Applies to SUSE Linux Enterprise Server 15 SP6

19 The systemd daemon

systemd initializes the system. It has the process ID 1. systemd is started directly by the kernel and resists signal 9, which normally terminates processes. All other programs are started directly by systemd or by one of its child processes. systemd is a replacement for the System V init daemon and is fully compatible with System V init (by supporting init scripts).

The main advantage of systemd is that it considerably speeds up boot time by parallelizing service starts. Furthermore, systemd only starts a service when it is really needed. Daemons are not started unconditionally at boot time, but when being required for the first time. systemd also supports Kernel Control Groups (cgroups), creating snapshots, and restoring the system state. For more details see https://www.freedesktop.org/wiki/Software/systemd/.

Tip: systemd inside WSL

Windows Subsystem for Linux (WSL) enables running Linux applications and distributions under the Microsoft Windows operating system. WSL uses its init process instead of systemd. To enable systemd in SLES running in WSL, install the wsl_systemd pattern that automates the process:

> sudo zypper in -t pattern wsl_systemd

Alternatively, you can edit /etc/wsl.conf and add the following lines manually:


Keep in mind that the support for systemd in WSL is partial—systemd unit files must have reasonable process management behavior.

19.1 The systemd concept

The following section explains the concept behind systemd.

systemd is a system and session manager for Linux, compatible with System V and LSB init scripts. The main features of systemd include:

  • parallelization capabilities

  • socket and D-Bus activation for starting services

  • on-demand starting of daemons

  • tracking of processes using Linux cgroups

  • creating snapshots and restoring of the system state

  • maintains mount and automount points

  • implements an elaborate transactional dependency-based service control logic

19.1.1 Unit file

A unit configuration file contains information about a service, a socket, a device, a mount point, an automount point, a swap file or partition, a start-up target, a watched file system path, a timer controlled and supervised by systemd, a temporary system state snapshot, a resource management slice or a group of externally created processes.

Unit file is a generic term used by systemd for the following:

  • Service.  Information about a process (for example, running a daemon); file ends with .service

  • Targets.  Used for grouping units and as synchronization points during start-up; file ends with .target

  • Sockets.  Information about an IPC or network socket or a file system FIFO, for socket-based activation (like inetd); file ends with .socket

  • Path.  Used to trigger other units (for example, running a service when files change); file ends with .path

  • Timer.  Information about a timer controlled, for timer-based activation; file ends with .timer

  • Mount point.  Normally auto-generated by the fstab generator; file ends with .mount

  • Automount point.  Information about a file system automount point; file ends with .automount

  • Swap.  Information about a swap device or file for memory paging; file ends with .swap

  • Device.  Information about a device unit as exposed in the sysfs/udev(7) device tree; file ends with .device

  • Scope / slice.  A concept for hierarchically managing resources of a group of processes; file ends with .scope/.slice

For more information about systemd unit files, see https://www.freedesktop.org/software/systemd/man/latest/systemd.unit.html

19.2 Basic usage

The System V init system uses several commands to handle services—the init scripts, insserv, telinit and others. systemd makes it easier to manage services, because there is only one command to handle most service related tasks: systemctl. It uses the command plus subcommand notation like git or zypper:


See man 1 systemctl for a complete manual.

Tip: Terminal output and Bash completion

If the output goes to a terminal (and not to a pipe or a file, for example), systemd commands send long output to a pager by default. Use the --no-pager option to turn off paging mode.

systemd also supports bash-completion, allowing you to enter the first letters of a subcommand and then press →|. This feature is only available in the bash shell and requires the installation of the package bash-completion.

19.2.1 Managing services in a running system

Subcommands for managing services are the same as for managing a service with System V init (start, stop, ...). The general syntax for service management commands is as follows:

systemctl reload|restart|start|status|stop|... MY_SERVICE(S)
System V init
rcMY_SERVICE(S) reload|restart|start|status|stop|...

systemd allows you to manage several services in one go. Instead of executing init scripts one after the other as with System V init, execute a command like the following:

> sudo systemctl start MY_1ST_SERVICE MY_2ND_SERVICE

To list all services available on the system:

> sudo systemctl list-unit-files --type=service

The following table lists the most important service management commands for systemd and System V init:

Table 19.1: Service management commands


systemd Command

System V init Command





Restarting.  Shuts down services and starts them afterward. If a service is not yet running, it is started.


Restarting conditionally.  Restarts services if they are currently running. Does nothing for services that are not running.


Reloading.  Tells services to reload their configuration files without interrupting operation. Use case: tell Apache to reload a modified httpd.conf configuration file. Not all services support reloading.


Reloading or restarting.  Reloads services if reloading is supported, otherwise restarts them. If a service is not yet running, it is started.


Reloading or restarting conditionally.  Reloads services if reloading is supported, otherwise restarts them if currently running. Does nothing for services that are not running.


Getting detailed status information.  Lists information about the status of services. The systemd command shows details such as description, executable, status, cgroup, and messages last issued by a service (see Section 19.6.9, “Debugging services”). The level of details displayed with the System V init differs from service to service.


Getting short status information.  Shows whether services are active or not.


19.2.2 Permanently enabling/disabling services

The service management commands mentioned in the previous section let you manipulate services for the current session. systemd also lets you permanently enable or disable services, so they are automatically started when requested or are always unavailable. You can either do this by using YaST, or on the command line. Enabling/disabling services on the command line

The following table lists enabling and disabling commands for systemd and System V init:

Important: Service start

When enabling a service on the command line, it is not started automatically. It is scheduled to be started with the next system start-up or runlevel/target change. To immediately start a service after having enabled it, explicitly run systemctl start MY_SERVICE or rc MY_SERVICE start.

Table 19.2: Commands for enabling and disabling services


systemd Command

System V init Command


systemctl enable MY_SERVICE(S)

insserv MY_SERVICE(S), chkconfig -a MY_SERVICE(S)


systemctl disable MY_SERVICE(S).service

insserv -r MY_SERVICE(S), chkconfig -d MY_SERVICE(S)

Checking.  Shows whether a service is enabled or not.

systemctl is-enabled MY_SERVICE

chkconfig MY_SERVICE

Re-enabling.  Similar to restarting a service, this command first disables and then enables a service. Useful to re-enable a service with its defaults.

systemctl reenable MY_SERVICE


Masking.  After disabling a service, it can still be started manually. To disable a service, you need to mask it. Use with care.

systemctl mask MY_SERVICE


Unmasking.  A service that has been masked can only be used again after it has been unmasked.

systemctl unmask MY_SERVICE


19.3 System start and target management

The entire process of starting the system and shutting it down is maintained by systemd. From this point of view, the kernel can be considered a background process to maintain all other processes and adjust CPU time and hardware access according to requests from other programs.

19.3.1 Targets compared to runlevels

With System V init the system was booted into a so-called Runlevel. A runlevel defines how the system is started and what services are available in the running system. Runlevels are numbered; the most commonly known ones are 0 (shutting down the system), 3 (multiuser with network) and 5 (multiuser with network and display manager).

systemd introduces a new concept by using so-called target units. However, it remains fully compatible with the runlevel concept. Target units are named rather than numbered and serve specific purposes. For example, the targets local-fs.target and swap.target mount local file systems and swap spaces.

The target graphical.target provides a multiuser system with network and display manager capabilities and is equivalent to runlevel 5. Complex targets, such as graphical.target act as meta targets by combining a subset of other targets. Since systemd makes it easy to create custom targets by combining existing targets, it offers great flexibility.

The following list shows the most important systemd target units. For a full list refer to man 7 systemd.special.

Selected systemd target units

The target that is booted by default. Not a real target, but rather a symbolic link to another target like graphic.target. Can be permanently changed via YaST (see Section 19.4, “Managing services with YaST”). To change it for a session, use the kernel parameter systemd.unit=MY_TARGET.target at the boot prompt.


Starts a minimal emergency root shell on the console. Only use it at the boot prompt as systemd.unit=emergency.target.


Starts a system with network, multiuser support and a display manager.


Shuts down the system.


Starts all services necessary for sending and receiving mails.


Starts a multiuser system with network.


Reboots the system.


Starts a single-user root session without network. Basic tools for system administration are available. The rescue target is suitable for solving multiple system problems, for example, failing logins or fixing issues with a display driver.

To remain compatible with the System V init runlevel system, systemd provides special targets named runlevelX.target mapping the corresponding runlevels numbered X.

To inspect the current target, use the command: systemctl get-default

Table 19.3: System V runlevels and systemd target units

System V runlevel

systemd target



runlevel0.target, halt.target, poweroff.target

System shutdown

1, S

runlevel1.target, rescue.target,

Single-user mode


runlevel2.target, multi-user.target,

Local multiuser without remote network


runlevel3.target, multi-user.target,

Full multiuser with network





runlevel5.target, graphical.target,

Full multiuser with network and display manager


runlevel6.target, reboot.target,

System reboot

Important: systemd ignores /etc/inittab

The runlevels in a System V init system are configured in /etc/inittab. systemd does not use this configuration. Refer to Section 19.5.5, “Creating custom targets” for instructions on how to create your own bootable target. Commands to change targets

Use the following commands to operate with target units:


systemd Command

System V init Command

Change the current target/runlevel

systemctl isolate MY_TARGET.target

telinit X

Change to the default target/runlevel

systemctl default


Get the current target/runlevel

systemctl list-units --type=target

With systemd, there is usually more than one active target. The command lists all currently active targets.

who -r



persistently change the default runlevel

Use the Services Manager or run the following command:

ln -sf /usr/lib/systemd/system/ MY_TARGET.target /etc/systemd/system/default.target

Use the Services Manager or change the line

id: X:initdefault:

in /etc/inittab

Change the default runlevel for the current boot process

Enter the following option at the boot prompt

systemd.unit= MY_TARGET.target

Enter the desired runlevel number at the boot prompt.

Show a target's/runlevel's dependencies

systemctl show -p "Requires" MY_TARGET.target

systemctl show -p "Wants" MY_TARGET.target

Requires lists the hard dependencies (the ones that must be resolved), whereas Wants lists the soft dependencies (the ones that get resolved if possible).


19.3.2 Debugging system start-up

systemd offers the means to analyze the system start-up process. You can review the list of all services and their status (rather than having to parse /var/log/). systemd also allows you to scan the start-up procedure to find out how much time each service start-up consumes. Review start-up of services

To review the complete list of services that have been started since booting the system, enter the command systemctl. It lists all active services like shown below (shortened). To get more information on a specific service, use systemctl status MY_SERVICE.

Example 19.1: List active services
# systemctl
UNIT                        LOAD   ACTIVE SUB       JOB DESCRIPTION
iscsi.service               loaded active exited    Login and scanning of iSC+
kmod-static-nodes.service   loaded active exited    Create list of required s+
libvirtd.service            loaded active running   Virtualization daemon
nscd.service                loaded active running   Name Service Cache Daemon
chronyd.service             loaded active running   NTP Server Daemon
polkit.service              loaded active running   Authorization Manager
postfix.service             loaded active running   Postfix Mail Transport Ag+
rc-local.service            loaded active exited    /etc/init.d/boot.local Co+
rsyslog.service             loaded active running   System Logging Service
LOAD   = Reflects whether the unit definition was properly loaded.
ACTIVE = The high-level unit activation state, i.e. generalization of SUB.
SUB    = The low-level unit activation state, values depend on unit type.

161 loaded units listed. Pass --all to see loaded but inactive units, too.
To show all installed unit files use 'systemctl list-unit-files'.

To restrict the output to services that failed to start, use the --failed option:

Example 19.2: List failed services
# systemctl --failed
apache2.service        loaded failed failed     apache
NetworkManager.service loaded failed failed     Network Manager
plymouth-start.service loaded failed failed     Show Plymouth Boot Screen

[...] Debug start-up time

To debug system start-up time, systemd offers the systemd-analyze command. It shows the total start-up time, a list of services ordered by start-up time and can also generate an SVG graphic showing the time services took to start in relation to the other services.

Listing the system start-up time
# systemd-analyze
Startup finished in 2666ms (kernel) + 21961ms (userspace) = 24628ms
Listing the services start-up time
# systemd-analyze blame
    15.000s backup-rpmdb.service
    14.879s mandb.service
     7.646s backup-sysconfig.service
     4.940s postfix.service
     4.921s logrotate.service
     4.640s libvirtd.service
     4.519s display-manager.service
     3.921s btrfsmaintenance-refresh.service
     3.466s lvm2-monitor.service
     2.774s plymouth-quit-wait.service
     2.591s firewalld.service
     2.137s initrd-switch-root.service
     1.954s ModemManager.service
     1.528s rsyslog.service
     1.378s apparmor.service
Services start-up time graphics
# systemd-analyze plot > jupiter.example.com-startup.svg
Image Review the complete start-up process

The commands above list the services that are started and their start-up times. For a more detailed overview, specify the following parameters at the boot prompt to instruct systemd to create a verbose log of the complete start-up procedure.

systemd.log_level=debug systemd.log_target=kmsg

Now systemd writes its log messages into the kernel ring buffer. View that buffer with dmesg:

> dmesg -T | less

19.3.3 System V compatibility

systemd is compatible with System V, allowing you to still use existing System V init scripts. However, there is at least one known issue where a System V init script does not work with systemd out of the box: starting a service as a different user via su or sudo in init scripts results in a failure of the script, producing an Access denied error.

When changing the user with su or sudo, a PAM session is started. This session will be terminated after the init script is finished. As a consequence, the service that has been started by the init script is also terminated. To work around this error, proceed as follows:

  1. Create a service file wrapper with the same name as the init script plus the file name extension .service:

    ExecStart=PATH TO INIT SCRIPT start
    ExecStop=PATH TO INIT SCRIPT stop
    ExecStopPost=/usr/bin/rm -f PATH TO PID FILE1

    Replace all values written in UPPERCASE LETTERS with appropriate values.


    Optional—only use if the init script starts a daemon.


    multi-user.target also starts the init script when booting into graphical.target. If it should only be started when booting into the display manager, use graphical.target.

  2. Start the daemon with systemctl start APPLICATION.

19.4 Managing services with YaST

Basic service management can also be done with the YaST Services Manager module. It supports starting, stopping, enabling and disabling services. It also lets you show a service's status and change the default target. Start the YaST module with YaST › System › Services Manager.

Services Manager
Figure 19.1: Services Manager
Changing the Default system target

To change the target the system boots into, choose a target from the Default System Target drop-down box. The most often used targets are Graphical Interface (starting a graphical login screen) and Multi-User (starting the system in command line mode).

Starting or stopping a service

Select a service from the table. The State column shows whether it is currently running (Active) or not (Inactive). Toggle its status by choosing Start or Stop.

Starting or stopping a service changes its status for the currently running session. To change its status throughout a reboot, you need to enable or disable it.

Defining service start-up behavior

Services can either be started automatically at boot time or manually. Select a service from the table. The Start column shows whether it is currently started Manually or On Boot. Toggle its status by choosing Start Mode.

To change a service status in the current session, you need to start or stop it as described above.

View a status messages

To view the status message of a service, select it from the list and choose Show Details. The output is identical to the one generated by the command systemctl -l status MY_SERVICE.

19.5 Customizing systemd

The following sections describe how to customize systemd unit files.

19.5.1 Where are unit files stored?

systemd unit files shipped by SUSE are stored in /usr/lib/systemd/. Customized unit files and unit file drop-ins are stored in /etc/systemd/.

Warning: Preventing your customization from being overwritten

When customizing systemd, always use the directory /etc/systemd/ instead of /usr/lib/systemd/. Otherwise your changes will be overwritten by the next update of systemd.

19.5.2 Override with drop-in files

Drop-in files (or drop-ins) are partial unit files that override only specific settings of the unit file. Drop-ins have higher precedence over main configuration files. The command systemctl edit SERVICE starts the default text editor and creates a directory with an empty override.conf file in /etc/systemd/system/NAME.service.d/. The command also ensures that the running systemd process is notified about the changes.

For example, to change the amount of time that the system waits for MariaDB to start, run sudo systemctl edit mariadb.service and edit the opened file to include the modified lines only:

# Configures the time to wait for start-up/stop

Adjust the TimeoutSec value and save the changes. To enable the changes, run sudo systemctl daemon-reload.

For further information, refer to the man pages that can be evoked with the man 1 systemctl command.

Warning: Creating a copy of a full unit file

If you use the --full option in the systemctl edit --full SERVICE command, a copy of the original unit file is created where you can modify specific options. We do not recommend such customization because when the unit file is updated by SUSE, its changes are overridden by the customized copy in the /etc/systemd/system/ directory. Moreover, if SUSE provides updates to distribution drop-ins, they override the copy of the unit file created with --full. To prevent this confusion and always have your customization valid, use drop-ins.

19.5.3 Creating drop-in files manually

Apart from using the systemctl edit command, you can create drop-ins manually to have more control over their priority. Such drop-ins let you extend both unit and daemon configuration files without having to edit or override the files themselves. They are stored in the following directories:

/etc/systemd/*.conf.d/, /etc/systemd/system/*.service.d/

Drop-ins added and customized by system administrators.

/usr/lib/systemd/*.conf.d/, /usr/lib/systemd/system/*.service.d/

Drop-ins installed by customization packages to override upstream settings. For example, SUSE ships systemd-default-settings.


See the man page man 5 systemd.unit for the full list of unit search paths.

For example, to disable the rate limiting that is enforced by the default setting of systemd-journald, follow these steps:

  1. Create a directory called /etc/systemd/journald.conf.d.

    > sudo mkdir /etc/systemd/journald.conf.d

    The directory name must follow the service name that you want to patch with the drop-in file.

  2. In that directory, create a file /etc/systemd/journald.conf.d/60-rate-limit.conf with the option that you want to override, for example:

    > cat /etc/systemd/journald.conf.d/60-rate-limit.conf
    # Disable rate limiting
  3. Save your changes and restart the service of the corresponding systemd daemon.

    > sudo systemctl restart systemd-journald
Note: Avoiding name conflicts

To avoid name conflicts between your drop-ins and files shipped by SUSE, it is recommended to prefix all drop-ins with a two-digit number and a dash, for example, 80-override.conf.

The following ranges are reserved:

  • 0-19 is reserved for systemd upstream.

  • 20-29 is reserved for systemd shipped by SUSE.

  • 30-39 is reserved for SUSE packages other than systemd.

  • 40-49 is reserved for third-party packages.

  • 50 is reserved for unit drop-in files created with systemctl set-property.

Use a two-digit number above this range to ensure that none of the drop-ins shipped by SUSE can override your own drop-ins.


You can use systemctl cat $UNIT to list and verify which files are taken into account in the units configuration.


Because the configuration of systemd components can be scattered across different places on the file system, it might be hard to get a global overview. To inspect the configuration of a systemd component, use the following commands:

  • systemctl cat UNIT_PATTERN prints configuration files related to one or more systemd units, for example:

    > systemctl cat atd.service
  • systemd-analyze cat-config DAEMON_NAME_OR_PATH copies the contents of a configuration file and drop-ins for a systemd daemon, for example:

    > systemd-analyze cat-config systemd/journald.conf

19.5.4 Converting xinetd services to systemd

Since the release of SUSE Linux Enterprise Server 15, the xinetd infrastructure has been removed. This section outlines how to convert existing custom xinetd service files to systemd sockets.

For each xinetd service file, you need at least two systemd unit files: the socket file (*.socket) and an associated service file (*.service). The socket file tells systemd which socket to create, and the service file tells systemd which executable to start.

Consider the following example xinetd service file:

# cat /etc/xinetd.d/example
service example
  socket_type = stream
  protocol = tcp
  port = 10085
  wait = no
  user = user
  group = users
  groups = yes
  server = /usr/libexec/example/exampled
  server_args = -auth=bsdtcp exampledump
  disable = no

To convert it to systemd, you need the following two matching files:

# cat /usr/lib/systemd/system/example.socket

# cat /usr/lib/systemd/system/example.service

ExecStart=/usr/libexec/example/exampled -auth=bsdtcp exampledump

For a complete list of the systemd socket and service file options, refer to the systemd.socket and systemd.service manual pages (man 5 systemd.socket, man 5 systemd.service).

19.5.5 Creating custom targets

On System V init SUSE systems, runlevel 4 is unused to allow administrators to create their own runlevel configuration. systemd allows you to create any number of custom targets. It is suggested to start by adapting an existing target such as graphical.target.

  1. Copy the configuration file /usr/lib/systemd/system/graphical.target to /etc/systemd/system/MY_TARGET.target and adjust it according to your needs.

  2. The configuration file copied in the previous step already covers the required (hard) dependencies for the target. To also cover the wanted (soft) dependencies, create a directory /etc/systemd/system/MY_TARGET.target.wants.

  3. For each wanted service, create a symbolic link from /usr/lib/systemd/system into /etc/systemd/system/MY_TARGET.target.wants.

  4. When you have finished setting up the target, reload the systemd configuration to make the new target available:

    > sudo systemctl daemon-reload

19.6 Advanced usage

The following sections cover advanced topics for system administrators. For even more advanced systemd documentation, refer to Lennart Pöttering's series about systemd for administrators at https://0pointer.de/blog/projects/.

19.6.1 Cleaning temporary directories

systemd supports cleaning temporary directories regularly. The configuration from the previous system version is automatically migrated and active. tmpfiles.d—which is responsible for managing temporary files—reads its configuration from /etc/tmpfiles.d/*.conf, /run/tmpfiles.d/*.conf, and /usr/lib/tmpfiles.d/*.conf files. Configuration placed in /etc/tmpfiles.d/*.conf overrides related configurations from the other two directories (/usr/lib/tmpfiles.d/*.conf is where packages store their configuration files).

The configuration format is one line per path containing action and path, and optionally mode, ownership, age and argument fields, depending on the action. The following example unlinks the X11 lock files:

Type Path               Mode UID  GID  Age Argument
r    /tmp/.X[0-9]*-lock

To get the status the tmpfile timer:

> sudo systemctl status systemd-tmpfiles-clean.timer
systemd-tmpfiles-clean.timer - Daily Cleanup of Temporary Directories
 Loaded: loaded (/usr/lib/systemd/system/systemd-tmpfiles-clean.timer; static)
 Active: active (waiting) since Tue 2018-04-09 15:30:36 CEST; 1 weeks 6 days ago
   Docs: man:tmpfiles.d(5)

Apr 09 15:30:36 jupiter systemd[1]: Starting Daily Cleanup of Temporary Directories.
Apr 09 15:30:36 jupiter systemd[1]: Started Daily Cleanup of Temporary Directories.

For more information on temporary files handling, see man 5 tmpfiles.d.

19.6.2 System log

Section 19.6.9, “Debugging services” explains how to view log messages for a given service. However, displaying log messages is not restricted to service logs. You can also access and query the complete log messages written by systemd—the so-called Journal. Use the command journalctl to display the complete log messages starting with the oldest entries. Refer to man 1 journalctl for options such as applying filters or changing the output format.

19.6.3 Snapshots

You can save the current state of systemd to a named snapshot and later revert to it with the isolate subcommand. This is useful when testing services or custom targets, because it allows you to return to a defined state at any time. A snapshot is only available in the current session and will automatically be deleted on reboot. A snapshot name must end in .snapshot.

Create a snapshot
> sudo systemctl snapshot MY_SNAPSHOT.snapshot
Delete a snapshot
> sudo systemctl delete MY_SNAPSHOT.snapshot
View a snapshot
> sudo systemctl show MY_SNAPSHOT.snapshot
Activate a snapshot
> sudo systemctl isolate MY_SNAPSHOT.snapshot

19.6.4 Loading kernel modules

With systemd, kernel modules can automatically be loaded at boot time via a configuration file in /etc/modules-load.d. The file should be named MODULE.conf and have the following content:

# load module MODULE at boot time

In case a package installs a configuration file for loading a kernel module, the file gets installed to /usr/lib/modules-load.d. If two configuration files with the same name exist, the one in /etc/modules-load.d tales precedence.

For more information, see the modules-load.d(5) man page.

19.6.5 Performing actions before loading a service

With System V init actions that need to be performed before loading a service, needed to be specified in /etc/init.d/before.local . This procedure is no longer supported with systemd. If you need to do actions before starting services, do the following:

Loading kernel modules

Create a drop-in file in /etc/modules-load.d directory (see man modules-load.d for the syntax)

Creating Files or Directories, Cleaning-up Directories, Changing Ownership

Create a drop-in file in /etc/tmpfiles.d (see man tmpfiles.d for the syntax)

Other tasks

Create a system service file, for example, /etc/systemd/system/before.service, from the following template:

# beware, executable is run directly, not through a shell, check the man pages
# systemd.service and systemd.unit for full syntax
# target in which to start the service

When the service file is created, you should run the following commands (as root):

> sudo systemctl daemon-reload
> sudo systemctl enable before

Every time you modify the service file, you need to run:

> sudo systemctl daemon-reload

19.6.6 Kernel control groups (cgroups)

On a traditional System V init system, it is not always possible to match a process to the service that spawned it. Certain services, such as Apache, spawn a lot of third-party processes (for example, CGI or Java processes), which themselves spawn more processes. This makes a clear assignment difficult or even impossible. Additionally, a service may not finish correctly, leaving certain children alive.

systemd solves this problem by placing each service into its own cgroup. cgroups are a kernel feature that allows aggregating processes and all their children into hierarchical organized groups. systemd names each cgroup after its service. Since a non-privileged process is not allowed to leave its cgroup, this provides an effective way to label all processes spawned by a service with the name of the service.

To list all processes belonging to a service, use the command systemd-cgls, for example:

Example 19.3: List all processes belonging to a service
# systemd-cgls --no-pager
├─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 20
│ └─user-1000.slice
│   ├─session-102.scope
│   │ ├─12426 gdm-session-worker [pam/gdm-password]
│   │ ├─15831 gdm-session-worker [pam/gdm-password]
│   │ ├─15839 gdm-session-worker [pam/gdm-password]
│   │ ├─15858 /usr/lib/gnome-terminal-server


  │ └─17616 /usr/lib/systemd/systemd-hostnamed
  │ └─1689 /usr/sbin/cron -n
  │ ├─ 1676 /usr/lib/postfix/master -w
  │ ├─ 1679 qmgr -l -t fifo -u
  │ └─15590 pickup -l -t fifo -u
  │ └─1436 /usr/sbin/sshd -D


See Chapter 10, Kernel control groups for more information about cgroups.

19.6.7 Terminating services (sending signals)

As explained in Section 19.6.6, “Kernel control groups (cgroups)”, it is not always possible to assign a process to its parent service process in a System V init system. This makes it difficult to stop a service and its children. Child processes that have not been terminated remain as zombie processes.

systemd's concept of confining each service into a cgroup makes it possible to identify all child processes of a service and therefore allows you to send a signal to each of these processes. Use systemctl kill to send signals to services. For a list of available signals refer to man 7 signals.

Sending SIGTERM to a service

SIGTERM is the default signal that is sent.

> sudo systemctl kill MY_SERVICE
Sending SIGNAL to a service

Use the -s option to specify the signal that should be sent.

> sudo systemctl kill -s SIGNAL MY_SERVICE
Selecting processes

By default the kill command sends the signal to all processes of the specified cgroup. You can restrict it to the control or the main process. The latter is, for example, useful to force a service to reload its configuration by sending SIGHUP:

> sudo systemctl kill -s SIGHUP --kill-who=main MY_SERVICE

19.6.8 Important notes on the D-Bus service

The D-Bus service is the message bus for communication between systemd clients and the systemd manager that is running as pid 1. Even though dbus is a stand-alone daemon, it is an integral part of the init infrastructure.

Stopping dbus or restarting it in the running system is similar to an attempt to stop or restart PID 1. It breaks the systemd client/server communication and makes most systemd functions unusable.

Therefore, terminating or restarting dbus is neither recommended nor supported.

Updating the dbus or dbus-related packages requires a reboot. When in doubt whether a reboot is necessary, run the sudo zypper ps -s. If dbus appears among the listed services, you need to reboot the system.

Keep in mind that dbus is updated even when automatic updates are configured to skip the packages that require reboot.

19.6.9 Debugging services

By default, systemd is not overly verbose. If a service was started successfully, no output is produced. In case of a failure, a short error message is displayed. However, systemctl status provides a means to debug the start-up and operation of a service.

systemd comes with its own logging mechanism (The Journal) that logs system messages. This allows you to display the service messages together with status messages. The status command works similar to tail and can also display the log messages in different formats, making it a powerful debugging tool.

Show service start-up failure

Whenever a service fails to start, use systemctl status MY_SERVICE to get a detailed error message:

# systemctl start apache2
Job failed. See system journal and 'systemctl status' for details.
# systemctl status apache2
   Loaded: loaded (/usr/lib/systemd/system/apache2.service; disabled)
   Active: failed (Result: exit-code) since Mon, 04 Apr 2018 16:52:26 +0200; 29s ago
   Process: 3088 ExecStart=/usr/sbin/start_apache2 -D SYSTEMD -k start (code=exited, status=1/FAILURE)
   CGroup: name=systemd:/system/apache2.service

Apr 04 16:52:26 g144 start_apache2[3088]: httpd2-prefork: Syntax error on line
205 of /etc/apache2/httpd.conf: Syntax error on li...alHost>
Show last N service messages

The default behavior of the status subcommand is to display the last ten messages a service issued. To change the number of messages to show, use the --lines=N parameter:

> sudo systemctl status chronyd
> sudo systemctl --lines=20 status chronyd
Show service messages in append mode

To display a live stream of service messages, use the --follow option, which works like tail -f:

> sudo systemctl --follow status chronyd
Messages output format

The --output=MODE parameter allows you to change the output format of service messages. The most important modes available are:


The default format. Shows the log messages with a human readable time stamp.


Full output with all fields.


Terse output without time stamps.

19.7 systemd timer units

Similar to cron, systemd timer units provide a mechanism for scheduling jobs on Linux. Although systemd timer units serve the same purpose as cron, they offer several advantages.

  • Jobs scheduled using a timer unit can depend on other systemd services.

  • Timer units are treated as regular systemd services, so can be managed with systemctl.

  • Timers can be realtime and monotonic.

  • Time units are logged to the systemd journal, which makes it easier to monitor and troubleshoot them.

systemd timer units are identified by the .timer file name extension.

19.7.1 systemd timer types

Timer units can use monotonic and realtime timers.

  • Similar to cronjobs, realtime timers are triggered on calendar events. Realtime timers are defined using the option OnCalendar.

  • Monotonic timers are triggered at a specified time elapsed from a certain starting point. The latter could be a system boot or system unit activation event. There are several options for defining monotonic timers including OnBootSec, OnUnitActiveSec, and OnTypeSec. Monotonic timers are not persistent, and they are reset after each reboot.

19.7.2 systemd timers and service units

Every timer unit must have a corresponding systemd unit file it controls. In other words, a .timer file activates and manages the corresponding .service file. When used with a timer, the .service file does not require an [Install] section, as the service is managed by the timer.

19.7.3 Practical example

To understand the basics of systemd timer units, we set up a timer that triggers the foo.sh shell script.

First step is to create a systemd service unit that controls the shell script. To do this, open a new text file for editing and add the following service unit definition:

Description="Foo shell script"


Save the file under the name foo.service in the directory /etc/systemd/system/.

Next, open a new text file for editing and add the following timer definition:

Description="Run foo shell script"



The [Timer] section in the example above specifies what service to trigger (foo.service) and when to trigger it. In this case, the option OnBootSec specifies a monotonic timer that triggers the service five minutes after the system boot, while the option OnUnitActiveSec triggers the service 24 hours after the service has been activated (that is, the timer triggers the service once a day). Finally, the option WantedBy specifies that the timer should start when the system has reached the multiuser target.

Instead of a monotonic timer, you can specify a real-time one using the option OnCalendar. The following realtime timer definition triggers the related service unit once a week, starting on Monday at 12:00.


The option Persistent=true indicates that the service is triggered immediately after the timer activation if the timer missed the last start time (for example, because of the system being powered off).

The option OnCalendar can also be used to define specific dates times for triggering a service using the following format: DayOfWeek Year-Month-Day Hour:Minute:Second. The example below triggers a service at 5am every day:

OnCalendar=*-*-* 5:00:00

You can use an asterisk to specify any value, and commas to list possible values. Use two values separated by .. to indicate a contiguous range. The following example triggers a service at 6pm on Friday of every month:

OnCalendar=Fri *-*-1..7 18:00:00

To trigger a service at different times, you can specify several OnCalendar entries:

OnCalendar=Mon..Fri 10:00
OnCalendar=Sat,Sun 22:00

In the example above, a service is triggered at 10am on week days and at 10pm on weekends.

When you are done editing the timer unit file, save it under the name foo.timer in the /etc/systemd/system/ directory. To check the correctness of the created unit files, run the following command:

> sudo  systemd-analyze verify /etc/systemd/system/foo.*

If the command returns no output, the files have passed the verification successfully.

To start the timer, use the command sudo systemctl start foo.timer. To enable the timer on boot, run the command sudo systemctl enable foo.timer.

19.7.4 Managing systemd timers

Since timers are treated as regular systemd units, you can manage them using systemctl. You can start a timer with systemctl start, enable a timer with systemctl enable, and so on. Additionally, you can list all active timers using the command systemctl list-timers. To list all timers, including inactive ones, run the command systemctl list-timers --all.

19.8 More information

For more information on systemd refer to the following online resources:



systemd for administrators

Lennart Pöttering, one of the systemd authors, has written a series of blog entries (13 at the time of writing this chapter). Find them at https://0pointer.de/blog/projects/.