Cannot replicate. Are you sure this happens after
grep searchstring? The standard $PS2 is >, maybe this is what you saw. We can see it after grep 'searchstring (Enter) because of the unpaired quote. Normally Ctrl+C should help. Please confirm this happens after grep searchstring for you. If not, [edit] and give a real example. What is your OS? What is your shell? Terminal emulator? grep?