init -- the userland adventure begins

init is the grandmother of all processes, and usually runs as PID 1

/etc/inittab is its configuration file, which determines what processes it initially starts

• Simple ':'-delimited format:
• id:run levels:action type:action command

init has a notion of "run levels" corresponding to different levels of system functionality

• 1 or S: single-user mode, for system administration work
• 2, 3, 4, 5: multi-user mode (sometimes with different levels of functionality, i.e. non-networked vs. networked)
• 0: system halt
• 6: system boot

Normal boot: init does stuff for run level 1, then run level 2

Normal shutdown: init does stuff for run level 0, then system halts

Normal reboot: init does stuff for run level 6, then reboots

Ubuntu replaced the traditional init with a variant called upstart. The main distinguishing characteristic of upstart is that each service's configuration is in separate files in /etc/init rather than lines in /etc/inittab.

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