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24
Linux/systemd.md
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@ -4,14 +4,13 @@ categories:
tags: [systems-programming, systemd]
---
# `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.
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`.
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
@ -30,7 +29,7 @@ When activating a unit, `systemd` first activates the dependencies and then move
Units are organised into **unit types**. The main types that run at boot time are as follows:
- service units (`.service`)
- control service daemons
- control service daemons
- socket units (`.socket`)
- handle incoming network connection request locations
- device units (`.device)
@ -40,7 +39,6 @@ Units are organised into **unit types**. The main types that run at boot time ar
- 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
@ -54,7 +52,6 @@ System level `systemd` config files are located in the _system unit directory_ a
![](/_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)
@ -91,17 +88,17 @@ SystemCallFilter=@default @file-system @basic-io @system-service @signal @io-eve
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.
- 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.
`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
@ -118,6 +115,7 @@ $ systemctl list-units | grep bluetooth
```
### Get status of a specific unit
Here I have requested the status of the currently running `mongodb` unit:
```
@ -133,6 +131,7 @@ mongodb.service - MongoDB Database Server
└─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:
@ -171,6 +170,7 @@ Created symlink /etc/systemd/system/multi-user.target.wants/mongodb.service →
```
Then we can start:
```
systemctl start mongodb.service
```
@ -187,3 +187,7 @@ After this we should disable it, in order to remove any symbolic links it has cr
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/