Linux 101 : Useful "systemctl" commands - Unit, Unit files, Targets -

Units and unit files:

Units represent services and system resources - sockets, devices, mounts, ... -

Unit files represent the configuration of these services and system resources.

To list all running services, we use the below command:

To list the enabled services - services that start at boot time -, we use the below:

To list all the units loaded in a Linux system, we use:

Units have a name and a type and use the following format "name.type".

Services or daemons have unit files with the ".service" extension.

Targets:

We could group services and start them together using target unit files.

After the system boots up , the "default.target" unit manages and starts all the required services for the default target.

The "default.target" unit points to a target, for example the "graphical.target".

We could check what our system uses as its default target using the below command:

Below are the most used targets:

graphical.target : allows us to have multi-user environment, plus a graphical interface.
multi-user.target : offers a multi-user access to the system with no graphical interface.

Service unit files hold information about services and live in different directories.

The directories are shown in increasing order of priority:

To change a unit's configuration file, we make a copy of the file in the "/etc/systemd/system" directory and make the modification there.

For example "/etc/systemd/system/my_modified_service.service".

After we are done, we run the below command for the changes to take effect:

We might also need to restart the service as below:

IT hands-on