best suported systemd https://wiki.debian.org/systemd apt install systemd systemd needs systemd bus either dbus or dbus-broker polkitd | policykit-1 is needed for passwordless auth init=/lib/systemd/systemd
systemd-sysv makes symlink of /sbin/init apt-get install systemd-sysv
https://wiki.debian.org/OpenRC
https://wiki.debian.org/runit runit-init
http://www.yolinux.com/TUTORIALS/LinuxTutorialInitProcess.html
chroot mode / init=/bin/sh apt --download-only install sysvinit-core libpam-elogind
libelogind0 , libsystemd0 will pull systemd/elogind when necessary so you can swap one for other depending on need
sysvinit-core has init in /sbin/init depends on : initscripts, sysv-rc | openrc, sysvinit-utils /usr/sbin/init is symlinked to /lib/systemd/systemd by systemd-sysv package also provides /usr/sbin/halt,init,poweroff,reboot,runlevel,shutdown,telinit dracut has /init as systemd
init-system-helpers(essential)
/usr/bin/deb-systemd-helper,deb-systemd-invoke
/usr/sbin/invoke-rc.d,service,update-rc.d
sysvinit-core /usr/sbin/halt,init,poweroff,reboot,runlevel,shutdown,telinit /usr/share/sysvinit/inittab
sysvinit-utils(essential) /usr/bin/pidof /usr/lib/init/init-d-script /usr/lib/init/vars.sh /usr/lib/lsb/init-functions /usr/lib/lsb/init-functions.d/00-verbose /usr/sbin/fstab-decode /usr/sbin/killall5
reinstall udev without udev needs manually loading driver in /etc/modules , find /proc/modules depmod -a generates /lib/modules/kernel-version/modules.dep
https://packages.debian.org/stable/all/orphan-sysvinit-scripts/filelist
/etc/inittab
syntax : id:runlevels:action:process id: A unique identifier for each entry (up to four characters). runlevels: Specifies the runlevels at which this entry should be executed. Each runlevel is a single digit, so multiple runlevels can be specified without separators (e.g., 123). action: Defines the action that init should take. Common actions include:
respawn: Restart the process if it terminates.
wait: Start the process once when entering the specified runlevel and wait for it to finish.
once: Start the process once when entering the specified runlevel without waiting.
sysinit: Run the process during system initialization.
initdefault: default init script to run
ctrlaltdel: Execute the process when Ctrl+Alt+Del is pressed.
powerfail: Run the process if a power failure is detected (if supported).
process: The command to execute, which can be a path to a program or a script.
runlevels
debian has runkevel in folder /etc/rc0.d,rc1.d....rc6.d,rcS.d
scripts in runlevels are renamed SXX or KXX to enable or disable
example : id:2:initdefault:
si::sysinit:/etc/init.d/rcS empty runlevel here means that the process defined by this entry runs at the system initialization stage before the runlevel is set.
~~:S:wait:/sbin/sulogin --force
l0:0:wait:/etc/init.d/rc 0
ca:12345:ctrlaltdel:/sbin/shutdown -t1 -a -r now
ps aux | head
runit sudo apt install runit-systemd runit runit-run elogind libpam-elogind orphan-sysvinit-scripts systemctl procps