exec replaces the current shell process with the command you specify (see 'help exec' or 'man bash'.) If that command fails for any reason, and the shell that you were working in was a login shell, you will naturally be logged out. If for, example, you are logged in on the console, getty assumes that your session is over once your shell completes. I expect your windowmanager is failing for some reason, such as failing to find an X server (is it running? is $DISPLAY set correctly?)
-- Launching a windowmanager by exec logs you out https://launchpad.net/bugs/48359 -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
