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:
  •  /etc/systemd/system
  •  /run/systemd/system
  • /usr/lib/systemd/system
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:

Brando Sabatini and Ikbal C ©. Powered by Blogger.