how do I change default text viewer?
Make sure, are you asking for text viewer or text editor?
in which file can I access/edit from terminal the bash options and general linux options?
Are you asking about shell or envorinmental variables? If so, then by editing the .bashrc (but I won't recommend doing this). Instead you can set value of any variable using following syntax in bash:
Then to check value assigned to any variable, use either:
Or for full listing, invoke cmd: