In Unix-based computer operating systems, init (i.e. initialization) is the first process started during booting of the computer system. It is just a daemon that is starting just after the kernel and which reads the /etc/inittab file in order to know what programs/daemons to start and/or what else to do.
(i) If you have accidentally removed the package and forgot to re-emerge again
(ii) if you have rebooted your system
then you are screwed.
At boot-time, after the kernel is decompressed and loaded, the first program that is loaded is init. If it's missing then your console will display a message like this:
sh: cannot set terminal process group (-1): Inappropriate ioctl for device sh: no job control in this shell sh-4.2# _
If you take a look at your /etc/inittab file then you will see exactly what commands your init program, if it still exists, should run (read more here). By running manually these commands you can recover your system.
The following commands help you recovering after accidentally removing sysvinit:
/sbin/rc sysinit /sbin/rc boot /sbin/rc default /sbin/agetty 38400 tty1 linux
The first command initialize the devices. The second one starts those daemons which normally start at boot-time. The third one starts those daemons which should start with a normal user. The last command just open a TTY console.
After you have run each of these commands try to re-emerge your sysvinit or whatever package you need. It should be fine...
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