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Linux/Procedural/systemd.md

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+---
+categories: 
+  - Linux
+  - Operating_Systems
+tags: [user_space, 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. 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.
+
+### Unit types 
+
+Units are organised into **unit types**. The main types that run at boot time are as follows:
+
+- service units
+  -  control service daemons
+- target units
+  - control other units by organising them into groups
+- socket units
+  - handle incoming network connection request locations
+- mount units
+  - handle the attachment of filesystems to the system
+
+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_