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Linux/systemd.md
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@ -4,7 +4,7 @@ categories:
tags: [systems-programming] 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.
@ -16,7 +16,7 @@ On Linux systems `systemd` is used to implement `init`.
> `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 > `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 ## How systemd works
### Goal-directed units ### Goal-directed units
@ -41,7 +41,7 @@ Units are organised into **unit types**. The main types that run at boot time ar
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
Units are managed via `systemd` configuration files. Units are managed via `systemd` configuration files.
@ -92,7 +92,7 @@ Also=uuidd.socket
- `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.
@ -188,6 +188,6 @@ 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` ? ## Why use systemd over cron ?
https://mark.stosberg.com/2016-08-26-rsnapshot-and-systemd/ https://mark.stosberg.com/2016-08-26-rsnapshot-and-systemd/