So let's talk about System D. System D is started as the first process after loading
the kernel, and it is the manager of everything. It's more than a service manager. Really,
everything is managed by System D. So it is used for starting services like secure shell
server, web server, and anything else that your Linux machine should be providing. And
apart from that, it's also taking care of mounts, of timers, of sockets, and so much
more. Everything that is managed by System D is referred to as a unit. So the unit is
an item-specific configuration file. For instance, you have the SSHD service unit that starts
the SSHD service. You may have timer units that are starting stuff at specific moments.
You may have mount units that are mounting different items. System CTL is a main management
tool for System D. And to start with, you should get an overview of all types of units
that are available. Use systemctl-dhelp to do so. And if you want to see all the units
that your system starts, use systemctl listunits. Let me show you. System CTL is one of these
commands that has awesome tab completion as well. Here you can see what systemctl tab
tab is doing. So don't remember, just recognize. To start with, systemctl-dhelp is giving me
what? It's giving all the different unit types. So service is what you want to use if you
want to start processes while your Linux system is booting. Mount is taking care of mount.
Swap is taking care of swap space. Socket has System D listening on a specific TCP or
UDP port for incoming traffic. A target is a group of units. A device is for initializing
hardware devices. Auto-mount allows you to mount a device upon activation of a directory.
Timer is for running specific stuff at a specific time. Path is making sure that a service is
activated when a path is entered. And slices and scopes are about system DC groups, which is
about process priority and resource allocation. For an overview of what is currently used,
use systemctl listunits. And there you can see all the units that are active, their current
state. And also, if something hasn't gone right for this unit, you can see that it has failed.
This is a nice way to verify that the units that you intended to see really are available.
In the next video, you'll learn more about System D units.