Once the [boot process](/Operating_Systems/Boot_process.md) has completed and the bootloader has located the kernel and injected it into memory the first user space program runs: `init` (for _initialisation_). `init` is a [daemon](/Operating_Systems/Daemons.md) process that continues running until shutdown and is responsible for starting all the processes that are prerequisites for user space. For example: network connections, disk access, user logins etc.
`init` is the parent of all processes: PID1. Whilst it does a lot of its work in quick succession at boot time it is not limited to the this stage of the lifescycle but runs continuously in reponse to new user events.
`systemd` is directly accessible from user space and provides a straightforward way to enable/disable, start/stop system level processes
> `systemd` can track individual service daemons after they start, and group together multiple processes associated with a service, giving you more power and insight into exactly what is running on the system _How Linux Works: Third Edition_, Brian Ward 2021
`systemd` works on the basis of _goals_. Each goal is system task defined as a **unit**. A unit contains instructions and a specification of dependencies to other units.
When activating a unit, `systemd` first activates the dependencies and then moves onto the details of the unit itself. `init` as implemented by `systemd` does not follow a rigid sequence every time, initialising units in the same sequence and waiting for one to complete before starting another. Instead it activates units whenever they are ready. This, its parallelized nature, is one of the main advantages over previous programs that managed the `init` sequence on Linux (such as for example System V);
For example, at boot, a target unit called `default.target` groups together a number of service and mount units as dependencies. These then run in a graph-like dependency structure where a unit that comes late in the boot process can depend on several previous units making earlier branches of a dependency tree join back together.
System level `systemd` config files are located in the _system unit directory_ at `/usr/lib/systemd/system`. You shouldn't change or manipulate these files or attempt to add new config files here since they will be overwritten by the system.
Local definitions that relate to the specific user and where the user herself can define units are located in the _system configuration_ directory: `/etc/systemd/system`.
The `systemctl` command is the chief way of interacting with `systemd`. You use it to activate and deactivate services, list their status, reload the configuration and so.
`systemctl status` by itself will print a long list of units grouped by their dependency relations. It will also provide some metadata about the current systemd boot context.
Below I have listed the running units pertaining to bluetooth:
```
$ systemctl list-units | grep bluetooth
sys-devices-pci0000:00-0000:00:14.0-usb3-3\x2d10-3\x2d10:1.0-bluetooth-hci0-hci0:3585.device loaded active plugged /sys/devices/pci0000:00/0000:00:14.0/usb3/3-10/3-10:1.0/bluetooth/hci0/hci0:3585
sys-devices-pci0000:00-0000:00:14.0-usb3-3\x2d10-3\x2d10:1.0-bluetooth-hci0.device loaded active plugged /sys/devices/pci0000:00/0000:00:14.0/usb3/3-10/3-10:1.0/bluetooth/hci0
sys-subsystem-bluetooth-devices-hci0.device loaded active plugged /sys/subsystem/bluetooth/devices/hci0
sys-subsystem-bluetooth-devices-hci0:3585.device loaded active plugged /sys/subsystem/bluetooth/devices/hci0:3585
bluetooth.service loaded active running Bluetooth service
bluetooth.target loaded active active Bluetooth Support
Requests to activate, reactivate and restart units are called **jobs** in `systemd`. They can be thought of as unit state changes. Current jobs can be listed with `systemctl list-jobs`.
