A.P. Lawrence

This article is from a FAQ concerning SCO operating systems. While some of the information may be applicable to any OS, or any Unix or Linux OS, it may be specific to SCO Xenix, Open Desktop or Openserver.

There is lots of Linux, Mac OS X and general Unix info elsewhere on this site: Search this site is the best way to find anything.

Unix, Xenix and ODT General FAQ

How do I restrict logins?

For some reason, I often get requests to limit users to one login. I guess the people asking such questions have a reason for wanting to restrict logins this way. The only way to do it is to add a script to either /etc/profile or the particular user's .profile that tests to see if this user is logged in somewhere else. Something like this in /etc/profile will work:

IAM=`who am i | cut -d" " -f1`
COUNT=`w | cut -d" " -f1 | grep "^$IAM$" | wc -l`
[ $COUNT -gt 1 ] && exit 0


Similar tricks can restrict a user to a particular tty:

IAM=`who am i | cut -d" " -f1`
TTY=`tty`
[ $TTY != "/dev/tty07" ] && [ $IAM = "tony" ] && exit 0

And then there's always restricting login to root: put this in /etc/profile

IAM=`who am i | cut -d" " -f1`
[ -f /etc/nologin ] && [ $IAM != "root" ] && exit 0

When you need to restrict logins, just "touch /etc/nologin"; remove it when the need is over.

IN ALL CASES YOU HAVE TO CONSIDER THE EFFECT OF SINGLE USER LOGIN. Tests like this can prevent you from logging in from the CTRL-D single user mode prompt because "who am i" and similar commands will return nothing. Avoid that by using "who -r" to test the run level.

You can restrict root to a particular device by adding a line like

CONSOLE=/dev/tty01

to /etc/default/login (se "man M login").

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