Terminating a Partial Command
If you begin a command in error or change your mind about issuing a command, you can use the CONTROL-C key sequence to cancel what you've typed so far and continue with a new command prompt.
Try it now:
john-williams@codermerlin:~$ echo iThoughtIKnewWhatIWasDoingBut CONTROL-C
If you began issuing a command but need to do something else in the shell before actually issuing it, an alternative to the above is to temporarily store what you've typed so far, do something else, then return to the original command. You can do that with:
|Yank the original text back