sulogin [options] [tty]
sulogin is invoked by init when the system goes into single-user mode.
The user is prompted:
If the root account is locked and --force is specified, no password is required.
sulogin will be connected to the current terminal, or to the optional tty device that can be specified on the command line (typically /dev/console).
When the user exits from the single-user shell, or presses control-D at the prompt, the system will continue to boot.
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.
sulogin was written by Miquel van Smoorenburg for sysvinit and later ported to util-linux by Dave Reisner and Karel Zak.
The sulogin command is part of the util-linux package and is available from Linux Kernel Archive