Create a logoff script/task for Linux

"For remote SSH sessions, it's the job of your shell to handle any on-logout scripting. The Bash shell typically runs ~/.bash_logout.

For X sessions, the login manager controls on-logout scripting. File locations depend on which login manager is in use. The Xserver is not running when the on-logout script is executed, so don't include any programs in the script that try to access the display.

For Gnome, use /etc/gdm/PostSession/Default for a system-wide default logout script (runs no matter who's logging out). Unlike Bash's logout script, I don't think there's a user-specific version of a GDM PostSession script.
XDM uses a Reset script is installed to /etc/X11/xdm/Xreset.
For KDE (using KDM), you want the Reset script. KDE3 uses /etc/kde3/kdm/Xreset. The location may be specified in KDM's configuration file, /etc/kde3/kdm/kdmrc, with the Reset key.
WDM apparently uses a pair of Reset scripts: a per-display script at /etc/X11/wdm/Xreset_0 (for display :0), and a global script at /etc/X11/wdm/Xreset. The WDM configuration file is in /etc/X11/wdm/wdm-config.

The file locations in this answer are from Debian packages. Ubuntu packages will generally use the same files, but other distributions may use alternate locations."

The C-shell, a.k.a. csh, has the .logout functionality, and Bash (the more current default) has .bash_logout, but I don't remember any other shells (ksh, sh, etc) with a similar concept.