What is it? (was Re: Current Run-Level: How can you tell?)

[Jeff Leyser]

In post <1991Apr25.171617.13505 at ssd.kodak.com>, weimer at ssd.kodak.com says:
!!
!!In article <dfs.672587346 at pulaski>, dfs at doe.carleton.ca (David F. Skoll)
!!writes:
!!|> I use a Sun system, and I've never heard of "Run-Level."
!!|> 
!!|> What is it?
!!
!!A brief description (since I've only had brief exposure to it...)
I deal with it daily.
!!
!!BSD Unix (?) has three "run levels": Single-user, Multi-user and Halt.
!!Usually we say the system is: 
!!     in Single-user mode
!!     in Multi-user mode
!!     Down ( 1/2 :-) )
!!
!!System V (?) Unix has many levels:
!!     0 == Halt
!!     3 == normal Multi-user mode
!!     others (this is were it gets brief :-) )

Well, sorta.  In SysV, init has 11 levels, numbered 0-6,a,b,c,S.  Each
line in the /etc/inittab file has a corresponding set of identifiers,
e.g:
s2:23:wait:/etc/rc2 >/dev/console 2>&1 </dev/console
   ^^
   run-levels

What each run-level "means" is somewhat vendor-specific (at least in SysVr3).
On our Motorola boxes, 0 and 6 are shutdown, 1 and S are single users, 2 is
multi-user, 3 is multi-user + RFS, and the rest are user defined.  On
out NCR iron, 0 and S are single-user, 1 is multi-user, 2 is multi-user
+ RFS, 6 is shutdown, and the rest are user defined.  Yes, I have often
shutdown the Motorola when trying to go single-user, and I have also
powered-down NCR boxes when they were only single-user, and hadn't been
sync'ed :^)
-- 
Jeff Leyser                                     jeffl at NCoast.ORG