What is the default character encoding?

The default character encoding is UTF-8 (Unicode), though almost all (quite possibly all on a default install) file names are regular ASCII characters, common to most encodings.

I don't know what you mean by "how many strings are represented by a bash or python script". You can use Unicode characters in bash scripts on Ubuntu, but usually with a bash script, you call other programs, and whether those other programs will handle them is another matter. It's certainly possible to do so with Python too, though you'll want to familiarize yourself with the packages and settings related thereto.

• Encoding of filenames on the filesystem is utf-8.
• Bash thinks in bytes, not with strings-with-encoding-knowledge. So no default encoding. gnome-terminal's default encoding is utf-8
• Python's default encoding is ascii