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Date:      Wed, 13 Jun 2018 15:46:49 +0200
From:      Polytropon <freebsd@edvax.de>
To:        Manish Jain <jude.obscure@yandex.com>
Cc:        Shane Ambler <FreeBSD@ShaneWare.Biz>, freebsd-questions@freebsd.org
Subject:   Re: How to detect single user mode in FreeBSD ?
Message-ID:  <20180613154649.6e49eeda.freebsd@edvax.de>
In-Reply-To: <f66e1b62-a6d5-c969-78ca-3ae9eb82efc5@yandex.com>
References:  <e9731c0f-1269-8919-836a-29b9a2f6b0dc@yandex.com> <c0718db9-8b46-2301-a770-cd334cbf0f07@ShaneWare.Biz> <f66e1b62-a6d5-c969-78ca-3ae9eb82efc5@yandex.com>

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On Wed, 13 Jun 2018 18:04:08 +0530, Manish Jain wrote:
> On 06/13/18 15:55, Shane Ambler wrote:
> > When in single user mode PID 1 should be "/sbin/init -s" which becomes
> > "/sbin/init --" in multi user mode.
> > 
> > The next closet to knowing would be looking at mount, in single user
> > mode you will only have / mounted read only and /dev. It is rare for any
> > system to be past single user mode with only that mount setup but the
> > user can mount the filesystems before starting your script and still be
> > in single user mode.
> 
> Hi Shane,
> 
> Tx for replying. But don't you think there should ideally be a sysctl to 
> be detect the runlevel, particularly single user mode ?

Currently, there is no such sysctl availablr (or another
utility that prints the current runlevel - see runlevel-
related comment later on).



> It makes things 
> easily documentible, just as when we can use sysctl to find out if the 
> OS is virtualized (I think kern.vm_guest).

You can probably work with the information from "man 8 init", such as

     If the system security level (see security(7)) is initially nonzero, then
     init leaves it unchanged.  Otherwise, init raises the level to 1 before
     going multi-user for the first time.  Since the level cannot be reduced,
     it will be at least 1 for subsequent operation, even on return to single-
     user.  If a level higher than 1 is desired while running multi-user, it
     can be set before going multi-user, e.g., by the startup script rc(8),
     using sysctl(8) to set the kern.securelevel variable to the required
     security level.

[...]

     If run as a user process as shown in the second synopsis line, init will
     emulate AT&T System V UNIX behavior, i.e., super-user can specify the
     desired run-level on a command line, and init will signal the original
     (PID 1) init as follows:

     Run-level    Signal     Action
     0            SIGUSR2    Halt and turn the power off
     1            SIGTERM    Go to single-user mode
     6            SIGINT     Reboot the machine
     c            SIGTSTP    Block further logins
     q            SIGHUP     Rescan the ttys(5) file

See "man 8 init" for further details.



For details about the run-levels (which aren't that common to
the FreeBSD boot & startup mechanism, which is BSD-style init,
whereas runlevels appear in System V sysvinit-like mechanisms),
check:

https://docs.freebsd.org/doc/7.4-RELEASE/usr/share/doc/en_US.ISO8859-1/articles/linux-users/startup.html

https://www.freebsd.org/doc/en_US.ISO8859-1/articles/linux-users/startup.html

You could summarize it as "FreeBSD doesn't have runlevels". :-)



However, you can still check for a process that is typucally
run in multi-user mode. If it's _not_ running - well, you are
in single-user mode.

Such a test could look like this:

	if ! ps -auxww | grep -v "grep" | grep "adjkerntz" > /dev/null; then
	   echo "Single user";
	else
	   echo "Multi-user";
	fi

In this example, a running instance of adjkerntz is being
process-grepped for.

Suggestion found here:

https://superuser.com/questions/457111/how-to-get-run-level-in-freebsd






-- 
Polytropon
Magdeburg, Germany
Happy FreeBSD user since 4.0
Andra moi ennepe, Mousa, ...



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