sysvinit/man/sulogin.8

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.TH SULOGIN 8 "17 Jan 2006" "sysvinit " "Linux System Administrator's Manual"
.SH NAME
sulogin \- Single-user login
.SH SYNOPSIS
.B sulogin
[ \fB\-e\fP ]
[ \fB\-p\fP ]
[ \fB\-t\fP \fISECONDS\fP ]
[ \fITTY\fP ]
.SH DESCRIPTION
.I sulogin
is invoked by \fBinit\fP(8) when the system goes into single user mode.
(This is done through an entry in \fBinittab\fP(5).)
\fBinit\fP(8) also
tries to execute \fBsulogin\fP when
the boot loader (e.g., \fBgrub\fP(8))
passes it the \fB\-b\fP option.
.PP
The user is prompted:
.IP "" .5i
Give root password for system maintenance
.br
(or type Control\-D for normal startup):
.PP
\fBsulogin\fP will be connected to the current terminal, or to the
optional device that can be specified on the command line
(typically \fI/dev/console\fP).
.PP
If the \fB\-t\fP option is used then the program only waits
the given number of seconds for user input.
.PP
If the \fB\-p\fP option is used then the single-user shell is invoked
with a \fIdash\fP as the first character in \fIargv[0]\fP.
This causes the shell process to behave as a login shell.
The default is \fBnot\fP to do this,
so that the shell will \fBnot\fP read \fI/etc/profile\fP
or \fI$HOME/.profile\fP at startup.
.PP
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.
.SH ENVIRONMENT VARIABLES
\fBsulogin\fP looks for the environment variable \fBSUSHELL\fP or
\fBsushell\fP to determine what shell to start. If the environment variable
is not set, it will try to execute root's shell from \fI/etc/passwd\fP. If that
fails it will fall back to \fI/bin/sh\fP.
.PP
This is very valuable together with the \fB\-b\fP option to \fBinit\fP(8). 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)
.PP
boot: linux \-b rw sushell=/sbin/sash
.SH FALLBACK METHODS
\fBsulogin\fP checks the root password using the standard method (\fBgetpwnam\fP(3))
first.
Then, if the \fB\-e\fP option was specified,
\fBsulogin\fP examines these files directly to find the root password:
.PP
/etc/passwd,
.br
/etc/shadow (if present)
.PP
If they are damaged or nonexistent, \fBsulogin\fP will start a root shell
without asking for a password. Only use the \fB\-e\fP option if you
are sure the console is physically protected against unauthorized access.
.SH AUTHOR
.MT miquels@\:cistron\:.nl
Miquel van Smoorenburg
.ME
.SH SEE ALSO
.BR init (8),
.BR inittab (5)