add linux notes

docs/linux/init/open-rc.md

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+# [OpenRC](https://github.com/OpenRC/openrc/blob/master/user-guide.md)
+
+**OpenRC** is a dependency-based init system for Unix-like systems that maintains compatibility with the system-provided init system, normally located in /sbin/init. OpenRC is Gentoo's native init system, although other init systems are available.
+
+OpenRC will start necessary system services in the correct order at boot, manage them while the system is in use, and stop them at shutdown. It can manage daemons installed from the Gentoo repository, can optionally supervise the processes it launches, and has an the possibility to start processes in parallel - when possible - to shorten boot time.
+
+## Runlevels & Services
+
+OpenRC has a concept of **runlevels**. A runlevel is a collection of services that needs to be started. Instead of random numbers they are named, and users can create their own if needed.
+
+The `rc-status` helper will print all currently active runlevels and the state of services in them:
+
+```sh
+Runlevel: default
+  modules                     [  started  ]
+  lvm                         [  started  ]
+```
+
+All runlevels are represented as folders in `/etc/runlevels/` with symlinks to the actual service scripts.
+
+Calling `openrc` with an argument will switch to that runlevel; this will start and stop services as needed.
+
+### Managing Runlevels
+
+Managing runlevels is usually done through the `rc-update` helper, but could of course be done by hand if desired.
+
+```sh
+rc-update add <service> <runlevel>
+rc-update del <service> <runlevel>
+rc-update show <runlevel>
+```
+
+The default startup uses the runlevels `sysinit`, `boot`, and `default`, in that order. Shutdown uses the `shutdown` runlevel.
+
+### Managing Services
+
+Any service can, at any time, be started/stopped/restarted by executing with the `rc-service` helper  someservice start, rc-service someservice stop, etc.
+
+```sh
+rc-service <service> start
+rc-service <service> stop
+rc-service <service> restart
+rc-service <service> status
+rc-service <service> zap
+```
+
+Another, less preferred method, is to run the service script directly, e.g. /etc/init.d/service start, /etc/init.d/service stop, etc.
+
+```sh
+/etc/init.d/<service> start
+/etc/init.d/<service> stop
+/etc/init.d/<service> restart
+/etc/init.d/<service> status
+/etc/init.d/<service> zap
+```
+
+There is a special command `zap` that makes OpenRC 'forget' that a service is started. This is mostly useful to reset a crashed service to stopped state without invoking the (possibly broken) stop function of the service script.
+
+Calling `openrc` without any arguments will try to reset all services so that the current runlevel is satisfied.

docs/linux/init/systemd.md

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+# [Systemd](https://wiki.archlinux.org/title/Systemd)
+
+**systemd** is a suite of basic building blocks for a Linux system. It provides a _system_ and _service manager_ that runs as PID 1 and starts the rest of the system.
+
+`systemd` provides aggressive parallelization capabilities, uses socket and D-Bus activation for starting services, offers on-demand starting of daemons, keeps track of processes using Linux control groups, maintains mount and automount points, and implements an elaborate transactional dependency-based service control logic.