How can I safely shutdown/reboot/logout KDE from the command line?

Note that this answer was written in 2010 for KDE 4. It may not apply to modern systems.

For kde4:

qdbus org.kde.ksmserver /KSMServer org.kde.KSMServerInterface.logout -1 -1 -1

The three integer parameters are the confirm, sdtype and sdmode arguments to KWorkSpace::requestShutDown. Their values are explained at the top of the page. Since the page has disappeared, here are the values (still present in a cache).

enum ShutdownConfirm {
  ShutdownConfirmDefault = -1,
  ShutdownConfirmNo = 0,
  ShutdownConfirmYes = 1
}

• ShutdownConfirmDefault: Obey the user's confirmation setting.
• ShutdownConfirmNo: Don't confirm, shutdown without asking.
• ShutdownConfirmYes: Always confirm, ask even if the user turned it off.

enum ShutdownMode {
  ShutdownModeDefault = -1,
  ShutdownModeSchedule = 0,
  ShutdownModeTryNow = 1,
  ShutdownModeForceNow = 2,
  ShutdownModeInteractive = 3
}

• ShutdownModeDefault: Select previous mode or the default if it's the first time.
• ShutdownModeSchedule: Schedule a shutdown (halt or reboot) for the time all active sessions have exited.
• ShutdownModeTryNow: Shut down, if no sessions are active. Otherwise do nothing.
• ShutdownModeForceNow: Force shutdown. Kill any possibly active sessions.
• ShutdownModeInteractive: Pop up a dialog asking the user what to do if sessions are still active.

enum ShutdownType {
  ShutdownTypeDefault = -1,
  ShutdownTypeNone = 0,
  ShutdownTypeReboot = 1,
  ShutdownTypeHalt = 2,
  ShutdownTypeLogout = 3
}

• ShutdownTypeDefault: Select previous action or the default if it's the first time.
• ShutdownTypeNone: Only log out.
• ShutdownTypeReboot: Log out and reboot the machine.
• ShutdownTypeHalt: Log out and halt the machine.
• ShutdownTypeLogout: Temporary brain damage. Don't use. Same as ShutdownTypeNone

So I tried the answer presented by Gilles, but that only works for KDE4.

After a system-update with my graphics, I could no longer log out, reboot, or shutdown. Eventually found this command worked:

qdbus org.kde.ksmserver /KSMServer logout 0 0 0

My source is from here, where they discuss it a bit more. I'm not sure about he other optoins. Forum topic discussion KDE5 shutdown options. The above command seems to have shutdown my system gracefully. All my programs came back that were expected, in the right layout order, and I did not seem to be missing anything. If this doesn't work, please comment and I will adjust my answer but so far this is all that has worked for a graceful KDE5 shutdown when my is locked. (I obviously could have used the shutdown command or called init, but those aren't graceful.)

For any version of KDE (maybe also GNOME and others):

In KDE Control Center (KDE3.5/trinity) you can set a keyboard shortcut for "halt without confirmation" (should be a complicated one to avoid tragedy, like Ctrl+Shift+Alt+Delete) and then run xvkbd (virtual keyboard):

xvkbd -text '\C\S\A\d'

You can create an alias for this (e.g. kdehalt);

My favourite one: sleep 1h 20m && kdehalt or wget "http://something" ; kdehalt.