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.
On Linux systems `systemd` is used to implement `init`.
`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
## How `systemd` works
### Goal-directed units
`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.
## `systemd` configuration files
Units are managed via `systemd` configuration files.
### Configuration file locations
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.
_`systemd` global unit files_
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 `Unit` section provides metadata about the unit including which `systemd` dependencies it has
*`Service` constitutes the main specification for the unit
*`Install` is the call to set the dependencies running before the `Service` functions are accessible.
## `systemd` operations: `systemctl`
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
```
### Get status of a specific unit
Here I have requested the status of the currently running `mongodb` unit:
We can also view the journal entry for the given unit. This provides you with its diagnostic log messages:
```
journalctl --unit=mongodb.service
- Boot b9565dfe8aca4d069143209b3aa84d8e --
Aug 05 18:31:30 archbish systemd[1]: Started MongoDB Database Server.
Aug 06 14:27:33 archbish systemd[1]: mongodb.service: Deactivated successfully.
Aug 06 14:27:33 archbish systemd[1]: mongodb.service: Consumed 3min 17.598s CPU time.
-- Boot 01922f84c3bd4b3a8f11824cf05f7320 --
Aug 07 11:58:09 archbish systemd[1]: Started MongoDB Database Server.
Aug 08 14:43:01 archbish systemd[1]: mongodb.service: Deactivated successfully.
Aug 08 14:43:01 archbish systemd[1]: mongodb.service: Consumed 5min 28.760s CPU time.
-- Boot e52b735e115c43bdad8c00462aaff395 --
Aug 10 13:13:22 archbish systemd[1]: Started MongoDB Database Server.
Aug 11 07:46:40 archbish systemd[1]: mongodb.service: Deactivated successfully.
Aug 11 07:46:40 archbish systemd[1]: mongodb.service: Consumed 2min 16.629s CPU time.
```
Each entry is organised around specific boots.
### List jobs
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`.
This will most likely return `No jobs running` if the computer has been running for a while. Most jobs execute at boot.
### Enable/disable, start/stop units
If a unit has dependencies it is necessary to _enable_ it before starting it. This installs the dependencies.
