---
categories:
  - Linux
tags: [systems-programming]
---

# systemd

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);

### Unit types

Units are organised into **unit types**. The main types that run at boot time are as follows:

- service units (`.service`)
  - control service daemons
- socket units (`.socket`)
  - handle incoming network connection request locations
- device units (`.device)
  - disks and other devices
- mount units (`.mount`)
  - handle the attachment of filesystems to the system
- target units
  - control other units by organising them into groups

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.

![](/_img/systemd-global-files.png)
_`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`.

![](/_img/systemd-local-files.png)

_`systemd` local unit files, specific to the currently logged-in user_

### Example files

Below is the systemd service file for the creation of UUIDs:

```plain
[Unit]
Description=Daemon for generating UUIDs
Documentation=man:uuidd(8)
Requires=uuidd.socket

[Service]
ExecStart=/usr/sbin/uuidd --socket-activation
Restart=no
User=uuidd
Group=uuidd
ProtectSystem=strict
ProtectHome=yes
PrivateDevices=yes
PrivateUsers=yes
ProtectKernelTunables=yes
ProtectKernelModules=yes
ProtectControlGroups=yes
MemoryDenyWriteExecute=yes
ReadWritePaths=/var/lib/libuuid/
SystemCallFilter=@default @file-system @basic-io @system-service @signal @io-event @network-io

[Install]
Also=uuidd.socket
```

- 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.

### View the dependency graph

`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.

### Viewing active units

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:

```
$ systemctl status mongod
mongodb.service - MongoDB Database Server
     Loaded: loaded (/usr/lib/systemd/system/mongodb.service; enabled; preset: disabled)
     Active: active (running) since Wed 2022-08-17 07:25:27 BST; 24h ago
       Docs: https://docs.mongodb.org/manual
   Main PID: 931 (mongod)
     Memory: 304.1M
        CPU: 2min 18.630s
     CGroup: /system.slice/mongodb.service
             └─931 /usr/bin/mongod --config /etc/mongodb.conf
Aug 17 07:25:27 archbish systemd[1]: Started MongoDB Database Server.****
```

In addition to the core info it tells us the unit type. In this case it is a service.

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.

```bash
systemctl enable mongodb.service
Created symlink /etc/systemd/system/multi-user.target.wants/mongodb.service → /usr/lib/systemd/system/mongodb.service.
```

Then we can start:

```
systemctl start mongodb.service
```

To stop the service:

```
systemctl stop mongodb.service
```

After this we should disable it, in order to remove any symbolic links it has created on the system as part of the install process:

```bash
systemctl disable mongodb.service
Removed "/etc/systemd/system/multi-user.target.wants/mongodb.service".
```

## Why use systemd over cron ?

https://mark.stosberg.com/2016-08-26-rsnapshot-and-systemd/
