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18
Linux/systemd.md
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@ -4,7 +4,6 @@ categories:
tags: [systems-programming, systemd] tags: [systems-programming, systemd]
--- ---
# `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.
@ -40,7 +39,6 @@ Units are organised into **unit types**. The main types that run at boot time ar
- target units - target units
- control other units by organising them into groups - 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. 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 ## `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) ![](/_img/systemd-global-files.png)
_`systemd` global unit files_ _`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`. 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) ![](/_img/systemd-local-files.png)
@ -91,17 +88,17 @@ SystemCallFilter=@default @file-system @basic-io @system-service @signal @io-eve
Also=uuidd.socket Also=uuidd.socket
``` ```
* The `Unit` section provides metadata about the unit including which `systemd` dependencies it has - The `Unit` section provides metadata about the unit including which `systemd` dependencies it has
* `Service` constitutes the main specification for the unit - `Service` constitutes the main specification for the unit
* `Install` is the call to set the dependencies running before the `Service` functions are accessible. - `Install` is the call to set the dependencies running before the `Service` functions are accessible.
## `systemd` operations: `systemctl` ## `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. 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 ### 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 ### Viewing active units
@ -118,6 +115,7 @@ $ systemctl list-units | grep bluetooth
``` ```
### Get status of a specific unit ### Get status of a specific unit
Here I have requested the status of the currently running `mongodb` 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 └─931 /usr/bin/mongod --config /etc/mongodb.conf
Aug 17 07:25:27 archbish systemd[1]: Started MongoDB Database Server.**** 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. 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: 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: Then we can start:
``` ```
systemctl start mongodb.service 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 systemctl disable mongodb.service
Removed "/etc/systemd/system/multi-user.target.wants/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/