initramfs-kernel

kernel runs /init.

/init is a script which sets INITRAMFS=1 and then execs /sbin/init.

/sbin/init is ELF comes from procd package. What it does (as pid=1):

• early (different from procd early):
  • mount /proc, /sysfs, and create /dev nodes, create /tmp subdirs etc
  • set PATH
• kmodloader in subprocess (waits until done)
• preinit
  • plugd in subprocess (seem to deal with firmware)
  • /etc/preinit in subprocess
• when /etc/preinit is done then it execvp the "real" /sbin/procd

procd states:

• early (first state):
  • coldplug (remount /dev, etc)
    • start udevtrigger in subproc
    • when udevtrigger completes, hotplug
    • when hotplug completes, next state (ubus)
• ubus:
  • start attempts to connect to ubs
  • start ubus as service
  • once connection's done then next state (init)
• init - read /etc/inittab and run:
  • respawn - things that need to keep rerunning once quitted
  • askconsole
  • askfirst - the "Press Enter to activate this console" message
  • sysinit - (usually /etc/init.d/rcS with S) and then next state (running)
• running - waits for shutdown signal (usr1/usr2) or reboot (int/term)
• shutdown
  • runs shutdown command per inittab (also /etc/init.d/rcS but with K)
  • when done next state (halt)
• halt - kill everything and either enter sleep loop or reboots

Debian

Needs devtmpfs in kernel. This seem to be first thing that /sbin/init does. (Maybe systemd specific.)