How to input hash (#) symbol to terminal?

Alt is used as Meta on your system. That's why it behaves strangely in nano and bash.

On Ubuntu server (command-line only) with British keyboard layout, you need to press the right alt key to get the hash symbol #: Only the right alt is interpreted as alt-gr by your Linux system, and that's the modifier key required on non-Mac OS systems for the alternative characters on the keyboard.

I just came across this as I'm doing my LPIC and using a MB Pro running CentOS in a VM.

Ended up being CTRL+\

Ubuntu 12.04 with UK keyboard, installed on a MacBook Pro. Unfortunately the right-side Alt3 only emits a superscript "2", not the hash, And the left-side Alt3 prints nothing.

Going to System Preferences » Keyboard I can see that my current keyboard is listed as: English (UK, MacIntosh international).

Clicking on the little "keyboard" button at the bottom shows the position of all characters on each key. So for me the hash # is Right-Ctrl|\ (right Alt, pipe and backslash). One of the previous posts said Alt but it was Ctrl

In Terminal preferences, go to Profiles then Keyboard. Disable the Use Option as Meta Key option.

For me it's just the \ key.

(Jessie, Virtualbox on El Capitan, UK keyboard)