How to reboot the system if busybox run as init

[Laurent Bercot]

>> sync && sleep 1 && reboot -f

 This is the right way to do it, when your process 1 is a shell.
 Depending on your hardware and the way you're using it, you might
want to manually unmount - or remount read-only - a few filesystems
first.

 Generally, to shut down a Unix machine, you want to do the following
operations, in that order:

 - Cleanly stop all long-running processes. (This can be done easily
with a supervision system, not so easily with SystemV-style inits.)
 - Gently kill all processes (with a SIGTERM), and wait a few seconds.
 - Brutally kill all processes (with a SIGKILL), to catch processes
that have escaped the previous step.
 - Unmount all your filesystems. This is possible because you have
killed all the processes (save process 1), so you should not have any
remaining open files on other filesystems than /.
 - Remount the / filesystem read-only. My opinion is that / should
*always* be read-only, but most systems are badly configured and can't
work properly with a read-only /.
 If your process 1 is a shell, you might have trouble remounting /
read-only, because the shell might have some files open for writing.
There is no way around that, except design your system differently.
It also does not matter if your root filesystem lives in RAM (which
is a common way of working around a bad design).
 - Perform hardware-specific operations such as saving the system
clock, writing various states into NVRAM, etc.
 - Shut down the hardware.

 The SystemV-style "reboot" command does all this automatically, but
of course, it only works if process 1 is a SystemV-style init. Other
init schemes have their own way of doing it (runit uses the /etc/runit/3
script, s6 runs /service/.s6-svscan/finish as process 1...)
 If your process 1 is a shell, you will have to do it by hand. Harald's
command is a shortcut that works if your system is simple enough and
you don't have to worry about a random fsck.

 
> But, there should be an 'init 6' around too, shouldn't it?

 Yes, but that command only sends a signal (typically via the
/dev/initctl fifo) to process 1. If process 1 is a SystemV-style init,
it will listen to it and trigger its shutdown procedure. If process 1
is a shell, there is no such hook and the command will simply be ignored.

-- 
 Laurent