195 lines
7.1 KiB
Markdown
195 lines
7.1 KiB
Markdown
---
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tags:
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- systems-programming
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- Linux
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- procedural
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- systemd
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- operating-systems
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---
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# systemd
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Once the [boot process](Boot_process.md) has completed and the bootloader has
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located the kernel and injected it into memory the first user space program
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runs: `init` (for _initialisation_). `init` is a [daemon](Daemons.md)
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[process](./Processes.md) that continues running until shutdown and is
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responsible for starting all the processes that are prerequisites for user
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space. For example: network connections, disk access, user logins etc. `init` is
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the parent of all processes: PID1. Whilst it does a lot of its work in quick
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succession at boot time it is not limited to the this stage of the lifescycle
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but runs continuously in reponse to new user events.
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On Linux systems `systemd` is used to implement `init` and therefore `systemd`
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will always be listed as PID1.
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`systemd` is directly accessible from user space and provides a straightforward
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way to enable/disable, start/stop system level processes
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> `systemd` can track individual service daemons after they start, and group
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> together multiple processes associated with a service, giving you more power
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> and insight into exactly what is running on the system _How Linux Works: Third
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> Edition_, Brian Ward 2021
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## How systemd works
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### Goal-directed units
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`systemd` works on the basis of _goals_. Each goal is system task defined as a
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**unit**. A unit contains instructions and a specification of dependencies to
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other units.
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When activating a unit, `systemd` first activates the dependencies and then
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moves onto the details of the unit itself. `init` as implemented by `systemd`
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does not follow a rigid sequence every time, initialising units in the same
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sequence and waiting for one to complete before starting another. Instead it
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activates units whenever they are ready. This, its parallelized nature, is one
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of the main advantages over previous programs that managed the `init` sequence
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on Linux (such as for example System V);
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### Unit types
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Units are organised into **unit types**. The main types that run at boot time
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are as follows:
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- service units (`.service`)
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- control service daemons
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- socket units (`.socket`)
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- handle incoming network connection request locations
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- device units (`.device`)
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- disks and other devices
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- mount units (`.mount`)
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- handle the attachment of filesystems to the system
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- target units
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- control other units by organising them into groups
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For example, at boot, a target unit called `default.target` groups together a
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number of service and mount units as dependencies. These then run in a
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graph-like dependency structure where a unit that comes late in the boot process
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can depend on several previous units making earlier branches of a dependency
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tree join back together.
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## systemd configuration files
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Units are managed via `systemd` configuration files.
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### Configuration file locations
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System level `systemd` config files are located in the _system unit directory_
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at `/usr/lib/systemd/system`. You shouldn't change or manipulate these files or
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attempt to add new config files here since they will be overwritten by the
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system.
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 _`systemd` global unit files_
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Local definitions that relate to the specific user and where the user herself
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can define units are located in the _system configuration_ directory:
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`/etc/systemd/system`.
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_`systemd` local unit files, specific to the currently logged-in user_
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### Example files
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Below is the systemd service file for the creation of UUIDs:
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```plain
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[Unit]
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Description=Daemon for generating UUIDs
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Documentation=man:uuidd(8)
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Requires=uuidd.socket
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[Service]
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ExecStart=/usr/sbin/uuidd --socket-activation
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Restart=no
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User=uuidd
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Group=uuidd
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ProtectSystem=strict
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ProtectHome=yes
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PrivateDevices=yes
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PrivateUsers=yes
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ProtectKernelTunables=yes
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ProtectKernelModules=yes
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ProtectControlGroups=yes
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MemoryDenyWriteExecute=yes
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ReadWritePaths=/var/lib/libuuid/
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SystemCallFilter=@default @file-system @basic-io @system-service @signal @io-event @network-io
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[Install]
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Also=uuidd.socket
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```
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- The `Unit` section provides metadata about the unit including which `systemd`
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dependencies it has
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- `Service` constitutes the main specification for the unit
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- `Install` is the call to set the dependencies running before the `Service`
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functions are accessible.
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## systemd operations: systemctl
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The `systemctl` command is the chief way of interacting with `systemd`. You use
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it to activate and deactivate services, list their status, reload the
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configuration and so.
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### View the dependency graph
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`systemctl status` by itself will print a long list of units grouped by their
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dependency relations. It will also provide some metadata about the current
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systemd boot context.
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### Viewing active units
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Below I have listed the running units pertaining to bluetooth:
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```
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$ systemctl list-units | grep bluetooth
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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
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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
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sys-subsystem-bluetooth-devices-hci0.device loaded active plugged /sys/subsystem/bluetooth/devices/hci0
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sys-subsystem-bluetooth-devices-hci0:3585.device loaded active plugged /sys/subsystem/bluetooth/devices/hci0:3585
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bluetooth.service loaded active running Bluetooth service
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bluetooth.target loaded active active Bluetooth Support
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```
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### List jobs
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Requests to activate, reactivate and restart units are called **jobs** in
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`systemd`. They can be thought of as unit state changes. Current jobs can be
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listed with `systemctl list-jobs`.
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This will most likely return `No jobs running` if the computer has been running
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for a while. Most jobs execute at boot.
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### Enable/disable, start/stop units
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If a unit has dependencies it is necessary to _enable_ it before starting it.
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This installs the dependencies.
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```bash
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systemctl enable mongodb.service
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Created symlink /etc/systemd/system/multi-user.target.wants/mongodb.service → /usr/lib/systemd/system/mongodb.service.
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```
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Then we can start:
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```
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systemctl start mongodb.service
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```
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To stop the service:
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```
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systemctl stop mongodb.service
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```
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After this we should disable it, in order to remove any symbolic links it has
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created on the system as part of the install process:
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```bash
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systemctl disable mongodb.service
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Removed "/etc/systemd/system/multi-user.target.wants/mongodb.service".
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```
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## Why use systemd over cron ?
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https://mark.stosberg.com/2016-08-26-rsnapshot-and-systemd/
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