Being able to edit PS1 (the shell prompt) was an eye-opener and productivity transform. My prompt has, among much fancier things, two newlines prefixed on to it, plus colorization to make the prompt stand out from the command it's receiving. I cannot emphasize enough how much this changed my ability to catch errors and reliably connect them to the command that generated 'em. I'm a better programmer for having discovered that one small detail that PS1 is customizable.