How do I write code in Ubuntu terminal?

Using the shortcut Ctrl+Alt+N. Press F1 and then select or type Run Code. Right click the text editor and the click Run code from context menu.

Where do I write C++ code in Ubuntu?

You can use any text editors like Gedit, nano , Vi/Vim to write the code and compile it using gcc or g++ which are installed by default. Install the (sufficient) packages for once. You can know more about gcc and g++ from its man pages.