systemd organizes the different types of resources it manages into units. Most units are configured in unit configuration files that enable you to configure these units according to system needs. In addition to the files, you can also use systemd runtime commands to configure the units.

To display all the types of units available in systemd, use the following command:

sudo systemctl -t help

Available unit types:
service
mount
swap
socket
target
device
automount
timer
path
slice
scope

The following list describes some system units that you can manage on an Oracle Linux system by using systemd:

Services

Service unit configuration files have the file name format service_name.service, for example sshd.service, crond.service, and httpd.service.

Service units start and control daemons and the processes of which the daemons consist.

The following example shows how you might start the systemd service unit for the Apache HTTP server, httpd.service:

sudo systemctl start httpd.service

See Service Management for more information.

Targets

Target unit configuration files have the file name format target_name.target, for example graphical.target.

Targets are similar to runlevels. A system reaches different targets during the boot process as resources get configured. For example, a system reaches network-pre.target before it reaches the target network-online.target.

Many target units have dependencies. For example, the activation of graphical.target (for a graphical session) fails unless multi-user.target (for multiuser system) is also active.

See Targets for more information.

File System Mount Points

Mount unit configuration files have the file name format mount_point_name.mount.

Mount units enable you to mount file systems at boot time. For example, you can run the following command to mount the temporary file system (tmpfs) on /tmp at boot time:

sudo systemctl enable tmp.mount

Devices

Device unit configuration files have the file name format device_unit_name.device.

Device units are named after the /sys and /dev paths they control. For example, the device /dev/sda5 is exposed in systemd as dev-sda5.device.

Device units enable you to implement device-based activation.

Sockets

Socket unit configuration files have the file name format socket_unit_name.socket.

Each "*.socket" file needs a corresponding "*.service" file to configure the service to start on incoming traffic on the socket.

Socket units enable you to implement socket-based activation.

Timers

Timer unit configuration files have the file name format timer_unit_name.timer.

Each "*.timer" file needs a corresponding "*.service" file to configure the service to start at a configured timer event. A Unit configuration entry can be used to specify a service that's named differently to the timer unit, if required.

Timer units can control when service units are run and can act as an alternative to using the cron daemon. Timer units can be configured for calendar time events, monotonic time events, and can be run asynchronously.

See Working with Timers for more information.

Paths to systemd unit configuration files vary depending on their purpose and whether systemd is running in 'user' or 'system' mode. For example, configuration for units that are installed from packages might be available in /usr/lib/systemd/system or in /usr/local/lib/systemd/system, while a user mode configuration unit is likely to be stored in $HOME/.config/systemd/user. See the systemd.unit(5) manual page for more information.