xdmshell - Man Page

/usr/bin/xdmshell

Xdmshell can be used on systems that are configured to start a console in text mode by default, to allow users to start the X display manager by logging in as a special user with the shell set to the xdmshell program.

Many servers set the keyboard to do non-blocking I/O under the assumption that they are the only programs attempting to read from the keyboard. Unfortunately, some versions of getty(8) will immediately see a continuous stream of zero-length reads which they interpret as end-of-file indicators.  Eventually, init(8) will disable logins on that line until somebody types the following as root:

On some platforms, one alternative is to disable logins on the console and always run xdm(8) from /etc/inittab.

Another approach is to set up an account whose shell is the xdmshell program found in the xdm distribution.  This program is not installed by default so that site administrators will examine it to see if it meets their needs.  The xdmshell utility makes sure that it is being run from the appropriate type of terminal, starts xdm, waits for it to finish, and then resets the console if necessary.  If the xdm resources file (specified by the DisplayManager*resources entry in the xdm-config file) contains a binding to the abort-display action similar to the following

the console can then by restored by pressing the indicated key (Control-R in the above example) in the xdm login window.

The xdmshell program is usually installed setuid to root but executable only by members of a special group, of which the only member is the account which has xdmshell as its shell:

%  grep xdm /etc/passwd
x::101:51:Account for starting up X:/tmp:/usr/bin/xdmshell
%  grep 51 /etc/group
xdmgrp:*:51:
%  ls -lg /usr/bin/xdmshell
-rws--x---   1 root     xdmgrp     20338 Nov  1 01:32 /usr/bin/xdmshell

If the xdm resources have not been configured to have a key bound to the abort-display() action, there will be no way for general users to login to the console directly.  Whether or not this is desirable depends on the particular site.

X(7), xdm(8), xinit(1)

xdm(8).