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. 


We could group services and start them together using target unit files.
After the system boots up , the "" unit manages and starts  all the required services for the default target.

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

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

Below are the most used targets:
  • : allows us to have multi-user environment, plus a  graphical interface.
  • : 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:


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