Singler usermode through CD VS Single user mode system

mokkan used Ask the Experts™
I have a question regarding single user mode. We can go to single usermode using  CD image and through server, what are the differences between them?  Please let me know.

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There is no fundamental difference, both would grant you access to the file system and let you change the root password.

Single user mode = runlevel 1, and that runlevel is the same, no matter how you get to it:
If you are talking about Redhat, single user mode is different from run level 1, the one you refer to as single user mode with CD is a totally different concept.  Likewise emergency mode is another thing.  Unfortunately many documentations do not address it correctly.

RESCUE MODE--single user mode with CD-- Your system is not even up using your hard drive.  It is NOT a single user mode.  You can use CD/DVD (sort of live CD) to bring system up.  In addition, it would provide access to your actual file system as a mount, generally under /mnt/sysimage folder.  You should not write/create files in anything above /mnt/sysimage.  For example your actual /etc under rescue would be /mnt/sysimage/etc.  You can use chroot jail to make it look like /etc/ though.  There are too many details that I don't want to go to.  Unlike single user mode (or run level 1, you can also access NFS mounts or ftp servers to access packages over the network.

Single User Mode vs Run level 1 vs emergency--It is lesser than run level 1 but more than emergency.  the main difference is that if your /etc/inittab is corrupt, you can still get to the shell and correct the problem.  It is little more uncomfortable than run level 1 and if you go to emergency, you should really know what you are doing.  Generally you go to this mode when system doesn't come up with run level 1 or run level S (single user mode).  For example if I don't have basic scripts like /etc/fstab /etc/inittab , etc, and system simply doesn't come up I would go to run level emengency.  It gives you root shell without running any scripts.

You would not be able to know the difference it you have not rescued totally broken systems.

run level 5 > run level 3 > run level 1 > run level S > run level emergency
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Thank you this is the information I was looking for. If I undertand correctly you can' do NFS using single user mode using system, but using rescue CD we can use NFS right?

Yes.  Single user mode does NOT have networking so you cannot do NFS mounts.  However in rescue mode you can.

But in single user mode, system runs from your hard drive to run, so you can change it to run level 3 or run level 5 by issuing command (init 3 or init 5).

Rescue mode cannot be changed to any run level because you are not running system from hard driver OS.  You run it from CD/DVD.

To summarize, lesser the run level is, the lesser number of starting up scripts run.  If the system files are corrupted, altered, you would want to start with lesser run level so that it doesn't error out.  On the other hand the lesser number of scripts run, the lesser are the facilities--so you need to run at the most run level so that you can operate comfortably.
On Redhat to use NFS with rescue do this

boot: linux rescue askmethod


Thank you guys.

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