Linux 2.6 - man page for sulogin (linux section 8)
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SULOGIN(8) Linux System Administrator's Manual SULOGIN(8)
sulogin - Single-user login
sulogin [ -e ] [ -p ] [ -t SECONDS ] [ TTY ]
sulogin is invoked by init(8) when the system goes into single user mode. (This is done
through an entry in inittab(5).) Init also tries to execute sulogin when the boot loader
(e.g., grub(8)) passes it the -b option.
The user is prompted
Give root password for system maintenance
(or type Control-D for normal startup):
If the root account is locked, as is the default on Ubuntu, no password prompt is dis-
played and sulogin behaves as if the correct password were entered.
sulogin will be connected to the current terminal, or to the optional device that can be
specified on the command line (typically /dev/console).
If the -t option is used then the program only waits the given number of seconds for user
If the -p option is used then the single-user shell is invoked with a dash as the first
character in argv. This causes the shell process to behave as a login shell. The
default is not to do this, so that the shell will not read /etc/profile or $HOME/.profile
After the user exits the single-user shell, or presses control-D at the prompt, the system
will (continue to) boot to the default runlevel.
sulogin looks for the environment variable SUSHELL or sushell to determine what shell to
start. If the environment variable is not set, it will try to execute root's shell from
/etc/passwd. If that fails it will fall back to /bin/sh.
This is very valuable together with the -b option to init. To boot the system into single
user mode, with the root file system mounted read/write, using a special "fail safe" shell
that is statically linked (this example is valid for the LILO bootprompt)
boot: linux -b rw sushell=/sbin/sash
sulogin checks the root password using the standard method (getpwnam) first. Then, if the
-e option was specified, sulogin examines these files directly to find the root password:
/etc/shadow (if present)
If they are damaged or nonexistent, sulogin will start a root shell without asking for a
password. Only use the -e option if you are sure the console is physically protected
against unauthorized access.
Miquel van Smoorenburg <firstname.lastname@example.org>
17 Jan 2006 SULOGIN(8)
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