Why does the terminal show "^[[A" "^[[B" "^[[C" "^[[D" when pressing the arrow keys in Ubuntu?

Because that's what the keyboard actually sends to the PC (more precisely, what the terminal prints for what it actually receives from the keyboard). bash for example gets those values, deciphers them and understands that you want to move around, so it will either move the cursor (in case of left/right) or use its history to fetch previous commands (up/down). So you can't expect your program to magically support arrow keys.

However, reading from standard input from the terminal already supports left/right arrow keys (I believe, but I'm not in Linux right now to test and make sure). So my guess is that there is another issue interfering. One possible cause could be that one of your modifier keys is stuck? Perhaps ALT, CTRL or SUPER?

For those who are coming from the osx (mac) try changing the shells to bash

Terminal -> Preferences -> Shells open with -> [select] Command (complete path)

then paste

/bin/bash