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

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:

For more information about systemd.unit see http://www.freedesktop.org/software/systemd/man/systemd.unit.html

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:

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:

tux > sudo systemctl start MY_1ST_SERVICE MY_2ND_SERVICE

To list all services available on the system:

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

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

start

start

stop

stop

restart

restart

try-restart

try-restart

reload

reload

reload-or-restart

n/a

reload-or-try-restart

n/a

status

status

is-active

status

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.

10.2.2.1 Enabling/Disabling Services on the Command Line #Edit source

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 10.2: Commands for Enabling and Disabling Services #

Task	systemd Command	System V init Command
Enabling.	systemctl enable MY_SERVICE(S)	insserv MY_SERVICE(S), chkconfig -a MY_SERVICE(S)
Disabling.	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	n/a
Masking.  After “disabling” a service, it can still be started manually. To completely disable a service, you need to mask it. Use with care.	systemctl mask MY_SERVICE	n/a
Unmasking.  A service that has been masked can only be used again after it has been unmasked.	systemctl unmask MY_SERVICE	n/a

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.

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

If you want to know the current target, use the command: systemctl get-default

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 10.5.3, “Creating Custom Targets” for instructions on how to create your own bootable target.

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.

root # 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'.

root # systemctl --failed
UNIT                   LOAD   ACTIVE SUB    JOB DESCRIPTION
apache2.service        loaded failed failed     apache
NetworkManager.service loaded failed failed     Network Manager
plymouth-start.service loaded failed failed     Show Plymouth Boot Screen

[...]

systemd.log_level=debug systemd.log_target=kmsg

tux > dmesg -T | less

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 will result 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 will also be terminated. To work around this error, proceed as follows:

The systemd unit files are located in /usr/lib/systemd/system. If you want to customize them, proceed as follows:

The modified files in /etc/systemd will take precedence over the original files in /usr/lib/systemd/system, provided that their file name is the same.

root # 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
}

root # cat /usr/lib/systemd/system/example.socket
[Socket]
ListenStream=0.0.0.0:10085
Accept=false

[Install]
WantedBy=sockets.target

root # cat /usr/lib/systemd/system/example.service
[Unit]
Description=example

[Service]
ExecStart=/usr/libexec/example/exampled -auth=bsdtcp exampledump
User=user
Group=users
StandardInput=socket

If you only want to add a few lines to a configuration file or modify a small part of it, you can use so-called “drop-in” files. Drop-in files let you extend the configuration of unit files without having to edit or override the unit files themselves.

For example, to change one value for the FOOBAR service located in /usr/lib/systemd/system/FOOBAR.SERVICE, proceed as follows:

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.

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:

tux > 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)
         man:systemd-tmpfiles(8)

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.

Section 10.6.9, “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.

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.

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
MODULE

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.

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:

On a traditional System V init system it is not always possible to clearly assign a process to the service that spawned it. Some 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 terminate correctly, leaving some 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. The result will look like the following (shortened) example:

root # systemd-cgls --no-pager
├─1 /usr/lib/systemd/systemd --switched-root --system --deserialize 20
├─user.slice
│ └─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

[...]

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

[...]

See Book “System Analysis and Tuning Guide”, Chapter 9 “Kernel Control Groups” for more information about cgroups.

As explained in Section 10.6.6, “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 terminate a service and all of its children. Child processes that have not been terminated will remain as zombie processes.

systemd's concept of confining each service into a cgroup makes it possible to clearly 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.

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.

Terminating dbus or restarting it in the running system is similar to an attempt to terminate or restart pid 1. It will break systemd client/server communication and make 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.

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