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During boot the kernel locates /sbin/init. Init is the parent of all
processes. Its primary role is to create processes from a script stored
in the file /etc/inittab . The man page for inittab describes syntax
of inittab lines ("id:runlevels:action:process")
runlevels: (0-6,A-C,or empty)
0 & 6 are used for shutting the machine down
1 is reserved for single-user mode
2-5 are available to define custom operating configurations
A-C are used for "ondemand" actions (*not* a
runlevel change)
action values of particular interest (there are others, see man inittab)
- [initdefault]defines the default runlevel (eg "id:3:initdefault:")
- [sysinit]always processed, always before anything else (eg, "si::sysinit:/etc/rc/d/rc.sysinit")
- [once]used mainly for "any-level" processes (eg,
"ud::once:/sbin/update") [starts bdflush]
- [wait]used for the primary runlevel-specifc action script (eg "i3:3:wait:/etc/rc.d/rc3")
- [respawn]- used to provide login gettys for virtual consoles, serial
connections, xdm (eg, "6:2345:respawn:/sbin/mingetty tty6")
- [ctrlaltdel]process to be executed when the CTRL-ALT-DEL key combination
(three finger salute) is pressed. Typically some shutdown procedure.
This line in /etc/inittab instructs /sbin/init what to do next:
si::sysinit:/etc/rc.d/rc.sysinit
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