...
wm geometry . +0-0
button .halt -background #002040 -foreground #FFFFFF -borderwidth 1 -font monospace:size=12 -text Shutdown \
-command {exec /sbin/poweroff}
button .reboot -background #002040 -foreground #FFFFFF -width 8 -borderwidth 1 -font monospace:size=12 -text Reboot \
-command {exec /sbin/reboot}
pack .halt .reboot -side left -fill both
...
