I had deleted an important file once
and later i had changed the permissions of a file
which i am not able to revert now. Is there any way through which i can undo delete or revert permissions on files while working with terminal?

And one more question about Ubuntu ... Why do we never get a dialog box asking 'are you sure you want to delete' as compared to windows.I see it as a flaw in Ubuntu GUI. Once you press delete,there it is in the trash but never prompts to ask if we really want it to be deleted and ofcourse, I know ppl would be like -- why you want reassurance before deleting...it is a part of HCI and should be implemented(I am sorry,just a beginner).