Process Parentage

• Where do new processes come from?
  • New processes are typically fork(2)’ed from a parent proces…
  • …which can be traced back recursively to an init process initiated by the kernel during system boot.
• i.e., process parentage traces back to an init process initiated by the kernel.

System Startup

• The boot process involves firmware, bootloader, kernel, and the init system.
  • Power on
  • processor executing firmware at known address ([EEP]ROM connected to a address bus)
  • firmware loads bootloader
  • bootloader loads kernel
  • kernel starts init system, responsible for managing system’s startup tasks

init System

• init plays a crucial role in managing the system’s startup and initialisation process.
• The system (typically PID 1) serves as the root of the process tree and finishes booting the system by handling tasks:
  • mounts filesystems
  • starts terminal (tty) device managers
  • starts network services
  • starts graphical login services
  • etc.

BSD init

• Offers two -user modes for system startup:
  • Single-user mode: forks a shell (/bin/sh) for troubleshooting purposes.
  • Multi-user mode: the “normal” system startup
    • forks a shell for /etc/rc
      • loads settings from config files (/etc/defaults/rc.conf)
      • overrides config (/etc/rc.conf)
      • runs scripts from /etc/rc.d with rcorder(8)
    • starts terminal login services (read /etc/ttys: start getty(8) $\to$ login(1))

SysV init

• Uses runlevels with start/stop scripts for each level:

  Runlevel	Meaning
  0	Halt
  1	Single user mode
  2	Default for AIX, SVR3, SVR4
  3	Default for Gentoo, HP-UX, Slackware…
  5	Graphical (X)
  6	Reboot

• Startup scripts are help in /etc/init.d, with symbolic links to directories like /etc/rc.d/rcN.d indicating services associated with specific runlevels

• Each runlevel has a directory (like /etc/rc.d/rc4.d)

  • Symbolic links to scripts in /etc/init.d
  • symlink implies services “part of” runlevel

• Can change runlevel: init 3. init 4. etc.

  • Identifies set differences between runlevels
  • runs etc/rc.d/rcN.d/foo stop or start

launchd

• A process introduced by Apple in 2005 for Mac OSX 10.4 Tiger
• Replaces init and inetd
• Listens to network ports listed in /etc/inetd.conf and…
• …starts process to serve requests upon connection
  • Therefore unifying service discovery by name or connection

systemd

• A system and service manager that serves as the core manager for various system components:
  • services
  • sockets
  • mounts
  • timers
  • etc.
• Devekioed starting in 2010 and has been adopted by several Linux distros since 2011
• Incorporates elements of different system components:
  • init
  • inetd
  • syslog
  • login
  • cron
  • devices
  • networking
  • temporary files
  • boot
  • etc.
• Apparently hugely controversial…

Present Day…

• Today, there are ongoing developments in the realm of init systems.
• With alternatives to systemd being explored such as…
  • jobd
  • nosh
  • OpenRC
  • procd
  • runit
  • and S6
• The landscape remains dynamic and evolving with new possibilities