• Shutting down:
• Proper shutdown is important (we'll talk more about this when we cover file systems)
  
• shutdown -h now
  
• halt -h   
  
• Rebooting
• reboot
• shutdown -r
  
• Bootstrapping (booting)
  
• what gets done during a boot?
  
1. read the boot loader of the disk's master boot record
2. load/init the kernel
3. detect/configure devices
4. create kernel processes
5. Admin intervention if needed (single-user mode)
6. Execute startup scripts
• Unix supports a 'run level"   (type 'runlevel' to see current level)
  
• level 0: shutdown
• level 1: single user mode
• levels 2 - 5 various levels -  level 2 is normal.  Other levels might be admin specific.  Details vary on platform.
• level 6 reboot level
• What's up with this?    Seems to be based on historic telecom phone switches supporting a number of run levels
• boot step 1:  we assume the boot loader is GRUB
• Grand Unified Boot Loader
  
• Config in /boot/grub
  
• boot steps 2 -6:
• 2: load/init the kernel:
• issue a 'uname -a' to see the exact kernel boot image
• located in /boot
• eventually the init process is created which then starts other kernel processes.
• 6:  startup scripts
• historically /etc/inittab was used for init configuration, Ubuntu uses upstart (the equivalent to the init process) and config files in /etc/init.d
• Because there are init files from upstart as well as from traditional /etc/rc scripts,  the command update-rc.d helps maintain compatibility
  

For Ubuntu 16.04, runlevels are slightly altered.

• systemd - provides an interface to startup and init of the system
  • Try : systemd --test ;should dump the startup sequence but NOT actually run it
  • --log-level : sets log level (emerg, alert, ..info, debug
• systemctl - controls systemd system and service mgr
  • systemctl -a : shows all loaded modules
  • sudo systemctl isolate multi-user.target : switches run level
  • To change the default runlevel after a reboot:
    • sudo systemctl start graphical.target : switches to GUI mode
• Mapping between older runlevel and updated Target
  • 0 --- poweroff.target
  • 1 --- rescue.target (single user mode)
  • 2,3,4 --- multi-user.target
  • 5 --- graphical.target
  • 6 --- reboot.target
  
  
  
  

last update: 2/16/17