Why is ‘init 6’ the reboot command? (historic reasons)

documentationhistoryinitsignals

I would like to know why 6 is the number/code/signal associated with the reboot command in "init 6". I mean the history/stories/legend reasons, not in a technical way… If it was a "list related reason" or maybe a graphic thing about recursivity/circle-ouroboros/101 alike number.

I'm starting reading Design of the UNIX Operating System by Maurice Bach, but didn't find yet a reason or idea.

Best Answer

init 6 is the (or, a) reboot command because of the historical definitions of "runlevels", or general system states in which a host can be expected to be. These are generally defined as:

0 - Shut down / System halt
1 - Single User mode
2 - Reserved for administrative use
3 - Multi-User mode with networking and services
4 - Reserved for administrative use
5 - Multi-User mode with networking, services, and GUI login daemon
6 - Reboot

The init command tells the system to move to the specified runlevel. Because 6 is the commonly defined runlevel used to reboot the host, and init 6 (or telinit 6) is the means to go to that runlevel, this is why init 6 is generally understood to be a reboot command.

Technically speaking, because these can be redefined by a crafty or bored system administrator, it might be more advisable to use shutdown -r as a reboot command. This is in part because some distributions (e. g. Gentoo) eschew this convention entirely, and because of the proliferating deprecation of the System V Init system in favor of upstart and other "PID 1" daemons.

Related Solutions

Linux – Why do we need the reboot function in different binaries

We don't necessarily need them both, but we have them both because of the history of Unix, and its multiplicity of versions.

From their respective man pages:

The shutdown utility appeared in 4.0BSD.
A reboot utility appeared in Version 6 AT&T UNIX.

shutdown is more general-purpose, and more powerful, while reboot is friendlier and easier to remember.

shutdown allows you to specify a temporal argument (to restart in 5 minutes, for instance) and allows you to do many things besides reboot, including:

you can just kick off users and not actually shutdown
you can put the system to sleep instead of shutting down
you can simply shutdown without rebooting (like the halt command)
you can include a custom warning message for users on the system

However, if you just want to reboot the system now, it's easier to type reboot than shutdown -r now.

What will happen if I run the command init in the Ubuntu Gnome terminal

If you're not root (and you shouldn't normally be logging in as root), you'll just get a message:

bash$ init
init: Need to be root

if you are root, you can change the current run level of the system using init (it actually runs "telinit" to make the change).

bash$ sudo init
init: missing runlevel
Try `init --help' for more information.

if you really want to reboot your system, you can do it by telling init to switch to run level six:

bash$ sudo init 6

(I'm not actually going to do this because I don't want to really reboot my machine.)

For future reference, all of this stuff is well described in the man pages. Try running man init for example. Other interesting man pages include runlevel and telinit.

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Linux – Why do we need the reboot function in different binaries

What will happen if I run the command init in the Ubuntu Gnome terminal

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