Execute a command without saving it in the history
Prepending one or more spaces to your command won't be saved in history.
Useful for passwords (those long mysql connections..) on the commandline. For example:
root@bsfw3-1:# echo 1
1
root@bsfw3-1:# echo 2
2
root@bsfw3-1:# echo 3
3
root@bsfw3-1:# history
...
501 echo 1
502 echo 3
503 history